HyperSQL User Guide

HyperSQL Database Engine (HSQLDB) 2.3

Edited by

The HSQL Development Group

Edited by

Blaine Simpson

The HSQL Development Group

Edited by

Fred Toussi

The HSQL Development Group

Copyright 2002-2013 The HSQL Development Group. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. You are not allowed to distribute or display this document on the web in an altered form.

2014-02-13 18:21:41-0500


Table of Contents

Preface
Available formats for this document
1. Running and Using HyperSQL
Introduction
The HSQLDB Jar
Running Database Access Tools
A HyperSQL Database
In-Process Access to Database Catalogs
Server Modes
HyperSQL HSQL Server
HyperSQL HTTP Server
HyperSQL HTTP Servlet
Connecting to a Database Server
Security Considerations
Using Multiple Databases
Accessing the Data
Closing the Database
Creating a New Database
2. SQL Language
Standards Support
SQL Data and Tables
Temporary Tables
Persistent Tables
Short Guide to Data Types
Data Types and Operations
Numeric Types
Boolean Type
Character String Types
Binary String Types
Bit String Types
Lob Data
Storage and Handling of Java Objects
Type Length, Precision and Scale
Datetime types
Interval Types
Arrays
Array Definition
Array Reference
Array Operations
Indexes and Query Speed
Query Processing and Optimisation
Indexes and Conditions
Indexes and Operations
Indexes and ORDER BY, OFFSET and LIMIT
3. Sessions and Transactions
Overview
Session Attributes and Variables
Session Attributes
Session Variables
Session Tables
Transactions and Concurrency Control
Two Phase Locking
Two Phase Locking with Snapshot Isolation
Lock Contention in 2PL
Locks in SQL Routines and Triggers
MVCC
Choosing the Transaction Model
Schema and Database Change
Simultaneous Access to Tables
Viewing Sessions
Session and Transaction Control Statements
4. Schemas and Database Objects
Overview
Schemas and Schema Objects
Names and References
Character Sets
Collations
Distinct Types
Domains
Number Sequences
Tables
Views
Constraints
Assertions
Triggers
Routines
Indexes
Statements for Schema Definition and Manipulation
Common Elements and Statements
Renaming Objects
Commenting Objects
Schema Creation
Table Creation
Table Manipulation
View Creation and Manipulation
Domain Creation and Manipulation
Trigger Creation
Routine Creation
Sequence Creation
SQL Procedure Statement
Other Schema Object Creation
The Information Schema
Predefined Character Sets, Collations and Domains
Views in INFORMATION SCHEMA
Visibility of Information
Name Information
Data Type Information
Product Information
Operations Information
SQL Standard Views
5. Text Tables
Overview
The Implementation
Definition of Tables
Scope and Reassignment
Null Values in Columns of Text Tables
Configuration
Disconnecting Text Tables
Text File Usage
Text File Global Properties
Transactions
6. Access Control
Overview
Authorizations and Access Control
Built-In Roles and Users
Listing Users and Roles
Access Rights
Statements for Authorization and Access Control
7. Data Access and Change
Overview
Cursors And Result Sets
Columns and Rows
Navigation
Updatability
Sensitivity
Holdability
Autocommit
JDBC Overview
JDBC Parameters
JDBC and Data Change Statements
JDBC Callable Statement
JDBC Returned Values
Cursor Declaration
Syntax Elements
Literals
References, etc.
Value Expression
Predicates
Aggregate Functions
Other Syntax Elements
Data Access Statements
Select Statement
Table
Subquery
Query Specification
Table Expression
Joined Table
Selection
Projection
Computed Columns
Naming
Grouping Operations
Aggregation
Set Operations
With Clause and Recursive Queries
Query Expression
Ordering
Slicing
Data Change Statements
Delete Statement
Truncate Statement
Insert Statement
Update Statement
Merge Statement
Diagnostics and State
8. SQL-Invoked Routines
Routine Definition
Routine Characteristics
SQL Language Routines (PSM)
Advantages and Disadvantages
Routine Statements
Compound Statement
Table Variables
Variables
Cursors
Handlers
Assignment Statement
Select Statement : Single Row
Formal Parameters
Iterated Statements
Iterated FOR Statement
Conditional Statements
Return Statement
Control Statements
Raising Exceptions
Routine Polymorphism
Returning Data From Procedures
Recursive Routines
Java Language Routines (SQL/JRT)
Polymorphism
Java Language Procedures
Java Static Methods
Legacy Support
Securing Access to Classes
Warning
User Defined Aggregate Functions
Definition of Aggregate Functions
SQL PSM Aggregate Functions
Java Aggregate Functions
9. Triggers
Overview
BEFORE Triggers
AFTER Triggers
INSTEAD OF Triggers
Trigger Properties
Trigger Event
Granularity
Trigger Action Time
References to Rows
Trigger Condition
Trigger Action in SQL
Trigger Action in Java
Trigger Creation
10. Built In Functions
Overview
String and Binary String Functions
Numeric Functions
Date Time and Interval Functions
Functions to Report the Time Zone.
Functions to Report the Current Datetime
Functions to Extract an Element of a Datetime
Functions for Datetime Arithmetic
Functions to Convert or Format a Datetime
Array Functions
General Functions
System Functions
11. System Management
Mode of Operation and Tables
Mode of Operation
Database Types
Tables
Large Objects
Deployment context
ACID, Persistence and Reliability
Atomicity, Consistency, Isolation, Durability
System Operations
Backing Up Database Catalogs
Making Online Backups
Making Offline Backups
Examining Backups
Restoring a Backup
Encrypted Databases
Creating and Accessing an Encrypted Database
Speed Considerations
Security Considerations
Monitoring Database Operations
External Statement Level Monitoring
Internal Statement Level Monitoring
Internal Event Monitoring
Log4J and JDK logging
Server Operation Monitoring
Database Security
Security Defaults
Authentication Control
Compatibility with Other RDBMS
PostgreSQL Compatibility
MySQL Compatibility
Firebird Compatibility
Apache Derby Compatibility
Oracle Compatibility
DB2 Compatibility
MS SQLServer and Sybase Compatibility
Statements
System Operations
Database Settings
SQL Conformance Settings
Cache, Persistence and Files Settings
Authentication Settings
12. Properties
Connection URL
Variables In Connection URL
Connection properties
Database Properties in Connection URL and Properties
SQL Conformance Properties
Database Operations Properties
Database File and Memory Properties
Crypt Properties
System Properties
13. HyperSQL Network Listeners (Servers)
Listeners
HyperSQL Server
HyperSQL HTTP Server
HyperSQL HTTP Servlet
Server and Web Server Properties
Starting a Server from your Application
Allowing a Connection to Open or Create a Database
Specifying Database Properties at Server Start
TLS Encryption
Requirements
Encrypting your JDBC connection
Making a Private-key Keystore
Automatic Server or WebServer startup on UNIX
Network Access Control
14. HyperSQL on UNIX
Purpose
Installation
Setting up Database Catalog and Listener
Accessing your Database
Create additional Accounts
Shutdown
Running Hsqldb as a System Daemon
Portability of hsqldb init script
Init script Setup Procedure
Troubleshooting the Init Script
Upgrading
15. Deployment Guide
Memory and Disk Use
Table Memory Allocation
Result Set Memory Allocation
Temporary Memory Use During Operations
Data Cache Memory Allocation
Object Pool Memory Allocation
Lob Memory Usage
Disk Space
Managing Database Connections
Application Development and Testing
Tweaking the Mode of Operation
Embedded Databases in Desktop Applications
Embedded Databases in Server Applications
Mixed Mode : Embedding a HyperSQL Server (Listener)
Using HyperSQL Without Logging Data Change
Bulk Inserts, Updates and Deletes
Using NIO File Access
Server Databases
Upgrading Databases
Upgrading From Older Versions
Manual Changes to the *.script File
Backward Compatibility Issues
HyperSQL Dependency Settings for Applications
What version to Pull
Using the HyperSQL Snapshot Repository
Range Versioning
A. Lists of Keywords
List of SQL Standard Keywords
List of SQL Keywords Disallowed as HyperSQL Identifiers
Special Function Keywords
B. Building HyperSQL Jars
Purpose
Building with Gradle
Invoking a Gradle Build Graphically
Invoking a Gradle Build from the Command Line
Using Gradle
Building with Ant
Obtaining Ant
Building Hsqldb with Ant
Building for Older JDKs
Building with IDE Compilers
Hsqldb CodeSwitcher
Building Documentation
C. HyperSQL with OpenOffice
HyperSQL with OpenOffice
Using OpenOffice / LibreOffice as a Database Tool
Converting .odb files to use with HyperSQL Server
D. HyperSQL File Links
SQL Index
General Index

List of Tables

1. Available formats of this document
10.1. TO_CHAR, TO_DATE and TO_TIMESTAMP format elements
12.1. Memory Database URL
12.2. File Database URL
12.3. Resource Database URL
12.4. Server Database URL
12.5. User and Password
12.6. Closing old ResultSet when Statement is reused
12.7. Column Names in JDBC ResultSet
12.8. Creating New Database
12.9. Automatic Shutdown
12.10. Validity Check Property
12.11. SQL Keyword Use as Identifier
12.12. SQL Keyword Starting with the Underscore or Containing Dollar Characters
12.13. Reference to Columns Names
12.14. String Size Declaration
12.15. Type Enforcement in Comparison and Assignment
12.16. Foreign Key Triggered Data Change
12.17. Use of LOB for LONGVAR Types
12.18. Concatenation with NULL
12.19. NULL in Multi-Column UNIQUE Constraints
12.20. Truncation or Rounding in Type Conversion
12.21. Decimal Scale of Division and AVG Values
12.22. Support for NaN values
12.23. Sort order of NULL values
12.24. Sort order of NULL values with DESC
12.25. String comparison with padding
12.26. Case Insensitive Varchar columns
12.27. DB2 Style Syntax
12.28. MSSQL Style Syntax
12.29. MySQL Style Syntax
12.30. Oracle Style Syntax
12.31. PostgreSQL Style Syntax
12.32. Default Table Type
12.33. Transaction Control Mode
12.34. Default Isolation Level for Sessions
12.35. Transaction Rollback in Deadlock
12.36. Time Zone and Interval Types
12.37. Opening Database as Read Only
12.38. Opening Database Without Modifying the Files
12.39. Huge database files and tables
12.40. Event Logging
12.41. SQL Logging
12.42. Temporary Result Rows in Memory
12.43. Rows Cached In Memory
12.44. Rows Cached In Memory
12.45. Size of Rows Cached in Memory
12.46. Size Scale of Disk Table Storage
12.47. Size Scale of LOB Storage
12.48. Compression of BLOB and CLOB data
12.49. Internal Backup of Database Files
12.50. Use of Lock File
12.51. Logging Data Change Statements
12.52. Automatic Checkpoint Frequency
12.53. Automatic Defrag at Checkpoint
12.54. Compression of the .script file
12.55. Logging Data Change Statements Frequency
12.56. Logging Data Change Statements Frequency
12.57. Use of NIO for Disk Table Storage
12.58. Use of NIO for Disk Table Storage
12.59. Recovery Log Processing
12.60. Default Properties for TEXT Tables
12.61. Forcing Garbage Collection
12.62. Crypt Property For LOBs
12.63. Cipher Key for Encrypted Database
12.64. Crypt Provider Encrypted Database
12.65. Cipher Specification for Encrypted Database
12.66. Logging Framework
12.67. Text Tables
12.68. Java Functions
13.1. common server and webserver properties
13.2. server properties
13.3. webserver properties

List of Examples

1.1. Java code to connect to the local hsql Server
1.2. Java code to connect to the local http Server
1.3. Java code to connect to the local secure SSL hsql and http Servers
1.4. specifying a connection property to shutdown the database when the last connection is closed
1.5. specifying a connection property to disallow creating a new database
3.1. User-defined Session Variables
3.2. User-defined Temporary Session Tables
3.3. Setting Transaction Characteristics
3.4. Locking Tables
3.5. Rollback
3.6. Setting Session Characteristics
3.7. Setting Session Authorization
3.8. Setting Session Time Zone
4.1. inserting the next sequence value into a table row
4.2. numbering returned rows of a SELECT in sequential order
4.3. using the last value of a sequence
4.4. Column values which satisfy a 2-column UNIQUE constraint
11.1. Using CACHED tables for the LOB schema
11.2. Offline Backup Example
11.3. Listing a Backup with DbBackup
11.4. Restoring a Backup with DbBackup
11.5. SQL Log Example
11.6. Finding foreign key rows with no parents after a bulk import
13.1. Exporting certificate from the server's keystore
13.2. Adding a certificate to the client keystore
13.3. Specifying your own trust store to a JDBC client
13.4. Getting a pem-style private key into a JKS keystore
13.5. Validating and Testing an ACL file
14.1. example sqltool.rc stanza
15.1. Using CACHED tables for the LOB schema
15.2. MainInvoker Example
15.3. HyperSQL Snapshot Repository Definition
15.4. Sample Snapshot Ivy Dependency
15.5. Sample Snapshot Maven Dependency
15.6. Sample Snapshot Gradle Dependency
15.7. Sample Snapshot ivy.xml loaded by Ivyxml plugin
15.8. Sample Snapshot Groovy Dependency, using Grape
15.9. Sample Range Ivy Dependency
15.10. Sample Range Maven Dependency
15.11. Sample Range Gradle Dependency
15.12. Sample Range ivy.xml loaded by Ivyxml plugin
15.13. Sample Range Groovy Dependency, using Grape
B.1. Buiding the standard Hsqldb jar file with Ant
B.2. Example source code before CodeSwitcher is run
B.3. CodeSwitcher command line invocation
B.4. Source code after CodeSwitcher processing

Preface

HSQLDB (HyperSQL DataBase) is a modern relational database manager that conforms closely to the SQL:2008 Standard and JDBC 4 specifications. It supports all core features and many of the optional features of SQL:2008.

The first versions of HSQLDB were released in 2001. Version 2, first released in 2010, includes a complete rewrite of most parts of the database engine.

This documentation covers the latest HyperSQL version 2.3. This documentation is regularly improved and updated. The latest, updated version can be found at http://hsqldb.org/doc/2.0/

If you notice any mistakes in this document, or if you have problems with the procedures themselves, please use the HSQLDB support facilities which are listed at http://hsqldb.org/support

Available formats for this document

This document is available in several formats.

You may be reading this document right now at http://hsqldb.org/doc/2.0, or in a distribution somewhere else. I hereby call the document distribution from which you are reading this, your current distro.

http://hsqldb.org/doc/2.0 hosts the latest production versions of all available formats. If you want a different format of the same version of the document you are reading now, then you should try your current distro. If you want the latest production version, you should try http://hsqldb.org/doc/2.0.

Sometimes, distributions other than http://hsqldb.org/doc/2.0 do not host all available formats. So, if you can't access the format that you want in your current distro, you have no choice but to use the newest production version at http://hsqldb.org/doc/2.0.

Table 1. Available formats of this document


If you are reading this document now with a standalone PDF reader, the your distro links may not work.

Chapter 1. Running and Using HyperSQL

Fred Toussi

The HSQL Development Group

$Revision: 5240 $

Copyright 2002-2013 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Introduction

HyperSQL Database (HSQLDB ) is a modern relational database system. Version 2.3 is the latest release of the all-new version 2 code. Written from ground up to follow the international ISO SQL:2011 standard, it supports the complete set of the classic features, together with optional features such as stored procedures and triggers.

HyperSQL is used for development, testing and deployment of database applications.

Standard compliance is the most unique characteristic of HyperSQL. There are several other distinctive features. HyperSQL can provide database access within the user's application process, within an application server, or as a separate server process. HyperSQL can run entirely in memory using dedicated fast memory structures as opposed to ram disk. HyperSQL can use disk persistence in a flexible way, with reliable crash-recovery. HyperSQL is the only open-source relational database management system with a high performance dedicated lob storage system, suitable for gigabytes of lob data. It is also the only relational database that can create and access large comma delimited files as SQL tables. HyperSQL supports three live switchable transaction control models, including fully multi threaded MVCC, and is suitable for high performance transaction processing applications. HyperSQL is also suitable for business intelligence, ETL and other applications that process large data sets. HyperSQL has a wide range of enterprise deployment options, such as XA transactions, connection pooling data sources and remote authentication.

New SQL syntax compatibility modes have been added to HyperSQL. These modes allow a high degree of compatibility with several other database systems which use non-standard SQL syntax.

HyperSQL is written in the Java programming language and runs in a Java virtual machine (JVM). It supports the JDBC interface for database access.

An ODBC driver is also available as a separate download.

This guide covers the database engine features, SQL syntax and different modes of operation. The Server, JDBC interfaces, pooling and XA components are documented in the JavaDoc. Utilities such as SqlTool and DatabaseManager are covered in a separate Utilities Guide.

The HSQLDB Jar

The HSQLDB jar package, hsqldb.jar, is located in the /lib directory of the ZIP package and contains several components and programs.

Components of the Hsqldb jar package

  • HyperSQL RDBMS Engine (HSQLDB)

  • HyperSQL JDBC Driver

  • Database Manager (GUI database access tool, with Swing and AWT versions)

The HyperSQL RDBMS and JDBC Driver provide the core functionality. DatabaseManagers are general-purpose database access tools that can be used with any database engine that has a JDBC driver.

An additional jar, sqltool.jar, contains Sql Tool, command line database access tool. This is a general purpose command line database access tool that can be ued with other database engines as well.

Running Database Access Tools

The tools are used for interactive user access to databases, including creation of a database, inserting or modifying data, or querying the database. All tools are run in the normal way for Java programs. In the following example the Swing version of the Database Manager is executed. The hsqldb.jar is located in the directory ../lib relative to the current directory.

 java -cp ../lib/hsqldb.jar org.hsqldb.util.DatabaseManagerSwing

If hsqldb.jar is in the current directory, the command would change to:

 java -cp hsqldb.jar org.hsqldb.util.DatabaseManagerSwing

Main classes for the Hsqldb tools

  • org.hsqldb.util.DatabaseManager

  • org.hsqldb.util.DatabaseManagerSwing

When a tool is up and running, you can connect to a database (may be a new database) and use SQL commands to access and modify the data.

Tools can use command line arguments. You can add the command line argument --help to get a list of available arguments for these tools.

Double clicking the HSQLDB jar will start the DatabaseManagerSwing application.

A HyperSQL Database

Each HyperSQL database is called a catalog. There are three types of catalog depending on how the data is stored.

Types of catalog data

  • mem: stored entirely in RAM - without any persistence beyond the JVM process's life

  • file: stored in filesystem files

  • res: stored in a Java resource, such as a Jar and always read-only

All-in-memory, mem: catalogs can be used for test data or as sophisticated caches for an application. These databases do not have any files.

A file: catalog consists of between 2 to 6 files, all named the same but with different extensions, located in the same directory. For example, the database named "test" consists of the following files:

  • test.properties

  • test.script

  • test.log

  • test.data

  • test.backup

  • test.lobs

The properties file contains a few settings about the database. The script file contains the definition of tables and other database objects, plus the data for non-cached tables. The log file contains recent changes to the database. The data file contains the data for cached tables and the backup file is a compressed backup of the last known consistent state of the data file. All these files are essential and should never be deleted. For some catalogs, the test.data and test.backup files will not be present. In addition to those files, a HyperSQL database may link to any formatted text files, such as CSV lists, anywhere on the disk.

While the "test" catalog is open, a test.log file is used to write the changes made to data. This file is removed at a normal SHUTDOWN. Otherwise (with abnormal shutdown) this file is used at the next startup to redo the changes. A test.lck file is also used to record the fact that the database is open. This is deleted at a normal SHUTDOWN.

[Note]Note

When the engine closes the database at a shutdown, it creates temporary files with the extension .new which it then renames to those listed above. These files should not be deleted by the user. At the time of the next startup, all such files will be renamed or deleted by the database engine. In some circumstances, a test.data.xxx.old is created and deleted afterwards by the database engine. The user can delete these test.data.xxx.old files.

A res: catalog consists of the files for a small, read-only database that can be stored inside a Java resource such as a ZIP or JAR archive and distributed as part of a Java application program.

In-Process Access to Database Catalogs

In general, JDBC is used for all access to databases. This is done by making a connection to the database, then using various methods of the java.sql.Connection object that is returned to access the data. Access to an in-process database is started from JDBC, with the database path specified in the connection URL. For example, if the file: database name is "testdb" and its files are located in the same directory as where the command to run your application was issued, the following code is used for the connection:

 Connection c = DriverManager.getConnection("jdbc:hsqldb:file:testdb", "SA", "");

The database file path format can be specified using forward slashes in Windows hosts as well as Linux hosts. So relative paths or paths that refer to the same directory on the same drive can be identical. For example if your database directory in Linux is /opt/db/ containing a database testdb (with files named testdb.*), then the database file path is /opt/db/testdb. If you create an identical directory structure on the C: drive of a Windows host, you can use the same URL in both Windows and Linux:

 Connection c = DriverManager.getConnection("jdbc:hsqldb:file:/opt/db/testdb", "SA", "");

When using relative paths, these paths will be taken relative to the directory in which the shell command to start the Java Virtual Machine was executed. Refer to the Javadoc for JDBCConnection for more details.

Paths and database names for file databases are treated as case-sensitive when the database is created or the first connection is made to the database. But if a second connection is made to an open database, using a path and name that differs only in case, then the connection is made to the existing open database. This measure is necessary because in Windows the two paths are equivalent.

A mem: database is specified by the mem: protocol. For mem: databases, the path is simply a name. Several mem: databases can exist at the same time and distinguished by their names. In the example below, the database is called "mymemdb":

 Connection c = DriverManager.getConnection("jdbc:hsqldb:mem:mymemdb", "SA", "");

A res: database, is specified by the res: protocol. As it is a Java resource, the database path is a Java URL (similar to the path to a class). In the example below, "resdb" is the root name of the database files, which exists in the directory "org/my/path" within the classpath (probably in a Jar). A Java resource is stored in a compressed format and is decompressed in memory when it is used. For this reason, a res: database should not contain large amounts of data and is always read-only.

 Connection c = DriverManager.getConnection("jdbc:hsqldb:res:org.my.path.resdb", "SA", "");

The first time in-process connection is made to a database, some general data structures are initialised and a few helper threads are started. After this, creation of connections and calls to JDBC methods of the connections execute as if they are part of the Java application that is making the calls. When the SQL command "SHUTDOWN" is executed, the global structures and helper threads for the database are destroyed.

Note that only one Java process at a time can make in-process connections to a given file: database. However, if the file: database has been made read-only, or if connections are made to a res: database, then it is possible to make in-process connections from multiple Java processes.

Server Modes

For most applications, in-process access is faster, as the data is not converted and sent over the network. The main drawback is that it is not possible by default to connect to the database from outside your application. As a result you cannot check the contents of the database with external tools such as Database Manager while your application is running.

Server modes provide the maximum accessibility. The database engine runs in a JVM and opens one or more in-process catalogs. It listens for connections from programs on the same computer or other computers on the network. It translates these connections into in-process connections to the databases.

Several different programs can connect to the server and retrieve or update information. Applications programs (clients) connect to the server using the HyperSQL JDBC driver. In most server modes, the server can serve an unlimited number of databases that are specified at the time of running the server, or optionally, as a connection request is received.

A Sever mode is also the preferred mode of running the database during development. It allows you to query the database from a separate database access utility while your application is running.

There are three server modes, based on the protocol used for communications between the client and server. They are briefly discussed below. More details on servers is provided in the HyperSQL Network Listeners (Servers) chapter.

HyperSQL HSQL Server

This is the preferred way of running a database server and the fastest one. A proprietary communications protocol is used for this mode. A command similar to those used for running tools and described above is used for running the server. The following example of the command for starting the server starts the server with one (default) database with files named "mydb.*" and the public name of "xdb". The public name hides the file names from users.

 java -cp ../lib/hsqldb.jar org.hsqldb.server.Server --database.0 file:mydb --dbname.0 xdb

The command line argument --help can be used to get a list of available arguments.

HyperSQL HTTP Server

This method of access is used when the computer hosting the database server is restricted to the HTTP protocol. The only reason for using this method of access is restrictions imposed by firewalls on the client or server machines and it should not be used where there are no such restrictions. The HyperSQL HTTP Server is a special web server that allows JDBC clients to connect via HTTP. The server can also act as a small general-purpose web server for static pages.

To run an HTTP server, replace the main class for the server in the example command line above with the following:

 org.hsqldb.server.WebServer

The command line argument --help can be used to get a list of available arguments.

HyperSQL HTTP Servlet

This method of access also uses the HTTP protocol. It is used when a separate servlet engine (or application server) such as Tomcat or Resin provides access to the database. The Servlet Mode cannot be started independently from the servlet engine. The Servlet class, in the HSQLDB jar, should be installed on the application server to provide the connection. The database file path is specified using an application server property. Refer to the source file src/org/hsqldb/server/Servlet.java to see the details.

Both HTTP Server and Servlet modes can be accessed using the JDBC driver at the client end. They do not provide a web front end to the database. The Servlet mode can serve multiple databases.

Please note that you do not normally use this mode if you are using the database engine in an application server. In this situation, connections to a catalog are usually made in-process, or using a separate Server

Connecting to a Database Server

When a HyperSQL server is running, client programs can connect to it using the HSQLDB JDBC Driver contained in hsqldb.jar. Full information on how to connect to a server is provided in the Java Documentation for JDBCConnection (located in the /doc/apidocs directory of HSQLDB distribution). A common example is connection to the default port (9001) used for the hsql: protocol on the same machine:

Example 1.1. Java code to connect to the local hsql Server

 try {
     Class.forName("org.hsqldb.jdbc.JDBCDriver" );
 } catch (Exception e) {
     System.err.println("ERROR: failed to load HSQLDB JDBC driver.");
     e.printStackTrace();
     return;
 }

 Connection c = DriverManager.getConnection("jdbc:hsqldb:hsql://localhost/xdb", "SA", "");

If the HyperSQL HTTP server is used, the protocol is http: and the URL will be different:

Example 1.2. Java code to connect to the local http Server

 Connection c = DriverManager.getConnection("jdbc:hsqldb:http://localhost/xdb", "SA", "");

Note in the above connection URL, there is no mention of the database file, as this was specified when running the server. Instead, the public name defined for dbname.0 is used. Also, see the HyperSQL Network Listeners (Servers) chapter for the connection URL when there is more than one database per server instance.

Security Considerations

When a HyperSQL server is run, network access should be adequately protected. Source IP addresses may be restricted by use of our Access Control List feature, network filtering software, firewall software, or standalone firewalls. Only secure passwords should be used-- most importantly, the password for the default system user should be changed from the default empty string. If you are purposefully providing data to the public, then the wide-open public network connection should be used exclusively to access the public data via read-only accounts. (i.e., neither secure data nor privileged accounts should use this connection). These considerations also apply to HyperSQL servers run with the HTTP protocol.

HyperSQL provides two optional security mechanisms. The encrypted SSL protocol, and Access Control Lists. Both mechanisms can be specified when running the Server or WebServer. On the client, the URL to connect to an SSL server is slightly different:

Example 1.3. Java code to connect to the local secure SSL hsql and http Servers

 Connection c = DriverManager.getConnection("jdbc:hsqldb:hsqls://localhost/xdb", "SA", "");
 Connection c = DriverManager.getConnection("jdbc:hsqldb:https://localhost/xdb", "SA", "");


The security features are discussed in detail in the HyperSQL Network Listeners (Servers) chapter.

Using Multiple Databases

A server can provide connections to more than one database. In the examples above, more than one set of database names can be specified on the command line. It is also possible to specify all the databases in a .properties file, instead of the command line. These capabilities are covered in the HyperSQL Network Listeners (Servers) chapter

Accessing the Data

As shown so far, a java.sql.Connection object is always used to access the database. But the speed and performance depends on the type of connection.

Establishing a connection and closing it has some overheads, therefore it is not good practice to create a new connection to perform a small number of operations. A connection should be reused as much as possible and closed only when it is not going to be used again for a long while.

Reuse is more important for server connections. A server connection uses a TCP port for communications. Each time a connection is made, a port is allocated by the operating system and deallocated after the connection is closed. If many connections are made from a single client, the operating system may not be able to keep up and may refuse the connection attempt.

A java.sql.Connection object has some methods that return further java.sql.* objects. All these objects belong to the connection that returned them and are closed when the connection is closed. These objects can be reused, but if they are not needed after performing the operations, they should be closed.

A java.sql.DatabaseMetaData object is used to get metadata for the database.

A java.sql.Statement object is used to execute queries and data change statements. A java.sql.Statement can be reused to execute a different statement each time.

A java.sql.PreparedStatement object is used to execute a single statement repeatedly. The SQL statement usually contains parameters, which can be set to new values before each reuse. When a java.sql.PreparedStatement object is created, the engine keeps the compiled SQL statement for reuse, until the java.sql.PreparedStatement object is closed. As a result, repeated use of a java.sql.PreparedStatement is much faster than using a java.sql.Statement object.

A java.sql.CallableStatement object is used to execute an SQL CALL statement. The SQL CALL statement may contain parameters, which should be set to new values before each reuse. Similar to java.sql.PreparedStatement, the engine keeps the compiled SQL statement for reuse, until the java.sql.CallableStatement object is closed.

A java.sql.Connection object also has some methods for transaction control.

The commit() method performs a COMMIT while the rollback() method performs a ROLLBACK SQL statement.

The setSavepoint(String name) method performs a SAVEPOINT <name> SQL statement and returns a java.sql.Savepoint object. The rollback(Savepoint name) method performs a ROLLBACK TO SAVEPOINT <name> SQL statement.

The Javadoc for JDBCConnection, JDBCDriver, JDBCDatabaseMetadata JDBCResultSet, JDBCStatement, JDBCPreparedStatement list all the supported JDBC methods together with information that is specific to HSQLDB.

Closing the Database

All databases running in different modes can be closed with the SHUTDOWN command, issued as an SQL statement.

When SHUTDOWN is issued, all active transactions are rolled back. The catalog files are then saved in a form that can be opened quickly the next time the catalog is opened.

A special form of closing the database is via the SHUTDOWN COMPACT command. This command rewrites the .data file that contains the information stored in CACHED tables and compacts it to its minimum size. This command should be issued periodically, especially when lots of inserts, updates or deletes have been performed on the cached tables. Changes to the structure of the database, such as dropping or modifying populated CACHED tables or indexes also create large amounts of unused file space that can be reclaimed using this command.

Databases are not closed when the last connection to the database is explicitly closed via JDBC. A connection property, shutdown=true, can be specified on the first connection to the database (the connection that opens the database) to force a shutdown when the last connection closes.

Example 1.4. specifying a connection property to shutdown the database when the last connection is closed

 Connection c = DriverManager.getConnection(
         "jdbc:hsqldb:file:/opt/db/testdb;shutdown=true", "SA", "");


This feature is useful for running tests, where it may not be practical to shutdown the database after each test. But it is not recommended for application programs.

Creating a New Database

When a server instance is started, or when a connection is made to an in-process database, a new, empty database is created if no database exists at the given path.

With HyperSQL 2.0 the username and password that are specified for the connection are used for the new database. Both the username and password are case-sensitive. (The exception is the default SA user, which is not case-sensitive). If no username or password is specified, the default SA user and an empty password are used.

This feature has a side effect that can confuse new users. If a mistake is made in specifying the path for connecting to an existing database, a connection is nevertheless established to a new database. For troubleshooting purposes, you can specify a connection property ifexists=true to allow connection to an existing database only and avoid creating a new database. In this case, if the database does not exist, the getConnection() method will throw an exception.

Example 1.5. specifying a connection property to disallow creating a new database

 Connection c = DriverManager.getConnection(
         "jdbc:hsqldb:file:/opt/db/testdb;ifexists=true", "SA", "");


A database has many optional properties, described in the System Management chapter. You can specify most of these properties on the URL or in the connection properties for the first connection that creates the database. See the Properties chapter.

Chapter 2. SQL Language

Fred Toussi

The HSQL Development Group

$Revision: 5287 $

Copyright 2002-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Standards Support

HyperSQL 2 supports the dialect of SQL defined by SQL standards 92, 1999, 2003, 2008 and 2011. This means where a feature of the standard is supported, e.g. left outer join, the syntax is that specified by the standard text. Almost all syntactic features of SQL-92 up to Advanced Level are supported, as well as SQL:2011 core and many optional features of this standard. Work is in progress for a formal declaration of conformance.

At the time of this release, HyperSQL supports the widest range of SQL standard features among all open source RDBMS.

Various chapters of this guide list the supported syntax. When writing or converting existing SQL DDL (Data Definition Language), DML (Data Manipulation Language) or DQL (Data Query Language) statements for HSQLDB, you should consult the supported syntax and modify the statements accordingly. Some statements written for older versions may have to be modified.

Over 300 words are reserved by the standard and should not be used as table or column names. For example, the word POSITION is reserved as it is a function defined by the Standards with a similar role as String.indexOf() in Java. HyperSQL does not currently prevent you from using a reserved word if it does not support its use or can distinguish it. For example CUBE is a reserved words that is not currently supported by HyperSQL and is allowed as a table or column name. You should avoid using such names as future versions of HyperSQL are likely to support the reserved words and may reject your table definitions or queries. The full list of SQL reserved words is in the appendix Lists of Keywords .

There are several user-defined properties to control the strict application of the SQL Standard in different areas.

If you have to use a reserved keyword as the name of a database object, you can enclose it in double quotes.

HyperSQL also supports enhancements with keywords and expressions that are not part of the SQL standard. Expressions such as SELECT TOP 5 FROM .., SELECT LIMIT 0 10 FROM ... or DROP TABLE mytable IF EXISTS are among such constructs.

Many print books cover SQL Standard syntax and can be consulted.

In HyperSQL version 2, all features of JDBC4 that apply to the capabilities of HSQLDB are fully supported. The relevant JDBC classes are thoroughly documented with additional clarifications and HyperSQL specific comments. See the JavaDoc for the org.hsqldb.jdbc.* classes.

SQL Data and Tables

In an SQL system, all significant data is stored in tables and sequence generators. Therefore, the first step in creating a database is defining the tables and their columns. The SQL standard supports temporary tables, which are for temporary data, and permanent base tables, which are for persistent data.

Temporary Tables

Data in TEMPORARY tables is not saved and lasts only for the lifetime of the session. The contents of each TEMP table is visible only from the session that is used to populate it.

HyperSQL supports two types of temporary tables.

The GLOBAL TEMPORARY type is a schema object. It is created with the CREATE GLOBAL TEMPORARY TABLE statement. The definition of the table persists, and each session has access to the table. But each session sees its own copy of the table, which is empty at the beginning of the session.

The LOCAL TEMPORARY type is not a schema object. It is created with the DECLARE LOCAL TEMPORARY TABLE statement. The table definition lasts only for the duration of the session and is not persisted in the database. The table can be declared in the middle of a transaction without committing the transaction. If a schema name is needed to reference these tables in a given SQL statement, the pseudo schema names MODULE or SESSION can be used.

When the session commits, the contents of all temporary tables are cleared by default. If the table definition statements includes ON COMMIT PRESERVE ROWS, then the contents are kept when a commit takes place.

The rows in temporary tables are stored in memory by default. If the hsqldb.result_max_memory_rows property has been set or the SET SESSION RESULT MEMORY ROWS <row count> has been specified, tables with row count above the setting are stored on disk.

Persistent Tables

HyperSQL supports the Standard definition of persistent base table, but defines three types according to the way the data is stored. These are MEMORY tables, CACHED tables and TEXT tables.

Memory tables are the default type when the CREATE TABLE command is used. Their data is held entirely in memory but any change to their structure or contents is written to the *.log and *.script files. The *.script file and the *.log file are read the next time the database is opened, and the MEMORY tables are recreated with all their contents. So unlike TEMPORARY tables, MEMORY tables are persistent. When the database is opened, all the data for the memory tables is read and inserted. This process may take a long time if the database is larger than tens of megabytes. When the database is shutdown, all the data is saved. This can also take a long time.

CACHED tables are created with the CREATE CACHED TABLE command. Only part of their data or indexes is held in memory, allowing large tables that would otherwise take up to several hundred megabytes of memory. Another advantage of cached tables is that the database engine takes less time to start up when a cached table is used for large amounts of data. The disadvantage of cached tables is a reduction in speed. Do not use cached tables if your data set is relatively small. In an application with some small tables and some large ones, it is better to use the default, MEMORY mode for the small tables.

TEXT tables use a CSV (Comma Separated Value) or other delimited text file as the source of their data. You can specify an existing CSV file, such as a dump from another database or program, as the source of a TEXT table. Alternatively, you can specify an empty file to be filled with data by the database engine. TEXT tables are efficient in memory usage as they cache only part of the text data and all of the indexes. The Text table data source can always be reassigned to a different file if necessary. The commands are needed to set up a TEXT table as detailed in the Text Tables chapter.

With all-in-memory databases, both MEMORY table and CACHED table declarations are treated as declarations for non-persistent memory tables. In the latest versions of HyperSQL, TEXT table declarations are allowed in all-in-memory databases.

The default type of tables resulting from future CREATE TABLE statements can be specified with the SQL command:

 SET DATABASE DEFAULT TABLE TYPE { CACHED | MEMORY };

The type of an existing table can be changed with the SQL command:

 SET TABLE <table name> TYPE { CACHED | MEMORY };

SQL statements access different types of tables uniformly. No change to statements is needed to access different types of table.

Short Guide to Data Types

Most other RDBMS do not conform to the SQL Standard in all areas, but they are gradually moving towards Standard conformance. When switching from another SQL dialect, the following should be considered:

  • Numeric types TINYINT, SMALLINT, INTEGER and BIGINT are types with fixed binary precision. These types are more efficient to store and retrieve. NUMERIC and DECIMAL are types with user-defined decimal precision. They can be used with zero scale to store very large integers, or with a non-zero scale to store decimal fractions. The DOUBLE type is a 64 bit, approximate floating point types. HyperSQL even allows you to store infinity in this type.

  • The BOOLEAN type is for logical values and can hold TRUE, FALSE or UNKNOWN. Although HyperSQL allows you to use one and zero in assignment or comparison, you should use the standard values for this type.

  • Character string types are CHAR(L), VARCHAR(L) and CLOB. CHAR is for fixed width strings and any string that is assigned to this type is padded with spaces at the end. Do not use this type for general storage of strings. If you use CHAR without the length L, then it is interpreted as a single character string. Use VARCHAR(L) for general strings. There are only memory limits and performance implications for the maximum length of VARCHAR(L). If the strings are larger than a few kilobytes, consider using CLOB. The CLOB types is for very large strings. Do not use this type for short strings as there are performance implications. The CLOB type is a better choice for the storage of long strings. By default LONGVARCHAR is a synonym for a long VARCHAR and can be used without specifying the size. You can set LONGVARCHAR to map to CLOB, with the sql.longvar_is_lob connection property or the SET DATABASE SQL LONGVAR IS LOB TRUE statement.

  • Binary string types are BINARY(L), VARBINARY(L) and BLOB. Do not use BINARY(L) unless you are storing keys such as UUID. This type pads short binary strings with zero bytes. BINARY without the length L means a single byte. Use VARBINARY(L) for general binary strings, and BLOB for large binary objects. You should apply the same considerations as with the character string types. By default LONGVARBINARY is a synonym for a long VARCHAR and can be used without specifying the size. You can set LONGVARBINARY to map to BLOB, with the sql.longvar_is_lob connection property or the SET DATABASE SQL LONGVAR IS LOB TRUE statement.

  • The BIT(L) and BITVARYING(L) types are for bit maps. Do not use them for other types of data. BIT without the length L argument means a single bit and is sometimes used as a logical type. Use BOOLEAN instead of this type.

  • The datetime types DATE, TIME and TIMESTAMP, together with their WITH TIME ZONE variations are available. Read the details in this chapter on how to use these types.

  • The INTERVAL type is very powerful when used together with the datetime types. This is very easy to use, but is supported mainly by "big iron" database systems. Note that functions that add days or months to datetime values are not really a substitute for the INTERVAL type. Expressions such as (datecol - 7 DAY) > CURRENT_DATE are optimised to use indexes when it is possible, while the equivalent function calls are not optimised.

  • The OTHER type is for storage of Java objects. If your objects are large, serialize them in your application and store them as BLOB in the database.

  • The ARRAY type supports all base types except LOB and OTHER types. ARRAY data objects are held in memory while being processed. It is therefore not recommended to store more than about a thousand objects in an ARRAY in normal operations with disk based databases. For specialised applications, use ARRAY with as many elements as your memory allocation can support.

HyperSQL 2.3 has several compatibility modes which allow the type names that are used by other RDBMS to be accepted and translated into the closest SQL Standard type. For example the type TEXT, supported by MySQL and PostgreSQL is translated in these compatibility modes.

Data Types and Operations

HyperSQL supports all the types defined by SQL-92, plus BOOLEAN, BINARY and LOB types that were later added to the SQL Standard. It also supports the non-standard OTHER type to store serializable Java objects.

SQL is a strongly typed language. All data stored in specific columns of tables and other objects (such as sequence generators) have specific types. Each data item conforms to the type limits such as precision and scale for the column. It also conforms to any additional integrity constraints that are defined as CHECK constraints in domains or tables. Types can be explicitly converted using the CAST expression, but in most expressions they are converted automatically.

Data is returned to the user (or the application program) as a result of executing SQL statements such as query expressions or function calls. All statements are compiled prior to execution and the return type of the data is known after compilation and before execution. Therefore, once a statement is prepared, the data type of each column of the returned result is known, including any precision or scale property. The type does not change when the same query that returned one row, returns many rows as a result of adding more data to the tables.

Some SQL functions used within SQL statements are polymorphic, but the exact type of the argument and the return value is determined at compile time.

When a statement is prepared, using a JDBC PreparedStatement object, it is compiled by the engine and the type of the columns of its ResultSet and / or its parameters are accessible through the methods of PreparedStatement.

Numeric Types

TINYINT, SMALLINT, INTEGER, BIGINT, NUMERIC and DECIMAL (without a decimal point) are the supported integral types. They correspond respectively to byte, short, int, long, BigDecimal and BigDecimal Java types in the range of values that they can represent (NUMERIC and DECIMAL are equivalent). The type TINYINT is an HSQLDB extension to the SQL Standard, while the others conform to the Standard definition. The SQL type dictates the maximum and minimum values that can be held in a field of each type. For example the value range for TINYINT is -128 to +127. The bit precision of TINYINT, SMALLINT, INTEGER and BIGINT is respectively 8, 16, 32 and 64. For NUMERIC and DECIMAL, decimal precision is used.

DECIMAL and NUMERIC with decimal fractions are mapped to java.math.BigDecimal and can have very large numbers of digits. In HyperSQL the two types are equivalent. These types, together with integral types, are called exact numeric types.

In HyperSQL, REAL, FLOAT, DOUBLE are equivalent and all mapped to double in Java. These types are defined by the SQL Standard as approximate numeric types. The bit-precision of all these types is 64 bits.

The decimal precision and scale of NUMERIC and DECIMAL types can be optionally defined. For example, DECIMAL(10,2) means maximum total number of digits is 10 and there are always 2 digits after the decimal point, while DECIMAL(10) means 10 digits without a decimal point. The bit-precision of FLOAT can be defined but it is ignored and the default bit-precision of 64 is used. The default precision of NUMERIC and DECIMAL (when not defined) is 100.

Note: If a database has been set to ignore type precision limits with the SET DATABASE SQL SIZE FALSE command, then a type definition of DECIMAL with no precision and scale is treated as DECIMAL(100,10). In normal operation, it is treated as DECIMAL(100).

Integral Types

In expressions, TINYINT, SMALLINT, INTEGER, BIGINT, NUMERIC and DECIMAL (without a decimal point) can be freely combined and no data narrowing takes place. The resulting value is of a type that can support all possible values.

If the SELECT statement refers to a simple column or function, then the return type is the type corresponding to the column or the return type of the function. For example:

 CREATE TABLE t(a INTEGER, b BIGINT);
 SELECT MAX(a), MAX(b) FROM t;

will return a ResultSet where the type of the first column is java.lang.Integer and the second column is java.lang.Long. However,

 SELECT MAX(a) + 1, MAX(b) + 1 FROM t;

will return java.lang.Long and BigDecimal values, generated as a result of uniform type promotion for all the return values. Note that type promotion to BigDecimal ensures the correct value is returned if MAX(b) evaluates to Long.MAX_VALUE.

There is no built-in limit on the size of intermediate integral values in expressions. As a result, you should check for the type of the ResultSet column and choose an appropriate getXXXX() method to retrieve it. Alternatively, you can use the getObject() method, then cast the result to java.lang.Number and use the intValue() or longValue() methods on the result.

When the result of an expression is stored in a column of a database table, it has to fit in the target column, otherwise an error is returned. For example when 1234567890123456789012 / 12345687901234567890 is evaluated, the result can be stored in any integral type column, even a TINYINT column, as it is a small value.

In SQL Statements, an integer literal is treated as INTEGER, unless its value does not fit. In this case it is treated as BIGINT or DECIMAL, depending on the value.

Depending on the types of the operands, the result of the operation is returned in a JDBC ResultSet in any of the related Java types: Integer, Long or BigDecimal. The ResultSet.getXXXX() methods can be used to retrieve the values so long as the returned value can be represented by the resulting type. This type is deterministically based on the query, not on the actual rows returned.

Other Numeric Types

In SQL statements, number literals with a decimal point are treated as DECIMAL unless they are written with an exponent. Thus 0.2 is considered a DECIMAL value but 0.2E0 is considered a DOUBLE value.

When an approximate numeric type, REAL, FLOAT or DOUBLE (all synonymous) is part of an expression involving different numeric types, the type of the result is DOUBLE. DECIMAL values can be converted to DOUBLE unless they are beyond the Double.MIN_VALUE - Double.MAX_VALUE range. For example, A * B, A / B, A + B, etc. will return a DOUBLE value if either A or B is a DOUBLE.

Otherwise, when no DOUBLE value exists, if a DECIMAL or NUMERIC value is part an expression, the type of the result is DECIMAL or NUMERIC. Similar to integral values, when the result of an expression is assigned to a table column, the value has to fit in the target column, otherwise an error is returned. This means a small, 4 digit value of DECIMAL type can be assigned to a column of SMALLINT or INTEGER, but a value with 15 digits cannot.

When a DECIMAL value is multiplied by a DECIMAL or integral type, the resulting scale is the sum of the scales of the two terms. When they are divided, the result is a value with a scale (number of digits to the right of the decimal point) equal to the larger of the scales of the two terms. The precision for both operations is calculated (usually increased) to allow all possible results.

The distinction between DOUBLE and DECIMAL is important when a division takes place. For example, 10.0/8.0 (DECIMAL) equals 1.2 but 10.0E0/8.0E0 (DOUBLE) equals 1.25. Without division operations, DECIMAL values represent exact arithmetic.

REAL, FLOAT and DOUBLE values are all stored in the database as java.lang.Double objects. Special values such as NaN and +-Infinity are also stored and supported. These values can be submitted to the database via JDBC PreparedStatement methods and are returned in ResultSet objects. In order to allow division by zero of DOUBLE values in SQL statements (which returns NaN or +-Infinity) you should set the property hsqldb.double_nan as false (SET DATABASE SQL DOUBLE NAN FALSE). The double values can be retrieved from a ResultSet in the required type so long as they can be represented. For setting the values, when PreparedStatement.setDouble() or setFloat() is used, the value is treated as a DOUBLE automatically.

In short,

<numeric type> ::= <exact numeric type> | <approximate numeric type>

<exact numeric type> ::= NUMERIC [ <left paren> <precision> [ <comma> <scale> ] <right paren> ] | { DECIMAL | DEC } [ <left paren> <precision> [ <comma> <scale> ] <right paren> ] | SMALLINT | INTEGER | INT | BIGINT

<approximate numeric type> ::= FLOAT [ <left paren> <precision> <right paren> ] | REAL | DOUBLE PRECISION

<precision> ::= <unsigned integer>

<scale> ::= <unsigned integer>

Boolean Type

The BOOLEAN type conforms to the SQL Standard and represents the values TRUE, FALSE and UNKNOWN. This type of column can be initialised with Java boolean values, or with NULL for the UNKNOWN value.

The three-value logic is sometimes misunderstood. For example, x IN (1, 2, NULL) does not return true if x is NULL.

In previous versions of HyperSQL, BIT was simply an alias for BOOLEAN. In version 2, BIT is a single-bit bit map.

<boolean type> ::= BOOLEAN

The SQL Standard does not support type conversion to BOOLEAN apart from character strings that consists of boolean literals. Because the BOOLEAN type is relatively new to the Standard, several database products used other types to represent boolean values. For improved compatibility, HyperSQL allows some type conversions to boolean.

Values of BIT and BIT VARYING types with length 1 can be converted to BOOLEAN. If the bit is set, the result of conversion is the TRUE value, otherwise it is FALSE.

Values of TINYINT, SMALLINT, INTEGER and BIGINT types can be converted to BOOLEAN. If the value is zero, the result is the FALSE value, otherwise it is TRUE.

Character String Types

The CHARACTER, CHARACTER VARYING and CLOB types are the SQL Standard character string types. CHAR, VARCHAR and CHARACTER LARGE OBJECT are synonyms for these types. HyperSQL also supports LONGVARCHAR as a synonym for VARCHAR. If LONGVARCHAR is used without a length, then a length of 16M is assigned. You can set LONGVARCHAR to map to CLOB, with the sql.longvar_is_lob connection property or the SET DATABASE SQL LONGVAR IS LOB TRUE statement..

HyperSQL's default character set is Unicode, therefore all possible character strings can be represented by these types.

The SQL Standard behaviour of the CHARACTER type is a remnant of legacy systems in which character strings are padded with spaces to fill a fixed width. These spaces are sometimes significant while in other cases they are silently discarded. It would be best to avoid the CHARACTER type altogether. With the rest of the types, the strings are not padded when assigned to columns or variables of the given type. The trailing spaces are still considered discardable for all character types. Therefore if a string with trailing spaces is too long to assign to a column or variable of a given length, the spaces beyond the type length are discarded and the assignment succeeds (provided all the characters beyond the type length are spaces).

The VARCHAR and CLOB types have length limits, but the strings are not padded by the system. Note that if you use a large length for a VARCHAR or CLOB type, no extra space is used in the database. The space used for each stored item is proportional to its actual length.

If CHARACTER is used without specifying the length, the length defaults to 1. For the CLOB type, the length limit can be defined in units of kilobyte (K, 1024), megabyte (M, 1024 * 1024) or gigabyte (G, 1024 * 1024 * 1024), using the <multiplier>. If CLOB is used without specifying the length, the length defaults to 1GB.

<character string type> ::= { CHARACTER | CHAR } [ <left paren> <character length> <right paren> ] | { CHARACTER VARYING | CHAR VARYING | VARCHAR } <left paren> <character length> <right paren> | LONGVARCHAR [ <left paren> <character length> <right paren> ] | <character large object type>

<character large object type> ::= { CHARACTER LARGE OBJECT | CHAR LARGE OBJECT | CLOB } [ <left paren> <character large object length> <right paren> ]

<character length> ::= <unsigned integer> [ <char length units> ]

<large object length> ::= <length> [ <multiplier> ] | <large object length token>

<character large object length> ::= <large object length> [ <char length units> ]

<large object length token> ::= <digit>... <multiplier>

<multiplier> ::= K | M | G

<char length units> ::= CHARACTERS | OCTETS

Each character type has a collation. This is either a default collation or stated explicitly with the COLLATE clause. Collations are discussed in the Schemas and Database Objects chapter.

 CHAR(10)
 CHARACTER(10)
 VARCHAR(2)
 CHAR VARYING(2)
 CLOB(1000)
 CLOB(30K)
 CHARACTER LARGE OBJECT(1M)
 LONGVARCHAR

Binary String Types

The BINARY, BINARY VARYING and BLOB types are the SQL Standard binary string types. VARBINARY and BINARY LARGE OBJECT are synonyms for BINARY VARYING and BLOB types. HyperSQL also supports LONGVARBINARY as a synonym for VARBINARY. You can set LONGVARBINARY to map to BLOB, with the sql.longvar_is_lob connection property or the SET DATABASE SQL LONGVAR IS LOB TRUE statement.

Binary string types are used in a similar way to character string types. There are several built-in functions that are overloaded to support character, binary and bit strings.

The BINARY type represents a fixed width-string. Each shorter string is padded with zeros to fill the fixed width. Similar to the CHARACTER type, the trailing zeros in the BINARY string are simply discarded in some operations. For the same reason, it is best to avoid this particular type and use VARBINARY instead.

When two binary values are compared, if one is of BINARY type, then zero padding is performed to extend the length of the shorter string to the longer one before comparison. No padding is performed with other binary types. If the bytes compare equal to the end of the shorter value, then the longer string is considered larger than the shorter string.

If BINARY is used without specifying the length, the length defaults to 1. For the BLOB type, the length limit can be defined in units of kilobyte (K, 1024), megabyte (M, 1024 * 1024) or gigabyte (G, 1024 * 1024 * 1024), using the <multiplier>. If BLOB is used without specifying the length, the length defaults to 1GB.

<binary string type> ::= BINARY [ <left paren> <length> <right paren> ] | { BINARY VARYING | VARBINARY } <left paren> <length> <right paren> | LONGVARBINARY [ <left paren> <length> <right paren> ] | <binary large object string type>

<binary large object string type> ::= { BINARY LARGE OBJECT | BLOB } [ <left paren> <large object length> <right paren> ]

<length> ::= <unsigned integer>

 BINARY(10)
 VARBINARY(2)
 BINARY VARYING(2)
 BLOB(1000)
 BLOB(30G)
 BINARY LARGE OBJECT(1M)
 LONGVARBINARY

Bit String Types

The BIT and BIT VARYING types are the supported bit string types. These types were defined by SQL:1999 but were later removed from the Standard. Bit types represent bit maps of given lengths. Each bit is 0 or 1. The BIT type represents a fixed width-string. Each shorter string is padded with zeros to fill the fixed with. If BIT is used without specifying the length, the length defaults to 1. The BIT VARYING type has a maximum width and shorter strings are not padded.

Before the introduction of the BOOLEAN type to the SQL Standard, a single-bit string of the type BIT(1) was commonly used. For compatibility with other products that do not conform to, or extend, the SQL Standard, HyperSQL allows values of BIT and BIT VARYING types with length 1 to be converted to and from the BOOLEAN type. BOOLEAN TRUE is considered equal to B'1', BOOLEAN FALSE is considered equal to B'0'.

For the same reason, numeric values can be assigned to columns and variables of the type BIT(1). For assignment, the numeric value zero is converted to B'0', while all other values are converted to B'1'. For comparison, numeric values 1 is considered equal to B'1' and numeric value zero is considered equal to B'0'.

It is not allowed to perform other arithmetic or boolean operations involving BIT(1) and BIT VARYING(1). The kid of operations allowed on bit strings are analogous to those allowed on BINARY and CHARACTER strings. Several built-in functions support all three types of string.

<bit string type> ::= BIT [ <left paren> <length> <right paren> ] | BIT VARYING <left paren> <length> <right paren>

 BIT
 BIT(10)
 BIT VARYING(2)

Lob Data

BLOB and CLOB are lob types. These types are used for very long strings that do not necessarily fit in memory. Small lobs that fit in memory can be accessed just like BINARY or VARCHAR column data. But lobs are usually much larger and therefore accessed with special JDBC methods.

To insert a lob into a table, or to update a column of lob type with a new lob, you can use the setBinaryStream() and setCharacterStream() methods of JDBC java.sql.PreparedStatement. These are very efficient methods for long lobs. Other methods are also supported. If the data for the BLOB or CLOB is already a memory object, you can use the setBytes() or setString() methods, which are efficient for memory data. Another method is to obtain a lob with the getBlob() and getClob() methods of java.sql.Connection, populate its data, then use the setBlob() or setClob() methods of PreparedStatement. Yet another method allows to create instances of org.hsqldb.jdbc.JDBCBlobFile and org.hsqldb.jdbc.JDBCClobFile and construct a large lob for use with setBlob() and setClob() methods.

A lob is retrieved from a ResultSet with the getBlob() or getClob() method. The steaming methods of the lob objects are then used to access the data. HyperSQL also allows efficient access to chunks of lobs with getBytes() or getString() methods. Furthermore, parts of a BLOB or CLOB already stored in a table can be modified. An updatable ResultSet is used to select the row from the table. The getBlob() or getClob() methods of ResultSet are used to access the lob as a java.sql.Blob or java.sql.Clob object. The setBytes() and setString() methods of these objects can be used to modify the lob. Finally the updateRow() method of the ResultSet is used to update the lob in the row. Note these modifications are not allowed with compressed or encrypted lobs.

Lobs are logically stored in columns of tables. Their physical storage is a separate *.lobs file. This file is created as soon as a BLOB or CLOB is inserted into the database. The file will grow as new lobs are inserted into the database. In version 2, the *.lobs file is never deleted even if all lobs are deleted from the database. In this case you can delete the *.lobs file after a SHUTDOWN. When a CHECKPOINT happens, the space used for deleted lobs is freed and is reused for future lobs. By default, clobs are stored without compression. You can use a database setting to enable compression of clobs. This can significantly reduce the storage size of clobs.

Storage and Handling of Java Objects

Any serializable JAVA Object can be inserted directly into a column of type OTHER using any variation of PreparedStatement.setObject() methods.

For comparison purposes and in indexes, any two Java Objects are considered equal unless one of them is NULL. You cannot search for a specific object or perform a join on a column of type OTHER.

Please note that HSQLDB is not an object-relational database. Java Objects can simply be stored internally and no operations should be performed on them other than assignment between columns of type OTHER or tests for NULL. Tests such as WHERE object1 = object2 do not mean what you might expect, as any non-null object would satisfy such a tests. But WHERE object1 IS NOT NULL is perfectly acceptable.

The engine does not allow normal column values to be assigned to Java Object columns (for example, assigning an INTEGER or STRING to such a column with an SQL statement such as UPDATE mytable SET objectcol = intcol WHERE ...).

<java object type> ::= OTHER

Type Length, Precision and Scale

In older version of HyperSQL, all table column type definitions with a column length, precision or scale qualifier were accepted and ignored. HSQLDB 1.8 enforced correctness but included an option to enforce the length, precision or scale.

In HyperSQL 2, length, precision and scale qualifiers are always enforced. For backward compatibility, when older databases which had the property hsqldb.enforce_strict_size=false are converted to version 2, this property is retained. However, this is a temporary measure. You should test your application to ensure the length, precision and scale that is used for column definitions is appropriate for the application data. You can test with the default database setting, which enforces the sizes.

String types, including all BIT, BINARY and CHAR string types plus CLOB and BLOB, are generally defined with a length. If no length is specified for BIT, BINARY and CHAR, the default length is 1. For CLOB and BLOB an implementation defined length of 1M is used.

TIME and TIMESTAMP types can be defined with a fractional second precision between 0 and 9. INTERVAL type definition may have precision and, in some cases, fraction second precision. DECIMAL and NUMERIC types may be defined with precision and scale. For all of these types a default precision or scale value is used if one is not specified. The default scale is 0. The default fractional precision for TIME is 0, while it is 6 for TIMESTAMP.

Values can be converted from one type to another in two different ways: by using explicit CAST expression or by implicit conversion used in assignment, comparison and aggregation.

String values cannot be assigned to VARCHAR columns if they are longer than the defined type length. For CHARACTER columns, a long string can be assigned (with truncation) only if all the characters after the length are spaces. Shorter strings are padded with the space character when inserted into a CHARACTER column. Similar rules are applied to VARBINARY and BINARY columns. For BINARY columns, the padding and truncation rules are applied with zero bytes, instead of spaces.

Explicit CAST of a value to a CHARACTER or VARCHAR type will result in forced truncation or padding. So a test such as CAST (mycol AS VARCHAR(2)) = 'xy' will find the values beginning with 'xy'. This is the equivalent of SUBSTRING(mycol FROM 1 FOR 2)= 'xy'.

For all numeric types, the rules of explicit cast and implicit conversion are the same. If cast or conversion causes any digits to be lost from the fractional part, it can take place. If the non-fractional part of the value cannot be represented in the new type, cast or conversion cannot take place and will result in a data exception.

There are special rules for DATE, TIME, TIMESTAMP and INTERVAL casts and conversions.

Datetime types

HSQLDB fully supports datetime and interval types and operations, including all relevant optional features, as specified by the SQL Standard since SQL-92. The two groups of types are complementary.

The DATE type represents a calendar date with YEAR, MONTH and DAY fields.

The TIME type represents time of day with HOUR, MINUTE and SECOND fields, plus an optional SECOND FRACTION field.

The TIMESTAMP type represents the combination of DATE and TIME types.

TIME and TIMESTAMP types can include WITH TIME ZONE or WITHOUT TIME ZONE (the default) qualifiers. They can have fractional second parts. For example, TIME(6) has six fractional digits for the second field.

If fractional second precision is not specified, it defaults to 0 for TIME and to 6 for TIMESTAMP.

<datetime type> ::= DATE | TIME [ <left paren> <time precision> <right paren> ] [ <with or without time zone> ] | TIMESTAMP [ <left paren> <timestamp precision> <right paren> ] [ <with or without time zone> ]

<with or without time zone> ::= WITH TIME ZONE | WITHOUT TIME ZONE

<time precision> ::= <time fractional seconds precision>

<timestamp precision> ::= <time fractional seconds precision>

<time fractional seconds precision> ::= <unsigned integer>

 DATE
 TIME(6)
 TIMESTAMP(2) WITH TIME ZONE

Examples of the string literals used to represent date time values, some with time zone, some without, are below:

 DATE '2008-08-22'
 TIMESTAMP '2008-08-08 20:08:08'
 TIMESTAMP '2008-08-08 20:08:08+8:00' /* Beijing */
 TIME '20:08:08.034900'
 TIME '20:08:08.034900-8:00' /* US Pacific */

Time Zone

DATE values do not take time zones. For example United Nations designates 5 June as World Environment Day, which was observed on DATE '2008-06-05' in different time zones.

TIME and TIMESTAMP values without time zone, usually have a context that indicates some local time zone. For example, a database for college course timetables usually stores class dates and times without time zones. This works because the location of the college is fixed and the time zone displacement is the same for all the values. Even when the events take place in different time zones, for example international flight times, it is possible to store all the datetime information as references to a single time zone, usually GMT. For some databases it may be useful to store the time zone displacement together with each datetime value. SQL’s TIME WITH TIME ZONE and TIMESTAMP WITH TIME ZONE values include a time zone displacement value.

The time zone displacement is of the type INTERVAL HOUR TO MINUTE. This data type is described in the next section. The legal values are between '–14:00' and   '+14:00'.

Operations on Datetime Types

The expression <datetime expression> AT TIME ZONE <time displacement> evaluates to a datetime value representing exactly the same point of time in the specified <time displacement>. The expression, AT LOCAL is equivalent to AT TIME ZONE <local time displacement>. If AT TIME ZONE is used with a datetime operand of type WITHOUT TIME ZONE, the operand is first converted to a value of type WITH TIME ZONE at the session’s time displacement, then the specified time zone displacement is set for the value. Therefore, in these cases, the final value depends on the time zone of the session in which the statement was used.

AT TIME ZONE, modifies the field values of the datetime operand. This is done by the following procedure:

  1. determine the corresponding datetime at UTC.

  2. find the datetime value at the given time zone that corresponds with the UTC value from step 1.

Example a:

 TIME '12:00:00' AT TIME ZONE INTERVAL '1:00' HOUR TO MINUTE

If the session’s time zone displacement is -'8:00', then in step 1, TIME '12:00:00' is converted to UTC, which is TIME '20:00:00+0:00'. In step 2, this value is expressed as TIME '21:00:00+1:00'.

Example b:

 TIME '12:00:00-5:00' AT TIME ZONE INTERVAL '1:00' HOUR TO MINUTE

Because the operand has a time zone, the result is independent of the session  time zone displacement. Step 1 results in TIME '17:00:00+0:00', and step 2 results in TIME '18:00:00+1:00'

Note that the operand is not limited to datetime literals used in these examples. Any valid expression that evaluates to a datetime value can be the operand.

Type Conversion

CAST is used to for all other conversions. Examples:

 CAST (<value> AS TIME WITHOUT TIME ZONE)
 CAST (<value> AS TIME WITH TIME ZONE)

In the first example, if <value> has a time zone component, it is simply dropped. For example TIME '12:00:00-5:00' is converted to TIME '12:00:00'

In the second example, if <value> has no time zone component, the current time zone displacement of the session is added. For example TIME '12:00:00' is converted to TIME '12:00:00-8:00' when the session time zone displacement is '-8:00'.

Conversion between DATE and TIMESTAMP is performed by removing the TIME component of a TIMESTAMP value or by setting the hour, minute and second fields to zero. TIMESTAMP '2008-08-08 20:08:08+8:00' becomes DATE '2008-08-08', while DATE '2008-08-22' becomes TIMESTAMP '2008-08-22 00:00:00'.

Conversion between TIME and TIMESTAMP is performed by removing the DATE field values of a TIMESTAMP value or by appending the fields of the TIME value to the fields of the current session date value.

Assignment

When a value is assigned to a datetime target, e.g., a value is used to update a row of a table, the type of the value must be the same as the target, but the WITH TIME ZONE or WITHOUT TIME ZONE characteristics can be different. If the types are not the same, an explicit CAST must be used to convert the value into the target type.

Comparison

When values WITH TIME ZONE are compared, they are converted to UTC values before comparison. If a value WITH TIME ZONE is compared to another WITHOUT TIME ZONE, then the WITH TIME ZONE value is converted to AT LOCAL, then converted to WITHOUT TIME ZONE before comparison.

It is not recommended to design applications that rely on comparisons and conversions between TIME values WITH TIME ZONE. The conversions may involve normalisation of the time value, resulting in unexpected results. For example, the expression: BETWEEN(TIME '12:00:00-8:00', TIME '22:00:00-8:00') is converted to BETWEEN(TIME '20:00:00+0:00', TIME '06:00:00+0:00') when it is evaluated in the UTC zone, which is always FALSE.

Functions

Several functions return the current session timestamp in different datetime types:

CURRENT_DATE

DATE

CURRENT_TIME

TIME WITH TIME ZONE

CURRENT_TIMESTAMP

TIMESTAMP WITH TIME ZONE

LOCALTIME

TIME WITHOUT TIME ZONE

LOCALTIMESTAMP

TIMESTAMP WITHOUT TIME ZONE

HyperSQL supports a very extensive range of functions for conversion, extraction and manipulation of DATE and TIMESTAMP values. See the Built In Functions chapter.

Session Time Zone Displacement

When an SQL session is started (with a JDBC connection) the local time zone of the client JVM (including any seasonal time adjustments such as daylight saving time) is used as the session time zone displacement. Note that the SQL session time displacement is not changed when a seasonal time adjustment takes place while the session is open. To change the SQL session time zone displacement use the following commands:

SET TIME ZONE <time displacement>

SET TIME ZONE LOCAL

The first command sets the displacement to the given value. The second command restores the original, real time zone displacement of the session.

Datetime Values and Java

When datetime values are sent to the database using the PreparedStatement or CallableStatement interfaces, the Java object is converted to the type of the prepared or callable statement parameter. This type may be DATE, TIME, or TIMESTAMP (with or without time zone). The time zone displacement is the time zone of the JDBC session.

When datetime values are retrieved from the database using the ResultSet interface, there are two representations. The getString(…) methods of the ResultSet interface, return an exact representation of the value in the SQL type as it is stored in the database. This includes the correct number of digits for the fractional second field, and for values with time zone displacement, the time zone displacement. Therefore if TIME '12:00:00' is stored in the database, all users in different time zones will get '12:00:00' when they retrieve the value as a string. The getTime(…) and getTimestamp(…) methods of the ResultSet interface return Java objects that are corrected for the session time zone. The UTC millisecond value contained the java.sql.Time or java.sql.Timestamp objects will be adjusted to the time zone of the session, therefore the toString() method of these objects return the same values in different time zones.

If you want to store and retrieve UTC values that are independent of any session's time zone, you can use a TIMESTAMP WITH TIME ZONE column. The setTime(...) and setTimestamp(...) methods of the PreparedStatement interface which have a Calendar parameter can be used to assign the values. The time zone of the given Calendar argument is used as the time zone. Conversely, the getTime(...) and getTimestamp(...) methods of the ResultSet interface which have a Calendar parameter can be used with a Calendar argument to retrieve the values.

JDBC has an unfortunate limitation and does not include type codes for SQL datetime types that have a TIME ZONE property. Therefore, for compatibility with database tools that are limited to the JDBC type codes, HyperSQL reports these types by default as datetime types without TIME ZONE. You can use the URL property hsqldb.translate_dti_types=false to override the default behaviour.

Non-Standard Extensions

HyperSQL version 2.3.0 supports some extenstions to the SQL standard treatment of datetime and interval types. For example, the Standard expression to add a number of days to a date has an explicit INTERVAL value but HSQLDB also allows an integer to be used without specifying DAY. Examples of some Standard expressions and their non-standard alternatives are given below:

 -- standard forms
 CURRENT_DATE + '2' DAY
 SELECT (LOCALTIMESTAMP - atimestampcolumn) DAY TO SECOND FROM atable

 -- non-standard forms
 CURRENT_DATE + 2
 SELECT LOCALTIMESTAMP - atimestampcolumn FROM atable

It is recommended to use the SQL Standard syntax as it is more precise and avoids ambiguity.

Interval Types

Interval types are used to represent differences between date time values. The difference between two date time values can be measured in seconds or in months. For measurements in months, the units YEAR and MONTH are available, while for measurements in seconds, the units DAY, HOUR, MINUTE, SECOND are available. The units can be used individually, or as a range. An interval type can specify the precision of the most significant field and the second fraction digits of the SECOND field (if it has a SECOND field). The default precision is 2. The default second precision is 0.

<interval type> ::= INTERVAL <interval qualifier>

<interval qualifier> ::= <start field> TO <end field> | <single datetime field>

<start field> ::= <non-second primary datetime field> [ <left paren> <interval leading field precision> <right paren> ]

<end field> ::= <non-second primary datetime field> | SECOND [ <left paren> <interval fractional seconds precision> <right paren> ]

<single datetime field> ::= <non-second primary datetime field> [ <left paren> <interval leading field precision> <right paren> ] | SECOND [ <left paren> <interval leading field precision> [ <comma> <interval fractional seconds precision> ] <right paren> ]

<primary datetime field> ::= <non-second primary datetime field> | SECOND

<non-second primary datetime field> ::= YEAR | MONTH | DAY | HOUR | MINUTE

<interval fractional seconds precision> ::= <unsigned integer>

<interval leading field precision> ::= <unsigned integer>

Examples of INTERVAL type definition:

 INTERVAL YEAR TO MONTH
 INTERVAL YEAR(3)
 INTERVAL DAY(4) TO HOUR
 INTERVAL MINUTE(4) TO SECOND(6)
 INTERVAL SECOND(4,6)

The word INTERVAL indicates the general type name. The rest of the definition is called an <interval qualifier>. This designation is important, as in most expressions <interval qualifier> is used without the word INTERVAL.

Interval Values

An interval value can be negative, positive or zero. An interval type has all the datetime fields in the specified range. These fields are similar to those in the TIMESTAMP type. The differences are as follows:

The first field of an interval value can hold any numeric value up to the specified precision. For example, the hour field in HOUR(2) TO SECOND can hold values above 23 (up to 99). The year and month fields can hold zero (unlike a TIMESTAMP value) and the maximum value of a month field that is not the most significant field, is 11.

The standard function ABS(<interval value expression>) can be used to convert a negative interval value to a positive one.

The literal representation of interval values consists of the type definition, with a string representing the interval value inserted after the word INTERVAL. Some examples of interval literal below:

 INTERVAL '145 23:12:19.345' DAY(3) TO SECOND(3)
 INTERVAL '3503:12:19.345' HOUR TO SECOND(3) /* equal to the first value */
 INTERVAL '19.345' SECOND(4,3) /* maximum number of digits for the second value is 4, and each value is expressed with three fraction digits. */
 INTERVAL '-23-10' YEAR(2) TO MONTH

Interval values of the types that are based on seconds can be cast into one another. Similarly those that are based on months can be cast into one another. It is not possible to cast or convert a value based on seconds to one based on months, or vice versa.

When a cast is performed to a type with a smaller least-significant field, nothing is lost from the interval value. Otherwise, the values for the missing least-significant fields are discarded. Examples:

 CAST ( INTERVAL '145 23:12:19' DAY TO SECOND AS INTERVAL DAY TO HOUR ) = INTERVAL '145 23' DAY TO HOUR
 CAST(INTERVAL '145 23' DAY TO HOUR AS INTERVAL DAY TO SECOND) = INTERVAL '145 23:00:00' DAY TO SECOND

A numeric value can be cast to an interval type. In this case the numeric value is first converted to a single-field INTERVAL type with the same field as the least significant field of the target interval type. This value is then converted to the target interval type For example CAST( 22 AS INTERVAL YEAR TO MONTH) evaluates to INTERVAL '22' MONTH and then INTERVAL '1 10' YEAR TO MONTH. Note that SQL Standard only supports casts to single-field INTERVAL types, while HyperSQL allows casting to multi-field types as well.

An interval value can be cast to a numeric type. In this case the interval value is first converted to a single-field INTERVAL type with the same field as the least significant filed of the interval value. The value is then converted to the target type. For example CAST (INTERVAL '1-11' YEAR TO MONTH AS INT) evaluates to INTERVAL '23' MONTH, and then 23.

An interval value can be cast into a character type, which results in an INTERVAL literal. A character value can be cast into an INTERVAL type so long as it is a string with a format compatible with an INTERVAL literal.

Two interval values can be added or subtracted so long as the types of both are based on the same field, i.e., both are based on MONTH or SECOND. The values are both converted to a single-field interval type with same field as the least-significant field between the two types. After addition or subtraction, the result is converted to an interval type that contains all the fields of the two original types.

An interval value can be multiplied or divided by a numeric value. Again, the value is converted to a numeric, which is then multiplied or divided, before converting back to the original interval type.

An interval value is negated by simply prefixing with the minus sign.

Interval values used in expressions are either typed values, including interval literals, or are interval casts. The expression: <expression> <interval qualifier> is a cast of the result of the <expression> into the INTERVAL type specified by the <interval qualifier>. The cast can be formed by adding the keywords and parentheses as follows: CAST ( <expression> AS INTERVAL <interval qualifier> ).

The examples below feature different forms of expression that represent an interval value, which is then added to the given date literal.

 DATE '2000-01-01' + INTERVAL '1-10' YEAR TO MONTH /* interval literal */
 DATE '2000-01-01' + '1-10' YEAR TO MONTH /* the string '1-10' is cast into INTERVAL YEAR TO MONTH */
 DATE '2000-01-01' + 22 MONTH /* the integer 22 is cast into INTERVAL MONTH, same value as above */
 DATE '2000-01-01' - 22 DAY /* the integer 22 is cast into INTERVAL DAY */
 DATE '2000-01-01' + COL2 /* the type of COL2 must be an INTERVAL type */
 DATE '2000-01-01' + COL2 MONTH /* COL2 may be a number, it is cast into a MONTH interval */

Datetime and Interval Operations

An interval can be added to or subtracted from a datetime value so long as they have some fields in common. For example, an INTERVAL MONTH cannot be added to a TIME value, while an INTERVAL HOUR TO SECOND can. The interval is first converted to a numeric value, then the value is added to, or subtracted from, the corresponding field of the datetime value.

If the result of addition or subtraction is beyond the permissible range for the field, the field value is normalised and carried over to the next significant field until all the fields are normalised. For example, adding 20 minutes to TIME '23:50:10' will result successively in '23:70:10', '24:10:10' and finally TIME '00:10:10'. Subtracting 20 minutes from the result is performed as follows: '00:-10:10', '-1:50:10', finally TIME '23:50:10'. Note that if DATE or TIMESTAMP normalisation results in the YEAR field value out of the range (1,1000), then an exception condition is raised.

If an interval value based on MONTH is added to, or subtracted from a DATE or TIMESTAMP value, the result may have an invalid day (30 or 31) for the given result month. In this case an exception condition is raised.

The result of subtraction of two datetime expressions is an interval value. The two datetime expressions must be of the same type. The type of the interval value must be specified in the expression, using only the interval field names. The two datetime expressions are enclosed in parentheses, followed by the <interval qualifier> fields. In the first example below, COL1 and COL2 are of the same datetime type, and the result is evaluated in INTERVAL YEAR TO MONTH type.

 (COL1 – COL2) YEAR TO MONTH /* the difference between two DATE or two TIEMSTAMP values in years and months */
 (CURRENT_DATE – COL3) DAY /* the number of days between the value of COL3 and the current date */
 (CURRENT_DATE - DATE '2000-01-01') YEAR TO MONTH /* the number of years and months since the beginning of this century */
 CURRENT_DATE - 2 DAY /* the date of the day before yesterday */
 (CURRENT_TIMESTAMP - TIMESTAMP '2009-01-01 00:00:00') DAY(4) TO SECOND(2) /* days to seconds since the given date */

The individual fields of both datetime and interval values can be extracted using the EXTRACT function. The same function can also be used to extract the time zone displacement fields of a datetime value.

EXTRACT ({YEAR | MONTH | DAY | HOUR | MINUTE | SECOND | TIMEZONE_HOUR | TIMEZONE_MINUTE | DAY_OF_WEEK | WEEK_OF_YEAR } FROM {<datetime value> | <interval value>})

The dichotomy between interval types based on seconds, and those based on months, stems from the fact that the different calendar months have different numbers of days. For example, the expression, “nine months and nine days since an event” is not exact when the date of the event is unknown. It can represent a period of around 284 days give or take one. SQL interval values are independent of any start or end dates or times. However, when they are added to or subtracted from certain date or timestamp values, the result may be invalid and cause an exception (e.g. adding one month to January 30 results in February 30, which is invalid).

JDBC has an unfortunate limitation and does not include type codes for SQL INTERVAL types. Therefore, for compatibility with database tools that are limited to the JDBC type codes, HyperSQL reports these types by default as VARCHAR. You can use the URL property hsqldb.translate_dti_types=false to override the default behaviour.

Arrays

Array are a powerful feature of SQL:2008 and can help solve many common problems. Arrays should not be used as a substitute for tables.

HyperSQL supports arrays of values according to the SQL:2008 Standard.

Elements of the array are either NULL, or of the same data type. It is possible to define arrays of all supported types, including the types covered in this chapter and user defined types, except LOB types. An SQL array is one dimensional and is addressed from position 1. An empty array can also be used, which has no element.

Arrays can be stored in the database, as well as being used as temporary containers of values for simplifying SQL statements. They facilitate data exchange between the SQL engine and the user's application.

The full range of supported syntax allows array to be created, used in SELECT or other statements, combined with rows of tables and used in routine calls.

Array Definition

The type of a table column, a routine parameter, a variable, or the return value of a function can be defined as an array.

<array type> ::= <data type> ARRAY [ <left bracket or trigraph> <maximum cardinality> <right bracket or trigraph> ]

The word ARRAY is added to any valid type definition except BLOB and CLOB type definitions. If the optional <maximum cardinality> is not used, the default value is 1024. The size of the array cannot be extended beyond maximum cardinality.

In the example below, the table contains a column of integer arrays and a column of varchar arrays. The VARCHAR array has an explicit maximum size of 10, which means each array can have between 0 and 10 elements. The INTEGER array has the default maximum size of 1024. The scores column has a default clause with an empty array. The default clause can be defined only as DEFAULT NULL or DEFAULT ARRAY[] and does not allow arrays containing elements.

 CREATE TABLE t (id INT PRIMARY KEY, scores INT ARRAY DEFAULT ARRAY[], names VARCHAR(20) ARRAY[10])

An array can be constructed from value expressions or a query expression.

<array value constructor by enumeration> ::= ARRAY <left bracket or trigraph> <array element list> <right bracket or trigraph>

<array element list> ::= <value expression> [ { <comma> <value expression> }... ]

<array value constructor by query> ::= ARRAY <left paren> <query expression> [ <order by clause> ] <right paren>

In the examples below, arrays are constructed from values, column references or variables, function calls, or query expressions.

 ARRAY [ 1, 2, 3 ]
 ARRAY [ 'HOT', 'COLD' ]
 ARRAY [ var1, var2, CURRENT_DATE ]
 ARRAY (SELECT lastname FROM namestable ORDER BY id)

Inserting and updating a table with an ARRAY column can use array constructors, not only for updated column values, but also in equality search conditions:

 INSERT INTO t VALUES 10, ARRAY[1,2,3], ARRAY['HOT', 'COLD']
 UPDATE t SET names = ARRAY['LARGE', 'SMALL'] WHERE id = 12
 UPDATE t SET names = ARRAY['LARGE', 'SMALL'] WHERE id < 12 AND scores = ARRAY[3,4]

When using a PreparedStatement with an ARRAY parameter, an object of the type java.sql.Array must be used to set the parameter. The org.hsqldb.jdbc.JDBCArrayBasic class can be used for constructing a java.sql.Array object in the user's application. Code fragment below:

 String sql = "UPDATE t SET names = ? WHERE id = ?";
 PreparedStatement ps = connection.prepareStatement(sql)
 Object[] data = new Object[]{"one", "two"};
 // default types defined in org.hsqldb.types.Type can be used
 org.hsqldb.types.Type type = org.hsqldb.types.Type.SQL_VARCHAR_DEFAULT;
 JDBCArrayBasic array = new JDBCArrayBasic(data, type);
 ps.setArray(1, array);
 ps.setInt(2, 1000);
 ps.executeUpdate();

Trigraph

A trigraph is a substitute for <left bracket> and <right bracket>.

<left bracket trigraph> ::= ??(

<right bracket trigraph> ::= ??)

The example below shows the use of trigraphs instead of brackets.

 INSERT INTO t VALUES 10, ARRAY??(1,2,3??), ARRAY['HOT', 'COLD']
 UPDATE t SET names = ARRAY ??('LARGE', 'SMALL'??) WHERE id = 12
 UPDATE t SET names = ARRAY['LARGE', 'SMALL'] WHERE id < 12 AND scores = ARRAY[3,4]

Array Reference

The most common operations on an array are element reference and assignment, which are used when reading or writing an element of the array. Unlike Java and many other languages, arrays are extended if an element is assigned to an index beyond the current length. This can result in gaps containing NULL elements. Array length cannot exceed the maximum cardinality.

Elements of all arrays, including those that are the result of function calls or other operations can be referenced for reading.

<array element reference> ::= <array value expression> <left bracket> <numeric value expression> <right bracket>

Elements of arrays that are table columns or routine variables can be referenced for writing. This is done in a SET statement, either inside an UPDATE statement, or as a separate statement in the case of routine variables, OUT and INOUT parameters.

<target array element specification> ::= <target array reference> <left bracket or trigraph> <simple value specification> <right bracket or trigraph>

<target array reference> ::= <SQL parameter reference> | <column reference>

Note that only simple values or variables are allowed for the array index when an assignment is performed. The examples below demonstrates how elements of the array are referenced in SELECT and an UPDATE statement.

 SELECT scores[ranking], names[ranking] FROM t JOIN t1 on (t.id = t1.tid)
 UPDATE t SET scores[2] = 123, names[2] = 'Reds' WHERE id = 10
 SELECT scores[ranking], names[ranking] FROM t JOIN t1 on (t.id = t1.tid)
 UPDATE t SET scores[2] = 123, names[2] = 'Reds' WHERE id = 10

Array Operations

Several SQL operations and functions can be used with arrays.

CONCATENATION

Array concatenation is performed similar to string concatenation. All elements of the array on the right are appended to the array on left.

<array concatenation> ::= <array value expression 1> <concatenation operator> <array value expression 2>

<concatenation operator> ::= ||

FUNCTIONS

Seven functions operate on arrays. Details are described in the Built In Functions chapter.

ARRAY_AGG is an aggregate function and produces an array containing values from differnt rows of a SELECT statement. Details are described in the Data Access and Change chapter.

SEQUENCE_ARRAY creates an array with sequential elements.

CARDINALITY <left paren> <array value expression> <right paren>

MAX_CARDINALITY <left paren> <array value expression> <right paren>

Array cardinality and max cardinality are functions that return an integer. CARDINALITY returns the element count, while MAX_CARDINALITY returns the maximum declared cardinality of an array.

POSITION_ARRAY <left paren> <value expression> IN <array value expression> [FROM <numeric value expression>] <right paren>

The POSITION_ARRAY function returns the position of the first match for the <value expression> from the start or from the given start position when <numeric value expression> is used.

TRIM_ARRAY <left paren> <array value expression> <comma> <numeric value expression> <right paren>

The TRIM_ARRAY function returns a copy of an array with the specified number of elements removed from the end of the array. The <array value expression> can be any expression that evaluates to an array.

SORT_ARRAY <left paren> <array value expression> [ { ASC | DESC } ] [ NULLS { FIRST | LAST } ] <right paren>

The SORT_ARRAY function returns a sorted copy of an array. NULL elements appear at the beginning of the new array. You can change the sort direction or the position of NULL elements with the option keywords.

CAST

An array can be cast into an array of a different type. Each element of the array is cast into the element type of the target array type.

UNNEST

Arrays can be converted into table references with the UNNEST keyword.

UNNEST(<array value expression>) [ WITH ORDINALITY ]

The <array value expression> can be any expression that evaluates to an array. A table is returned that contains one column when WITH ORDINALITY is not used, or two columns when WITH ORDINALITY is used. The first column contains the elements of the array (including all the nulls). When the table has two columns, the second column contains the ordinal position of the element in the array. When UNNEST is used in the FROM clause of a query, it implies the LATERAL keyword, which means the array that is converted to table can belong to any table that precedes the UNNEST in the FROM clause. This is explained in the Data Access and Change chapter.

INLINE CONSTRUCTOR

Array constructors can be used in SELECT and other statements. For example, an array constructor with a subquery can return the values from several rows as one array.

The example below shows an ARRAY constructor with a correlated subquery to return the list of order values for each customer. The CUSTOMER table that is included for tests in the DatabaseManager GUI app is the source of the data.

 SELECT FIRSTNAME, LASTNAME, ARRAY(SELECT INVOICE.TOTAL FROM INVOICE WHERE CUSTOMERID = CUSTOMER.ID) AS ORDERS FROM CUSTOMER

 FIRSTNAME LASTNAME  ORDERS                                    
 --------- --------- -------------------------------------- 
 Laura     Steel     ARRAY[2700.90,4235.70]                 
 Robert    King      ARRAY[4761.60]                         
 Robert    Sommer    ARRAY[]                                
 Michael   Smith     ARRAY[3420.30]

COMPARISON

Arrays can be compared for equality, but they cannot be compared for ordering values or range comparison. Array expressions are therefore not allowed in an ORDER BY clause, or in a comparison expression such as GREATER THAN. It is possible to define a UNIQUE constraint on a column of ARRAY type. Two arrays are equal if they have the same length and the values at each index position are either equal or both NULL.

USER DEFINED FUNCTIONS and PROCEDURES

Array parameters, variables and return values can be specified in user defined functions and procedures, including aggregate functions. An aggregate function can return an array that contains all the scalar values that have been aggregated. These capabilities allow a wider range of applications to be covered by user defined functions and easier data exchange between the engine and the user's application.

Indexes and Query Speed

HyperSQL supports PRIMARY KEY, UNIQUE and FOREIGN KEY constraints, which can span multiple columns.

The engine creates indexes internally to support PRIMARY KEY, UNIQUE and FOREIGN KEY constraints: a unique index is created for each PRIMARY KEY or UNIQUE constraint; an ordinary index is created for each FOREIGN KEY constraint.

HyperSQL allows defining indexes on single or multiple columns. You should not create duplicate user-defined indexes on the same column sets covered by constraints. This would result in unnecessary memory and speed overheads. See the discussion in the Deployment Guide chapter for more information.

Indexes are crucial for adequate query speed. When range or equality conditions are used e.g. SELECT ... WHERE acol > 10 AND bcol = 0, an index should exist on one of the columns that has a condition. In this example, the bcol column is the best candidate. HyperSQL always uses the best condition and index. If there are two indexes, one on acol, and another on bcol, it will choose the index on bcol.

Queries always return results whether indexes exist or not, but they return much faster when an index exists. As a rule of thumb, HSQLDB is capable of internal processing of queries at over 100,000 rows per second. Any query that runs into several seconds is clearly accessing thousands of rows. The query should be checked and indexes should be added to the relevant columns of the tables if necessary. The EXPLAIN PLAN FOR <query> statement can be used to see which indexes are used to process the query.

When executing a DELETE or UPDATE statement, the engine needs to find the rows that are to be deleted or updated. If there is an index on one of the columns in the WHERE clause, it is often possible to start directly from the first candidate row. Otherwise all the rows of the table have to be examined.

Indexes are even more important in joins between multiple tables. SELECT ... FROM t1 JOIN t2 ON t1.c1 = t2.c2 is performed by taking rows of t1 one by one and finding a matching row in t2. If there is no index on t2.c2 then for each row of t1, all the rows of t2 must be checked. Whereas with an index, a matching row can be found in a fraction of the time. If the query also has a condition on t1, e.g., SELECT ... FROM t1 JOIN t2 ON t1.c1 = t2.c2 WHERE t1.c3 = 4 then an index on t1.c3 would eliminate the need for checking all the rows of t1 one by one, and will reduce query time to less than a millisecond per returned row. So if t1 and t2 each contain 10,000 rows, the query without indexes involves checking 100,000,000 row combinations. With an index on t2.c2, this is reduced to 10,000 row checks and index lookups. With the additional index on t2.c2, only about 4 rows are checked to get the first result row.

Note that in HSQLDB an index on multiple columns can be used internally as a non-unique index on the first column in the list. For example: CONSTRAINT name1 UNIQUE (c1, c2, c3); means there is the equivalent of CREATE INDEX name2 ON atable(c1);. So you do not need to specify an extra index if you require one on the first column of the list.

In HyperSQL 2, a multi-column index will speed up queries that contain joins or values on the first n columns of the index. You need NOT declare additional individual indexes on those columns unless you use queries that search only on a subset of the columns. For example, rows of a table that has a PRIMARY KEY or UNIQUE constraint on three columns or simply an ordinary index on those columns can be found efficiently when values for all three columns, or the first two columns, or the first column, are specified in the WHERE clause. For example, SELECT ... FROM t1 WHERE t1.c1 = 4 AND t1.c2 = 6 AND t1.c3 = 8 will use an index on t1(c1,c2,c3) if it exists.

A multi-column index will not speed up queries on the second or third column only. The first column must be specified in the JOIN .. ON or WHERE conditions.

Sometimes query speed depends on the order of the tables in the JOIN .. ON or FROM clauses. For example the second query below should be faster with large tables (provided there is an index on TB.COL3). The reason is that TB.COL3 can be evaluated very quickly if it applies to the first table (and there is an index on TB.COL3):

 -- TB is a very large table with only a few rows where TB.COL3 = 4

 SELECT * FROM TA JOIN TB ON TA.COL1 = TB.COL2 AND TB.COL3 = 4;
 SELECT * FROM TB JOIN TA ON TA.COL1 = TB.COL2 AND TB.COL3 = 4;

The general rule is to put first the table that has a narrowing condition on one of its columns. In certain cases, HyperSQL 2.2.x reorders the joined tables if it is obvious that this will introduce a narrowing condition.

HyperSQL features automatic, on-the-fly indexes for views and subselects that are used in a query.

Indexes are used when a LIKE condition searches from the start of the string.

Indexes are used for ORDER BY clauses if the same index is used for selection and ordering of rows. It is possible to force the use of index for ORDER BY.

Query Processing and Optimisation

HyperSQL 2.2.x changes the order of tables in a query in order to optimise processing. This happens only when one of the tables has a narrowing condition and reordering does not change the result of the query.

Indexes and Conditions

HyperSQL optimises queries to use indexes, for all types of range and equality conditions, including IS NULL and NOT NULL conditions. Conditions can be in join or WHERE clauses, including all types of joins.

In addition, HyperSQL will use an index (if one exists) for IN conditions, whether constants, variable, or subqueries are used on the right hand side of the IN predicate. Multicolumn IN conditions can also use an index.

HyperSQL can always use indexes when several conditions are combined with the AND operator, choosing a conditions which can use an index. This now extended to all equality conditions on multiple columns that are part of an index.

HyperSQL will also use indexes when several conditions are combined with the OR operator and each condition can use an index (each condition may use a different index). For example, if a huge table has two separate columns for first name and last name, and both columns are indexed, a query such as the following example will use the indexes and complete in a short time:

 -- TC is a very large table

 SELECT * FROM TC WHERE TC.FIRSTNAME = 'John' OR TC.LASTNAME = 'Smith' OR TC.LASTNAME = 'Williams'

Each subquery is considered a separate SELECT statement and uses indexes when they are available.

In each SELECT statement, at least one index per table can be used if there is a query conditions that can use the index. When conditions on a table are combined with the OR operator, and each condition can use an index, multiple indexes per table are used.

Indexes and Operations

HyperSQL optimises simple row count queries in the form of SELECT COUNT(*) FROM <table> and returns the result immediately (this optimisation does not take place in MVCC mode).

HyperSQL can use an index on a column for SELECT MAX(<column>) FROM <table> and SELECT MIN(<column>) FROM <table> queries. There should be an index on the <column> and the query can have a WHERE condition on the same column. In the example below the maximum value for the TB.COL3 below 1000000 is returned.

 SELECT MAX(TB.COL3) FROM TB WHERE TB.COL < 1000000

HyperSQL can use an index for simple queries containing DISTINCT or GROUP BY to avoid checking all the rows of the table. Note that indexes are always used if the query has a condition, regardless of the use of DISTINCT or GROUP BY. This particular optimisation applies to cases in which all the columns in the SELECT list are from the same table and are covered by a single index, and any join or query condition uses this index.

For example, with the large table below, a DISTINCT or GROUP BY query to return all the last names, can use an the index on the TC.LASTNAME column. Similarly, a GROUP BY query on two columns can use an index that covers the two columns.

 -- TC is a very large table

 SELECT DISTINCT LASTNAME FROM TC WHERE TC.LASTNAME > 'F'
 SELECT STATE, LASTNAME FROM TC GROUP BY STATE, LASTNAME

Indexes and ORDER BY, OFFSET and LIMIT

HyperSQL can use an index on an ORDER BY clause if all the columns in ORDER BY are in a single-column or multi-column index (in the exact order). This is important if there is a LIMIT n (or FETCH n ROWS ONLY) clause. In this situation, the use of index allows the query processor to access only the number of rows specified in the LIMIT clause, instead of building the whole result set, which can be huge. This also works for joined tables when the ORDER BY clause is on the columns of the first table in a join. Indexes are used in the same way when ORDER BY ... DESC is specified in the query. Note that unlike some other RDBMS, HyperSQL does not need or create DESC indexes. It can use any ordinary, ascending index for ORDER BY ... DESC.

If there is an equality or range condition (e.g. EQUALS, GREATER THAN) condition on the columns specified in the ORDER BY clause, the index is still used.

In the two examples below, the index on TA.COL3 is used and only up to 1000 rows are processed and returned.

 -- TA is a very large table with an index on TA.COL3

 SELECT * FROM TA JOIN TB ON TA.COL2 = TB.COL1 WHERE TA.COL3 > 40000 ORDER BY TA.COL3 LIMIT 1000;
 SELECT * FROM TA JOIN TB ON TA.COL2 = TB.COL1 WHERE TA.COL3 > 40000 AND TA.COL3 < 100000 ORDER BY TA.COL3 DESC LIMIT 1000;

But if the query contains an equality condition on another indexed column in the table, this may take precedence and no index may be used for ORDER BY. In this case USING INDEX can be added to the end of the query to force the use of the index for the LIMIT operation. In the example below there is an index on TA.COL1 as well as the index on TA.COL3. Normally the index on TA.COL1 is used, but the USING INDEX hint results in the index on TB.COL3 to be used for selecting the first 1000 rows.

 -- TA is a very large table with an index on TA.COL3 and a separate index on TA.COL1

 SELECT * FROM TA JOIN TB ON TA.COL2 = TB.COL1 WHERE TA.COL1 = 'SENT' AND TB.COL3 > 40000 ORDER BY TB.COL3 LIMIT 1000 USING INDEX;

Chapter 3. Sessions and Transactions

Fred Toussi

The HSQL Development Group

$Revision: 5212 $

Copyright 2010-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Overview

All SQL statements are executed in sessions. When a connection is established to the database, a session is started. The authorization of the session is the name of the user that started the session. A session has several properties. These properties are set by default at the start according to database settings.

SQL Statements are generally transactional statements. When a transactional statement is executed, it starts a transaction if no transaction is in progress. If SQL Data is modified during a transaction, the change can be undone with a ROLLBACK statement. When a COMMIT statement is executed, the transaction is ended. If a single statement fails, the transaction is not normally terminated. However, some failures are caused by execution of statements that are in conflict with statements executed in other concurrent sessions. Such failures result in an implicit ROLLBACK, in addition to the exception that is raised.

Schema definition and manipulation statements are also transactional according to the SQL Standard. HyperSQL 2.0 performs automatic commits before and after the execution of such transactions. Therefore, schema-related statements cannot be rolled back. This is likely to change in future versions.

Some statements are not transactional. Most of these statements are used to change the properties of the session. These statements begin with the SET keyword.

If the AUTOCOMMIT property of a session is TRUE, then each transactional statement is followed by an implicit COMMIT.

The default isolation level for a session is READ COMMITTED. This can be changed using the JDBC java.sql.Connection object and its setTransactionIsolation(int level) method. The session can be put in read-only mode using the setReadOnly(boolean readOnly) method. Both methods can be invoked only after a commit or a rollback, but not during a transaction.

The isolation level and / or the readonly mode of a transaction can also be modified using an SQL statement. You can use the statement to change only the isolation mode, only the read-only mode, or both at the same time. This command can be issued only after a commit or rollback.

SET TRANSACTION <transaction characteristic> [ <comma> <transaction characteristic> ]

Details of the statement is described later in this chapter.

Session Attributes and Variables

Each session has several system attributes. A session can also have user-defined session variables.

Session Attributes

The system attributes reflect the current mode of operation for the session. These attributes can be accessed with function calls and can be referenced in queries. For example, they can be returned using the VALUES <attribute function>, ... statement.

The named attributes such as CURRENT_USER, CURRENT_SCHEMA, etc. are SQL Standard functions. Other attributes of the session, such as auto-commit or read-only modes can be read using other built-in functions. All these functions are listed in the Built In Functions chapter.

Session Variables

Session variables are user-defined variables created the same way as the variables for stored procedures and functions. Currently, these variables cannot be used in general SQL statements. They can be assigned to IN, INOUT and OUT parameters of stored procedures. This allows calling stored procedures which have INOUT or OUT arguments and is useful for development and debugging. See the example in the SQL-Invoked Routines chapter, under Formal Parameters.

Example 3.1. User-defined Session Variables

 DECLARE counter INTEGER DEFAULT 3;
 DECLARE result VARCHAR(20) DEFAULT NULL;
 SET counter=15;
 CALL myroutine(counter, result)

Session Tables

With necessary access privileges, sessions can access all table, including GLOBAL TEMPORARY tables, that are defined in schemas. Although GLOBAL TEMPORARY tables have a single name and definition which applies to all sessions that use them, the contents of the tables are different for each session. The contents are cleared either at the end of each transaction or when the session is closed.

Session tables are different because their definition is visible only within the session that defines a table. The definition is dropped when the session is closed. Session tables do not belong to schemas.

<temporary table declaration> ::= DECLARE LOCAL TEMPORARY TABLE <table name> <table element list> [ ON COMMIT { PRESERVE | DELETE } ROWS ]

The syntax for declaration is based on the SQL Standard. A session table cannot have FOREIGN KEY constraints, but it can have PRIMARY KEY, UNIQUE or CHECK constraints. A session table definition cannot be modified by adding or removing columns, indexes, etc.

It is possible to refer to a session table using its name, which takes precedence over a schema table of the same name. To distinguish a session table from schema tables, the pseudo schema names, MODULE or SESSION can be used. An example is given below:

Example 3.2. User-defined Temporary Session Tables

 DECLARE LOCAL TEMPORARY TABLE buffer (id INTEGER PRIMARY KEY, textdata VARCHAR(100)) ON COMMIT PRESERVE ROWS
 INSERT INTO module.buffer SELECT id, firstname || ' ' || lastname FROM customers
 -- do some more work
 DROP TABLE module.buffer
 -- or use alternative pseudo schema name
 DROP TABLE session.buffer


Session tables can be created inside a transaction. Automatic indexes are created and used on session tables when necessary for a query or other statement. By default, session table data is held in memory. This can be changed with the SET SESSION RESULT MEMORY ROWS statement.

Transactions and Concurrency Control

HyperSQL 2 has been fully redesigned to support different transaction isolation models. It no longer supports the old 1.8.x model with "dirty read". Although it is perfectly possible to add an implementation of the transaction manager that supports the legacy model, we thought this is no longer necessary. The new system allows you to select the transaction isolation model while the engine is running. It also allows you to choose different isolation levels for different simultaneous sessions.

HyperSQL 2 supports three concurrency control models, two-phase-locking (2PL), which is the default, multiversion concurrency control (MVCC) and a hybrid model, which is 2PL plus multiversion rows. Within each model, it supports some of the 4 standard levels of transaction isolation: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ and SERIALIZABLE. The concurrency control model is a strategy that governs all the sessions and is set for the database, as opposed for individual sessions. The isolation level is a property of each SQL session, so different sessions can have different isolation levels. In the new implementation, all isolation levels avoid the "dirty read" phenomenon and do not read uncommitted changes made to rows by other transactions.

HyperSQL is fully multi threaded in all transaction models. Sessions continue to work simultaneously and can fully utilise multi-core processors.

To concurrency control model of a live database can be changed. The SET DATABASE TRANSACTION CONTROL { LOCKS | MVLOCKS | MVCC } can be used by a user with the DBA role.

Two Phase Locking

The two-phase locking model is the default mode. It is referred to by the keyword, LOCKS. In the 2PL model, each table that is read by a transaction is locked with a shared lock (read lock), and each table that is written to is locked with an exclusive lock (write lock). If two sessions read and modify different tables then both go through simultaneously. If one session tries to lock a table that has been locked by the other, if both locks are shared locks, it will go ahead. If either of the locks is an exclusive lock, the engine will put the session in wait until the other session commits or rolls back its transaction. In some cases the engine will invalidate the transaction of the current session, if the action would result in deadlock.

HyperSQL also supports explicit locking of a group of tables for the duration of the current transaction. Use of this command blocks access to the locked tables by other sessions and ensures the current session can complete the intended reads and writes on the locked tables.

If a table is read-only, it will not be locked by any transaction.

The READ UNCOMMITTED isolation level can be used in 2PL modes for read-only operations. It is the same as READ COMMITTED plus read only.

The READ COMMITTED isolation level is the default. It keeps write locks on tables until commit, but releases the read locks after each operation.

The REPEATABLE READ level is upgraded to SERIALIZABLE. These levels keep both read and write locks on tables until commit.

It is possible to perform some critical operations at the SERIALIZABLE level, while the rest of the operations are performed at the READ COMMITTED level.

Note: two phase locking refers to two periods in the life of a transaction. In the first period, locks are acquired, in the second period locks are released. No new lock is acquired after releasing a lock.

Two Phase Locking with Snapshot Isolation

This model is referred to as MVLOCKS. It works the same way as normal 2PL as far as updates are concerned.

SNAPSHOT ISOLATION is a multiversion concurrency strategy which uses the snapshot of the whole database at the time of the start of the transaction. In this model, read only transactions use SNAPSHOT ISOLATION. While other sessions are busy changing the database, the read only session sees a consistent view of the database and can access all the tables even when they are locked by other sessions for updates.

There are many applications for this mode of operation. In heavily updated data sets, this mode allows uninterrupted read access to the data.

Lock Contention in 2PL

When multiple connections are used to access the database, the transaction manager controls their activities. When each transaction performs only reads or writes on a single table, there is no contention. Each transaction waits until it can obtain a lock then performs the operation and commits. All contentions occur when transactions perform reads and writes on more than one table, or perform a read, followed by a write, on the same table.

For example, when sessions are working at the SERIALIZABLE level, when multiple sessions first read from a table in order to check if a row exists, then insert a row into the same table when it doesn't exist, there will be regular contention. Transaction A reads from the table, then does Transaction B. Now if either Transaction A or B attempts to insert a row, it will have to be terminated as the other transaction holds a shared lock on the table. If instead of two operations, a single MERGE statement is used to perform the read and write, no contention occurs because both locks are obtained at the same time.

Alternatively, there is the option of obtaining the necessary locks with an explicit LOCK TABLE statement. This statement should be executed before other statements and should include the names of all the tables and the locks needed. After this statement, all the other statements in the transaction can be executed and the transaction committed. The commit will remove all the locks.

HyperSQL detects deadlocks before attempting to execute a statement. When a lock is released after the completion of the statement, the first transaction that is waiting for the lock is allowed to continue.

HyperSQL is fully multi threaded. It therefore allows different transactions to execute concurrently so long as they are not waiting to lock the same table for write.

Locks in SQL Routines and Triggers

In both LOCKS and MVLOCKS models, SQL routines (functions and procedures) and triggers obtain all the read and write locks at the beginning of the routine execution. SQL statements contained in the routine or trigger are all executed without deadlock as all the locks have already been obtained. At the end of execution of the routine or trigger, read locks are released if the session isolation level is READ COMMITTED.

MVCC

In the MVCC model, there are no shared, read locks. Exclusive locks are used on individual rows, but their use is different. Transactions can read and modify the same table simultaneously, generally without waiting for other transactions. The SQL Standard isolation levels are used by the user's application, but these isolation levels are translated to the MVCC isolation levels READ CONSISTENCY or SNAPSHOT ISOLATION.

When transactions are running at READ COMMITTED level, no conflict will normally occur. If a transaction that runs at this level wants to modify a row that has been modified by another uncommitted transaction, then the engine puts the transaction in wait, until the other transaction has committed. The transaction then continues automatically. This isolation level is called READ CONSISTENCY.

Deadlock is completely avoided. In theory conflict is possible if each transaction is waiting for a different row modified by the other transaction. In this case, one of the transactions is immediately terminated (rolled back) unless the setting has been changed with the <set database transaction rollback on conflict statement>. When this setting is changed to FALSE, the session that avoided executing the deadlock-causing statement returns an error, but without rolling back the previous actions. This will cause the other transaction to wait for the current transaction. The property should not be changed unless the application can quickly perform an alternative statement to continue or roll back the transaction. This allows maximum flexibility and compatibility with other database engines which do not roll back the transaction upon deadlock.

When transactions are running in REPEATABLE READ or SERIALIZABLE isolation levels, conflict is more likely to happen. There is no difference in operation between these two isolation levels. This isolation level is called SNAPSHOT ISOLATION.

In this mode, when the duration of two transactions overlaps, if one of the transactions has modified a row and the second transaction wants to modify the same row, the action of the second transaction will fail. The engine will invalidate the second transaction and roll back all its changes. If the setting is changed to false with the <set database transaction rollback on conflict statement>, then the second transaction will just return an error without rolling back. The application must perform an alternative statement to continue or roll back the transaction.

In the MVCC model, READ UNCOMMITTED is promoted to READ COMMITTED, as the new architecture is based on multi-version rows for uncommitted data and more than one version may exist for some rows.

With MVCC, when a transaction only reads data, then it will go ahead and complete regardless of what other transactions may do. This does not depend on the transaction being read-only or the isolation modes.

Choosing the Transaction Model

The SQL Standard defines the isolation levels as modes of operation that avoid the three unwanted phenomena, "dirty read", "fuzzy read" and "phantom row". The "dirty read" phenomenon occurs when a session can read a row that has been changed by another session. The "fuzzy read" phenomenon occurs when a row that was read by a session is modified by another session, then the first session reads the row again. The "phantom row" phenomenon occurs when a session performs an operation that affects several rows, for example, counts the rows or modifies them using a search condition, then another session adds one or more rows that fulfil the same search condition, then the first session performs an operation that relies on the results of its last operation. According to the Standard, the SERIALIZABLE isolation level avoids all three phenomena and also ensures that all the changes performed during a transaction can be considered as a series of uninterrupted changes to the database without any other transaction changing the database at all for the duration of these actions. The changes made by other transactions are considered to occur before the SERIALIZABLE transaction starts, or after it ends. The READ COMMITTED level avoids "dirty read" only, while the REPEATABLE READ level avoids "dirty read" and "fuzzy read", but not "phantom row".

The Standard allows the engine to return a higher isolation level than requested by the application. HyperSQL promotes a READ UNCOMMITTED request to READ COMMITTED and promotes a REPEATABLE READ request to SERIALIZABLE.

The MVCC model is not covered directly by the Standard. Research has established that the READ CONSISTENCY level fulfils the requirements of (and is stronger than) the READ COMMITTED level. The SNAPSHOT ISOLATION level is stronger than the READ CONSISTENCY level. It avoids the three anomalies defined by the Standard, and is therefore stronger than the REPEATABLE READ level as defined by the Standard. When operating with the MVCC model, HyperSQL treats a REPEATABLE READ or SERIALIZABLE setting for a transaction as SNAPSHOT ISOLATION.

All modes can be used with as many simultaneous connections as required. The default 2PL model is fine for applications with a single connection, or applications that do not access the same tables heavily for writes. With multiple simultaneous connections, MVCC can be used for most applications. Both READ CONSISTENCY and SNAPSHOT ISOLATION levels are stronger than the corresponding READ COMMITTED level in the 2PL mode. Some applications require SERIALIZABLE transactions for at least some of their operations. For these applications, one of the 2PL modes can be used. It is possible to switch the concurrency model while the database is operational. Therefore, the model can be changed for the duration of some special operations, such as synchronization with another data source.

All concurrency models are very fast in operation. When data change operations are mainly on the same tables, the MVCC model may be faster, especially with multi-core processors.

Schema and Database Change

There are a few SQL statements that must access a consistent state of the database during their executions. These statements, which include CHECKPOINT and BACKUP, put an exclusive lock on all the tables of the database when they start.

Some schema manipulation statements put an exclusive lock on one or more tables. For example changing the columns of a table locks the table exclusively.

In the MVCC model, all statements that need an exclusive lock on one or more tables, put an exclusive lock on the database catalog until they complete.

The effect of these exclusive locks is similar to the execution of data manipulation statements with write locks. The session that is about to execute the schema change statement waits until no other session is holding a lock on any of the objects. At this point it starts its operation and locks the objects to prevents any other session from accessing the locked objects. As soon as the operation is complete, the locks are all removed.

Simultaneous Access to Tables

It was mentioned that there is no limit on the number of sessions that can access the tables and all sessions work simultaneously in multi threaded execution. However there are internal resources that are shared. Simultaneous access to these resources can reduce the overall efficiency of the system. MEMORY and TEXT tables do not share resources and do not block multi threaded access. With CACHED tables, each row change operation blocks the file and its cache momentarily until the operation is finished. This is done separately for each row, therefore a multi-row INSERT, UPDATE, or DELETE statement will allow other sessions to access the file during its execution. With CACHED tables, SELECT operations do not block each other, but selecting from different tables and different parts of a large table causes the row cache to be updated frequently and will reduce overall performance.

The new access pattern is the opposite of the access pattern of version 1.8.x. In the old version, even when 20 sessions are actively reading and writing, only a single session at a time performs an SQL statement completely, before the next session is allowed access. In the new version, while a session is performing a SELECT statement and reading rows of a CACHED table to build a result set, another session may perform an UPDATE statement that reads and writes rows of the same table. The two operations are performed without any conflict, but the row cache is updated more frequently than when one operation is performed after the other operation has finished.

Viewing Sessions

As HyperSQL is multithreaded, you can view the current sessions and their state from any admin session. The INFORMATION_SCHEMA.SYSTEM_SESSIONS table contains the list of open sessions, their unique ids and the statement currently executed or waiting to be executed by each session. For each session, it displays the list of sessions that are waiting for it to commit, or the session that this session is waiting for.

Session and Transaction Control Statements

ALTER SESSION

alter session statement

<alter session statement> ::= ALTER SESSION <numeric literal> { CLOSE | RELEASE }

<alter current session statement> ::= ALTER SESSION RESET { ALL | RESULT SETS | TABLE DATA }

The <alter session statement> is used by an administrator to close another session or to release the transaction in another session. When a session is released, its current transaction is terminated with a failure. The session remains open. This statement is different from the other statements discussed in this chapter as it is not used for changing the settings of the current session.

The session ID is used as a <numeric literal> in this statement. The administrator can use the INFORMATION_SCHEMA.SYSTEM_SESSIONS table to find the session IDs of other sessions.

The <alter current session statement> is used to clear and reset different states of the current session. When ALL is specified, the current transaction is rolled back, the session settings such as time zone, current schema etc. are restored to their original state at the time the session was opened and all open result sets are closed and temporary tables cleared. When RESULT SETS is specified, all currently open result sets are closed and the resources are released. When TABLE DATA is specified, the data in all temporary tables is cleared.

SET AUTOCOMMIT

set autocommit command

<set autocommit statement> ::= SET AUTOCOMMIT { TRUE | FALSE }

When an SQL session is started by creating a JDBC connection, it is in AUTOCOMMIT mode. In this mode, after each SQL statement a COMMIT is performed automatically. This statement changes the mode. It is equivalent to using the setAutoCommit( boolean autoCommit) method of the JDBC Connection object.

START TRANSACTION

start transaction statement

<start transaction statement> ::= START TRANSACTION [ <transaction characteristics> ]

Start an SQL transaction and set its characteristics. All transactional SQL statements start a transaction automatically, therefore using this statement is not necessary. If the statement is called in the middle of a transaction, an exception is thrown.

SET TRANSACTION

set next transaction characteristics

<set transaction statement> ::= SET [ LOCAL ] TRANSACTION <transaction characteristics>

Set the characteristics of the next transaction in the current session. This statement has an effect only on the next transactions and has no effect on the future transactions after the next.

transaction characteristics

transaction characteristics

<transaction characteristics> ::= [ <transaction mode> [ { <comma> <transaction mode> }... ] ]

<transaction mode> ::= <isolation level> | <transaction access mode> | <diagnostics size>

<transaction access mode> ::= READ ONLY | READ WRITE

<isolation level> ::= ISOLATION LEVEL <level of isolation>

<level of isolation> ::= READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE

<diagnostics size> ::= DIAGNOSTICS SIZE <number of conditions>

<number of conditions> ::= <simple value specification>

Specify transaction characteristics.

Example 3.3. Setting Transaction Characteristics

 SET TRANSACTION READ ONLY
 SET TRANSACTION ISOLATION LEVEL SERIALIZABLE
 SET TRANSACTION READ WRITE, ISOLATION LEVEL READ COMMITTED

SET CONSTRAINTS

set constraints mode statement

<set constraints mode statement> ::= SET CONSTRAINTS <constraint name list> { DEFERRED | IMMEDIATE }

<constraint name list> ::= ALL | <constraint name> [ { <comma> <constraint name> }... ]

If the statement is issued during a transaction, it applies to the rest of the current transaction. If the statement is issued when a transaction is not active then it applies only to the next transaction in the current session. HyperSQL does not yet support this feature.

LOCK TABLE

lock table statement

<lock table statement> ::= LOCK TABLE <table name> { READ | WRITE} [, <table name> { READ | WRITE} ...]}

In some circumstances, where multiple simultaneous transactions are in progress, it may be necessary to ensure a transaction consisting of several statements is completed, without being terminated due to possible deadlock. When this statement is executed, it waits until it can obtain all the listed locks, then returns. If obtaining the locks would result in a deadlock an error is raised. The SQL statements following this statements use the locks already obtained (and obtain new locks if necessary) and can proceed without waiting. All the locks are released when a COMMIT or ROLLBACK statement is issued.

When the isolation level of a session is READ COMMITTED, read locks are released immediately after the execution of the statement, therefore you should use only WRITE locks in this mode. Alternatively, you can switch to the SERIALIZABLE isolation mode before locking the tables for the specific transaction that needs to finish consistently and without a deadlock. It is best to execute this statement at the beginning of the transaction with the complete list of required read and write locks.

Currently, this command does not have any effect when the database transaction control model is MVCC.

Example 3.4. Locking Tables

 LOCK TABLE table_a WRITE, table_b READ

SAVEPOINT

savepoint statement

<savepoint statement> ::= SAVEPOINT <savepoint specifier>

<savepoint specifier> ::= <savepoint name>

Establish a savepoint. This command is used during an SQL transaction. It establishes a milestone for the current transaction. The SAVEPOINT can be used at a later point in the transaction to rollback the transaction to the milestone.

RELEASE SAVEPOINT

release savepoint statement

<release savepoint statement> ::= RELEASE SAVEPOINT <savepoint specifier>

Destroy a savepoint. This command is rarely used as it is not very useful. It removes a SAVEPOINT that has already been defined.

COMMIT

commit statement

<commit statement> ::= COMMIT [ WORK ] [ AND [ NO ] CHAIN ]

Terminate the current SQL-transaction with commit. This make all the changes to the database permanent.

ROLLBACK

rollback statement

<rollback statement> ::= ROLLBACK [ WORK ] [ AND [ NO ] CHAIN ]

Rollback the current SQL transaction and terminate it. The statement rolls back all the actions performed during the transaction. If NO CHAIN is specified, a new SQL transaction is started just after the rollback. The new transaction inherits the properties of the old transaction.

ROLLBACK TO SAVEPOINT

rollback statement

<rollback statement> ::= ROLLBACK [ WORK ] TO SAVEPOINT <savepoint specifier>

Rollback part of the current SQL transaction and continue the transaction. The statement rolls back all the actions performed after the specified SAVEPOINT was created. The same effect can be achieved with the rollback( Savepoint savepoint) method of the JDBC Connection object.

Example 3.5. Rollback

 -- perform some inserts, deletes, etc.
 SAVEPOINT A
 -- perform some inserts, deletes, selects etc.
 ROLLBACK WORK TO SAVEPOINT A
 -- all the work after the declaration of SAVEPOINT A is rolled back

DISCONNECT

disconnect statement

<disconnect statement> ::= DISCONNECT

Terminate the current SQL session. Closing a JDBC connection has the same effect as this command.

SET SESSION CHARACTERISTICS

set session characteristics statement

<set session characteristics statement> ::= SET SESSION CHARACTERISTICS AS <session characteristic list>

<session characteristic list> ::= <session characteristic> [ { <comma> <session characteristic> }... ]

<session characteristic> ::= <session transaction characteristics>

<session transaction characteristics> ::= TRANSACTION <transaction mode> [ { <comma> <transaction mode> }... ]

Set one or more characteristics for the current SQL-session. This command is used to set the transaction mode for the session. This endures for all transactions until the session is closed or the next use of this command. The current read-only mode can be accessed with the ISREADONLY() function.

Example 3.6. Setting Session Characteristics

 SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY
 SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL SERIALIZABLE
 SET SESSION CHARACTERISTICS AS TRANSACTION READ WRITE, ISOLATION LEVEL READ COMMITTED

SET SESSION AUTHORIZATION

set session user identifier statement

<set session user identifier statement> ::= SET SESSION AUTHORIZATION <value specification>

Set the SQL-session user identifier. This statement changes the current user. The user that executes this command must have the CHANGE_AUTHORIZATION role, or the DBA role. After this statement is executed, all SQL statements are executed with the privileges of the new user. The current authorisation can be accessed with the CURRENT_USER and SESSION_USER functions.

Example 3.7. Setting Session Authorization

 SET SESSION AUTHORIZATION 'FELIX'
 SET SESSION AUTHORIZATION SESSION_USER

SET ROLE

set role statement

<set role statement> ::= SET ROLE <role specification>

<role specification> ::= <value specification> | NONE

Set the SQL-session role name and the current role name for the current SQL-session context. The user that executes this command must have the specified role. If NONE is specified, then the previous CURRENT_ROLE is eliminated. The effect of this lasts for the lifetime of the session. The current role can be accessed with the CURRENT_ROLE function.

SET TIME ZONE

set local time zone statement

<set local time zone statement> ::= SET TIME ZONE <set time zone value>

<set time zone value> ::= <interval value expression> | LOCAL

Set the current default time zone displacement for the current SQL-session. When the session starts, the time zone displacement is set to the time zone of the client. This command changes the time zone displacement. The effect of this lasts for the lifetime of the session. If LOCAL is specified, the time zone displacement reverts to the local time zone of the session.

Example 3.8. Setting Session Time Zone

 SET TIME ZONE LOCAL
 SET TIME ZONE INTERVAL '+6:00' HOUR TO MINUTE

SET CATALOG

set catalog statement

<set catalog statement> ::= SET <catalog name characteristic>

<catalog name characteristic> ::= CATALOG <value specification>

Set the default schema name for unqualified names used in SQL statements that are prepared or executed directly in the current sessions. As there is only one catalog in the database, only the name of this catalog can be used. The current catalog can be accessed with the CURRENT_CATALOG function.

SET SCHEMA

set schema statement

<set schema statement> ::= SET <schema name characteristic>

<schema name characteristic> ::= SCHEMA <value specification> | <schema name>

Set the default schema name for unqualified names used in SQL statements that are prepared or executed directly in the current sessions. The effect of this lasts for the lifetime of the session. The SQL Standard form requires the schema name as a single-quoted string. HyperSQL also allows the use of the identifier for the schema. The current schema can be accessed with the CURRENT_SCHEMA function.

SET PATH

set path statement

<set path statement> ::= SET <SQL-path characteristic>

<SQL-path characteristic> ::= PATH <value specification>

Set the SQL-path used to determine the subject routine of routine invocations with unqualified routine names used in SQL statements that are prepared or executed directly in the current sessions. The effect of this lasts for the lifetime of the session.

SET MAXROWS

set max rows statement

<set max rows statement> ::= SET MAXROWS <unsigned integer literal>

The normal operation of the session has no limit on the number of rows returned from a SELECT statement. This command set the maximum number of rows of the result returned by executing queries.

This statement has a similar effect to the setMaxRows(int max) method of the JDBC Statement interface, but it affects the results returned from the next statement execution only. After the execution of the next statement, the MAXROWS limit is removed.

Only zero or positive values can be used with this command. The value overrides any value specified with setMaxRows(int max) method of a JDBC statement. The statement SET MAXROWS 0 means no limit.

It is possible to limit the number of rows returned from SELECT statements with the FETCH <n> ROWS ONLY, or its alternative, LIMIT <n>. Therefore this command is not recommended for general use. The only legitimate use of this command is for checking and testing queries that may return very large numbers of rows.

SET SESSION RESULT MEMORY ROWS

set session result memory rows statement

<set session result memory rows statement> ::= SET SESSION RESULT MEMORY ROWS <unsigned integer literal>

By default the session uses memory to build result sets, subquery results and temporary tables. This command sets the maximum number of rows of the result (and temporary tables) that should be kept in memory. If the row count of the result or temporary table exceeds the setting, the result is stored on disk. The default is 0, meaning all result sets are held in memory.

This statement applies to the current session only. The general database setting is:

SET DATABASE DEFAULT RESULT MEMORY ROWS <unsigned integer literal>

SET IGNORECASE

set ignore case statement

<set ignore case statement> ::= SET IGNORECASE { TRUE | FALSE }

This is a legacy method for creating case-insensitive columns. Still supported but not recommended for use.

Sets the type used for new VARCHAR table columns. By default, character columns in new databases are case-sensitive. If SET IGNORECASE TRUE is used, all VARCHAR columns in new tables are set to use a collation that converts strings to uppercase for comparison. In the latest versions of HyperSQL you can specify the collations for the database and for each column and have some columns case-sensitive and some not, even in the same table. The collation's strength is used to force case-insensitive comparison. Collations are discussed in the Schemas and Database Objects chapter.

This statement must be switched before creating tables. Existing tables and their data are not affected.

Chapter 4. Schemas and Database Objects

Fred Toussi

The HSQL Development Group

$Revision: 5243 $

Copyright 2009-2013 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Overview

The persistent elements of an SQL environment are database objects. The database consists of catalogs plus authorizations.

A catalog contains schemas, while schemas contain the objects that contain data or govern the data.

Each catalog contains a special schema called INFORMATION_SCHEMA. This schema is read-only and contains some views and other schema objects. The views contain lists of all the database objects that exist within the catalog, plus all authorizations.

Each database object has a name. A name is an identifier and is unique within its name-space.

Schemas and Schema Objects

In HyperSQL, there is only one catalog per database. The name of the catalog is PUBLIC. You can rename the catalog with the ALTER CATALOG RENAME TO statement. All schemas belong the this catalog. The catalog name has no relation to the file name of the database.

Each database has also an internal "unique" name which is automatically generated when the database is created. This name is used for event logging. You can also change this unique name.

Schema objects are database objects that contain data or govern or perform operations on data. By definition, each schema object belongs to a specific schema.

Schema objects can be divided into groups according to their characteristics.

  • Some kinds of schema objects can exist independently from other schema object. Other kinds can exist only as an element of another schema object. These dependent objects are automatically destroyed when the parent object is dropped.

  • Separate name-spaces exists for different kinds of schema object. Some name-spaces are shared between two similar kinds of schema objects.

  • There can be dependencies between various schema objects, as a schema object can include references to other schema objects. These references can cross schema boundaries. Interdependence and cross referencing between schema objects is allowed in some circumstances and disallowed in some others.

  • Schema objects can be destroyed with the DROP statement. If dependent schema objects exist, a DROP statement will succeed only if it has a CASCADE clause. In this case, dependent objects are also destroyed in most cases. In some cases, such as dropping DOMAIN objects, the dependent objects are not destroyed, but modified to remove the dependency.

A new HyperSQL catalog contains an empty schema called PUBLIC. By default, this schema is the initial schema when a new session is started. New schemas and schema objects can be defined and used in the PUBLIC schema, as well as any new schema that is created by the user. You can rename the PUBLIC schema.

HyperSQL allows all schemas to be dropped, except the schema that is the default initial schema for new sessions (by default, the PUBLIC schema). For this schema, a DROP SCHEMA ... CASCADE statement will succeed but will result in an empty schema, rather than no schema.

The statements for setting the initial schema for users are described in the Statements for Authorization and Access Control chapter.

Names and References

The name of a schema object is an <identifier>. The name belongs to the name-space for the particular kind of schema object. The name is unique within its name-space. For example, each schema has a separate name-space for TRIGGER objects.

In addition to the name-spaces in the schema. Each table has a name-space for the names of its columns.

Because a schema object is always in a schema and a schema always in a catalog, it is possible, and sometimes necessary, to qualify the name of the schema object that is being referenced in an SQL statement. This is done by forming an <identifier chain>. In some contexts, only a simple <identifier> can be used and the <identifier chain> is prohibited. While in some other contexts, the use of <identifier chain> is optional. An identifier chain is formed by qualifying each object with the name of the object that owns its name-space. Therefore a column name is prefixed with a table name, a table name is prefixed with a schema name, and a schema name is prefixed with a catalog name. A fully qualified column name is in the form <catalog name>.<schema name>.<table name>.<column name>, likewise, a fully qualified sequence name is in the form <catalog name>.<schema name>.<sequence name>.

HyperSQL extends the SQL standard to allow renaming all database objects. The ALTER ... RENAME TO command has slightly different forms depending on the type of object. If an object is referenced in a VIEW or ROUTINE definition, it is not always possible to rename it.

Character Sets

A CHARACTER SET is the whole or a subset of the UNICODE character set.

A character set name can only be a <regular identifier>. There is a separate name-space for character sets.

There are several predefined character sets. These character sets belong to INFORMATION_SCHEMA. However, when they are referenced in a statement, no schema prefix is necessary.

The following character sets, together with some others, have been specified by the SQL Standard:

SQL_TEXT, SQL_IDENTIFIER, SQL_CHARACTER

The SQL_CHARACTER consists of ASCII letters, digits and the symbols used in the SQL language. SQL_TEXT and SQL_IDENTIFIER are implementation defined. HyperSQL defines SQL_TEXT as the UNICODE character set and SQL_IDENTIFIER as the UNICODE character set minus the SQL language special characters.

SQL_TEXT consists of the full set of Unicode characters. These characters can be used in strings and clobs stored in the database. The character repertoire of HyperSQL is the UTF16 character set, which covers all possible character sets.

If a predefined character set is specified for a table column, then any string stored in the column must contain only characters from the specified character set. HyperSQL does not enforce the CHARACTER SET that is specified for a column and may accept any character string supported by SQL_TEXT.

Collations

A COLLATION is the method used for ordering character strings in ordered sets and to determine equivalence of two character strings.

The system collation is called SQL_TEXT. This collation sorts according to the Unicode code of the characters, UNICODE_SIMPLE. The system collation is always used for INFORMATION_SCHEMA tables.

The default database collation is the same as the system collation. You can change this default, either with a language collation, or with the SQL_TEXT_UCC. This collation is a case-insensitive form of the UNICODE_SIMPLE collation.

Collations for a large number of languages are supported by HyperSQL. These collations belong to INFORMATION_SCHEMA. However, when they are referenced in a statement, there is no need for a schema prefix.

A different collation than the default collation can be specified for each table column that is defined as CHAR or VARCHAR.

A collation can also be used in an ORDER BY clause.

A collation can be used in the GROUP BY clause.

 CREATE TABLE t (id INTEGER PRIMARY KEY, name VARCHAR(20) COLLATE "English")
 SELECT * FROM t ORDER BY name COLLATE "French"
 SELECT COUNT(*), name FROM t GROUP BY name COLLATE "English 0"

In the examples above, the collation for the column is already specified when it is defined. In the first SELECT statement, the column is sorted using the French collation. In the second SELECT, the "English 0" collation is used in the GROUP BY clause. This collation is case insensitive, so the same name with different uses of upper and lower case letters is considered the same and counted together.

The supported collations are named according to the language. You can see the list in the INFORMATION_SCHEMA.COLLATIONS view. You can use just the name in double quotes for the default form of the collation. If you add a strength between 0, 1, 2, 3, the case sensitivity and accent sensitivity changes. The value 0 indicates least sensitivity to differences. At this strength the collation is case-insensitive and ignores differences between accented letters. At strength 1, differences between accented letters are taken into account. At strength 2, both case and accent are significant. Finally 3 indicates additional sensitivity to different punctuation. A second parameter can also be used with values 0 or 1, to indicate how decomposition of accented characters for comparison is handled for languages that support such characters. See the Java and ICU (International Components for Unicode) collation documentation for more details on these values. For example, possible forms of the French collation are "French", "French 0", "French 1", etc. and "French 2 1", etc. When the collation is specified without strength, it seems the system defaults to strength 2, which is case and accent sensitive.

When a collation is not explicitly used in the CREATE TABLE statement for a column, then the database default collation is used for this column. If you change the database default collation afterwards, the new collation will be used.

With the older versions of HyperSQL the special type VARCHAR_IGNORECASE was used as the column type for case-insensitive comparison. Any column already defined as VARCHAR_IGNORECASE will be compared exactly as before. In version 2.3.0 and later, this form is represented by the addition of UCC after the collation name, for example "French UCC". You can still use the SET IGNORECASE TRUE statement in your session to force the UCC to be applied to the collation for the VARCHAR columns of new tables. UCC stands for Upper Case Comparison. Before comparing two strings, both are converted to uppercase using the current collation. This is exactly how VARCHAR_IGNORECASE worked.

It is recommended to use the default SQL_TEXT collation for your general CHAR or VARCHAR columns. For columns where a language collation is desirable, the choice should be made very carefully, because names that are very similar but only differ in the accents may be considered equal in searches.

When comparing two strings, HyperSQL 2 pads the shorter string with spaces in order to compare two strings of equal length. You can change the default database collation with one that does not pad the string with spaces before comparison. This method of comparison was used in versions older than 2.

User defined collations can be created based on existing collations to control the space padding. These collations are part of the current schema.

See the COLLATE keyword and SET DATABASE COLLATION statement in the System Management chapter. The PAD SPACE or NO PAD clause is used to control padding.

[Important]Important

If you change the default collation of a database when there are tables containing data with CHAR or VARCHAR columns that are part of an index, a primary key or a unique constraint, you must execute SHUTDOWN COMPACT or SHUTDOWN SCRIPT after the change. If you do not do this, your queries and other statements will show erratic behaviour and may result in unrecoverable errors.

Distinct Types

A distinct, user-defined TYPE is simply based on a built-in type. A distinct TYPE is used in table definitions and in CAST statements.

Distinct types share a name-space with domains.

Domains

A DOMAIN is a user-defined type, simply based on a built-in type. A DOMAIN can have constraints that limit the values that the DOMAIN can represent. A DOMAIN can be used in table definitions and in CAST statements.

Distinct types share a name-space with domains.

Number Sequences

A SEQUENCE object produces INTEGER values in sequence. The SEQUENCE can be referenced in special contexts only within certain SQL statements. For each row where the object is referenced, its value is incremented.

There is a separate name-space for SEQUENCE objects.

IDENTITY columns are columns of tables which have an internal, unnamed SEQUENCE object. HyperSQL also supports IDENTITY columns that use a named SEQUENCE object.

SEQUENCE objects and IDENTITY columns are supported fully according to the latest SQL 2008 Standard syntax.

Sequences

The SQL:2008 syntax and usage is different from what is supported by many existing database engines. Sequences are created with the CREATE SEQUENCE command and their current value can be modified at any time with ALTER SEQUENCE. The next value for a sequence is retrieved with the NEXT VALUE FOR <name> expression. This expression can be used for inserting and updating table rows.

Example 4.1. inserting the next sequence value into a table row

 INSERT INTO mytable VALUES 2, 'John', NEXT VALUE FOR mysequence

You can also use it in select statements. For example, if you want to number the returned rows of a SELECT in sequential order, you can use:

Example 4.2. numbering returned rows of a SELECT in sequential order

 SELECT NEXT VALUE FOR mysequence, col1, col2 FROM mytable WHERE ...

In version 2.0, the semantics of sequences is exactly as defined by SQL:2008. If you use the same sequence twice in the same row in an INSERT statement, you will get the same value as required by the Standard.

The correct way to use a sequence value is the NEXT VALUE FOR expression.

HyperSQL adds an extension to Standard SQL to return the last value returned by the NEXT VALUE FOR expression in the current session. After a statement containing NEXT VALUE FOR is executed, the value that was returned for NEXT VALUE FOR is available using the CURRENT VALUE FOR expression. In the example below, the NEXT VALUE FOR expression is used to insert a new row. The value that was returned by NEXT VALUE FOR is retrieved with the CURRENT VALUE FOR in the next insert statements to populate two new rows in a different table that has a parent child relationship with the first table. For example if the value 15 was returned by the sequence, the same value 15 is inserted in the three rows.

Example 4.3. using the last value of a sequence

 INSERT INTO mytable VALUES 2, 'John', NEXT VALUE FOR mysequence
 INSERT INTO childtable VALUES 4, CURRENT VALUE FOR mysequence
 INSERT INTO childtable VALUES 5, CURRENT VALUE FOR mysequence


The INFORMATION_SCHEMA.SEQUENCES table contains the next value that will be returned from any of the defined sequences. The SEQUENCE_NAME column contains the name and the NEXT_VALUE column contains the next value to be returned. Note that this is only for getting information and you should not use it for accessing the next sequence value. When multiple sessions access the same sequence, the value returned from this table by one session could also be used by a different session, causing a sequence value to be used twice unintentionally.

Identity Auto-Increment Columns

Each table can contain a single auto-increment column, known as the IDENTITY column. An IDENTITY column is a SMALLINT, INTEGER, BIGINT, DECIMAL or NUMERIC column with its value generated by a sequence generator.

In HyperSQL 2.0, an IDENTITY column is not by default treated as the primary key for the table (as a result, multi-column primary keys are possible with an IDENTITY column present). Use the SQL standard syntax for declaration of the IDENTITY column.

The SQL standard syntax is used, which allows the initial value and other options to be specified.

<colname> [ INTEGER | BIGINT | DECIMAL | NUMERIC ] GENERATED { BY DEFAULT | ALWAYS} AS IDENTITY [( <options> )]

 /* this table has no primary key */
 CREATE TABLE vals (id INTEGER GENERATED BY DEFAULT AS IDENTITY, data VARBINARY(2000))
 
 /* in this table id becomes primary key because the old syntax is used - avoid this syntax */
 CREATE TABLE vals (id INTEGER IDENTITY, data VARBINARY(2000))

 /* use the standard syntax and explicity declare a primary key identity column */
 CREATE TABLE vals (id INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY, data VARBINARY(2000))

When you add a new row to such a table using an INSERT INTO <tablename> ... statement, you can use the DEFAULT keyword for the IDENTITY column, which results in an auto-generated value for the column.

The IDENTITY() function returns the last value inserted into any IDENTITY column by this session. Each session manages this function call separately and is not affected by inserts in other sessions. Use CALL IDENTITY() as an SQL statement to retrieve this value. If you want to use the value for a field in a child table, you can use INSERT INTO <childtable> VALUES (...,IDENTITY(),...);. Both types of call to IDENTITY() must be made before any additional update or insert statements are issued by the session.

In triggers and routines, the value returned by the IDENTITY() function is correct for the given context. For example, if a call to a stored procedure inserts a row into a table, causing a new identity value to be generated, a call to IDENTITY() inside the procedure will return the new identity, but a call outside the procedure will return the last identity value that was generated before a call was made to the procedure.

The last inserted IDENTITY value can also be retrieved via JDBC, by specifying the Statement or PreparedStatement object to return the generated value.

The next IDENTITY value to be used can be changed with the following statement. Note that this statement is not used in normal operation and is only for special purposes, for example resetting the identity generator:

 ALTER TABLE ALTER COLUMN <column name> RESTART WITH <new value>;

For backward compatibility, support has been retained for CREATE TABLE <tablename>(<colname> IDENTITY, ...) as a shortcut which defines the column both as an IDENTITY column and a PRIMARY KEY column. Also, for backward compatibility, it is possible to use NULL as the value of an IDENTITY column in an INSERT statement and the value will be generated automatically. You should avoid these compatibility features as they may be removed from future versions of HyperSQL.

In the following example, the identity value for the first INSERT statement is generated automatically using the DEFAULT keyword. The second INSERT statement uses a call to the IDENTITY() function to populate a row in the child table with the generated identity value.

 CREATE TABLE star (id INTEGER GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY, 
   firstname VARCHAR(20),
   lastname VARCHAR(20))
 CREATE TABLE movies (starid INTEGER, movieid INTEGER PRIMARY KEY, title VARCHAR(40)) 
 INSERT INTO star (id, firstname, lastname) VALUES (DEFAULT, 'Felix', 'the Cat')
 INSERT INTO movies (starid, movieid, title) VALUES (IDENTITY(), 10, 'Felix in Hollywood')

HyperSQL 2.1 also supports IDENTITY columns that use an external, named SEQUENCE object. This feature is not part of the SQL Standard. The example below uses this type of IDENTITY. Note the use of CURRENT VALUE FOR seq here is multi-session safe. The returned value is the last value used by this session when the row was inserted into the star table. This value is available until the transaction is committed. After commit, NULL is returned by the CURRENT VALUE FOR expression until the SEQUENCE is used again.

 CREATE SEQUENCE seq
 CREATE TABLE star (id INTEGER GENERATED BY DEFAULT AS SEQUENCE seq PRIMARY KEY, 
   firstname VARCHAR(20),
   lastname VARCHAR(20))
 CREATE TABLE movies (starid INTEGER, movieid INTEGER PRIMARY KEY, title VARCHAR(40)) 
 INSERT INTO star (id, firstname, lastname) VALUES (DEFAULT, 'Felix', 'the Cat')
 INSERT INTO movies (starid, movieid, title) VALUES (CURRENT VALUE FOR seq, 10, 'Felix in Hollywood')

Tables

In the SQL environment, tables are the most essential components, as they hold all persistent data.

If TABLE is considered as metadata (i.e. without its actual data) it is called a relation in relational theory. It has one or more columns, with each column having a distinct name and a data type. A table usually has one or more constraints which limit the values that can potentially be stored in the TABLE. These constraints are discussed in the next section.

A single column of the table can be defined as IDENTITY. The values stored in this column are auto-generated and are based on an (unnamed) identity sequence, or optionally, a named SEQUENCE object.

Views

A VIEW is similar to a TABLE but it does not permanently contain rows of data. A view is defined as a QUERY EXPRESSION, which is often a SELECT statement that references views and tables, but it can also consist of a TABLE CONSTRUCTOR that does not reference any tables or views.

A view has many uses:

  • Hide the structure and column names of tables. The view can represent one or more tables or views as a separate table. This can include aggregate data, such as sums and averages, from other tables.

  • Allow access to specific rows in a table. For example, allow access to records that were added since a given date, while hiding older records.

  • Allow access to specific columns. For example allow access to columns that contain non-confidential information. Note that this can also be achieved with the GRANT SELECT statement, using column-level privileges

A VIEW that returns the columns of a single ordinary TABLE is updatable if the query expression of the view is an updatable query expression as discussed in the Data Access and Change chapter. Some updatable views are insertable-into because the query expression is insertable-into. In these views, each column of the query expressions must be a column of the underlying table and those columns of the underlying table that are not in the view must have a default clause, or be an IDENTITY or GENERATED column. When rows of an updatable view are updated, or new rows are inserted, or rows are deleted, these changes are reflected in the base table. A VIEW definition may specify that the inserted or updated rows conform to the search condition of the view. This is done with the CHECK OPTION clause.

A view that is not updatable according to the above paragraph can be made updatable or insertable-into by adding INSTEAD OF triggers to the view. These triggers contain statements to use the submitted data to modify the contents of the underlying tables of the view separately. For example, a view that represents a SELECT statements that joins two tables can have an INSTEAD OF DELETE trigger with two DELETE statements, one for each table. Views that have an INSTEAD OF trigger are called TRIGGER INSERTABLE, TRIGGER UPDATABLE, etc. according to the triggers that have been defined.

Views share a name-space with tables.

Constraints

A CONSTRAINT is a child schema object and can belong to a DOMAIN or a TABLE. CONSTRAINT objects can be defined without specifying a name. In this case the system generates a name for the new object beginning with "SYS_".

In a DOMAIN, CHECK constraints can be defined that limits the value represented by the DOMAIN. These constraints work exactly like a CHECK constraint on a single column of a table as described below.

In a TABLE, a constraint takes three basic forms.

CHECK

A CHECK constraint consists of a <search condition> that must not be false (can be unknown) for each row of the table. The <search condition> can reference all the columns of the current row, and if it contains a <subquery>, other tables and views in the database (excluding its own table).

NOT NULL

A simple form of check constraint is the NOT NULL constraint, which applies to a single column.

UNIQUE

A UNIQUE constraint is based on an equality comparison of values of specific columns (taken together) of one row with the same values from each of the other rows. The result of the comparison must never be true (can be false or unknown). If a row of the table has NULL in any of the columns of the constraint, it conforms to the constraint. A unique constraint on multiple columns (c1, c2, c3, ..) means that in no two rows, the sets of values for the columns can be equal unless at lease one of them is NULL. Each single column taken by itself can have repeat values in different rows. The following example satisfies a UNIQUE constraint on the two columns

Example 4.4. Column values which satisfy a 2-column UNIQUE constraint

1,2
2,1
2,2
NULL,1
NULL,1
1,NULL
NULL,NULL
NULL,NULL

If the SET DATABASE SQL UNIQUE NULLS FALSE has been set, then if not all the values set of columns are null, the not null values are compared and it is disallowed to insert identical rows that contain at least one not-null value.

PRIMARY KEY

A PRIMARY KEY constraint is equivalent to a UNIQUE constraint on one or more NOT NULL columns. Only one PRIMARY KEY can be defined in each table.

FOREIGN KEY

A FOREIGN key constraint is based on an equality comparison between values of specific columns (taken together) of each row with the values of the columns of a UNIQUE constraint on another table or the same table. The result of the comparison must never be false (can be unknown). A special form of FOREIGN KEY constraint, based on its CHECK clause, allows the result to be unknown only if the values for all columns are NULL. A FOREIGN key can be declared only if a UNIQUE constraint exists on the referenced columns.

Constraints share a name space with assertions.

Assertions

An ASSERTION is a top-level schema objects. It consists of a <search condition> that must not be false (can be unknown). HyperSQL does not yet support assertions.

Assertions share a name-space with constraints

Triggers

A TRIGGER is a child schema object that always belongs to a TABLE or a VIEW.

Each time a DELETE, UPDATE or INSERT is performed on the table or view, additional actions are taken by the triggers that have been declared on the table or view.

Triggers are discussed in detail in Triggers chapter.

Routines

Routines are user-defined functions or procedures. The names and usage of functions and procedures are different. FUNCTION is a routine that can be referenced in many types of statements. PROCEDURE is a routine that can be referenced only in a CALL statement.

There is a separate name-space for routines.

Because of the possibility of overloading, each routine can have more than one name. The name of the routine is the same for all overloaded variants, but each variant has a specific name, different from all other routine names and specific names in the schema. The specific name can be specified in the routine definition statement. Otherwise it is assigned by the engine. The specific name is used only for schema manipulation statements, which need to reference a specific variant of the routine. For example, if a routine has two signatures, each signature has its own specific name. This allows the user to drop one of the signatures while keeping the other.

Routines are discussed in detail in chapter SQL-Invoked Routines .

Indexes

Indexes are an implementation-defined extension to the SQL Standard. HyperSQL has a dedicated name-space for indexes in each schema.

Statements for Schema Definition and Manipulation

Schemas and schema objects can be created, modified and dropped. The SQL Standard defines a range of statements for this purpose. HyperSQL supports many additional statements, especially for changing the properties of existing schema objects.

Common Elements and Statements

These elements and statements are used for different types of object. They are described here, before the statements that can use them.

identifier

definition of identifier

<identifier> ::= <regular identifier> | <delimited identifier> | <SQL language identifier>

<delimited identifier> ::= <double quote> <character sequence> <double quote>

<regular identifier> ::= <special character sequence>

<SQL language identifier> ::= <special character sequence>

A <delimited identifier> is a sequence of characters enclosed with double-quote symbols. All characters are allowed in the character sequence.

A <regular identifier> is a special sequence of characters. It consists of letters, digits and the underscore characters. It must begin with a letter. All the letters are translated to their upper-case version.

The database setting, SET DATABASE SQL REGULAR NAMES FALSE can be used to relax the rules for regular identifier. With this setting, an underscore character can appear at the start of the regular identifier, and the dollar sign character can be used in the identifier.

A <SQL language identifier> is similar to <regular identifier> but the letters can range only from A-Z in the ASCII character set. This type of identifier is used for names of CHARACTER SET objects.

If the character sequence of a delimited identifier is the same as an undelimited identifier, it represents the same identifier. For example "JOHN" is the same identifier as JOHN. In a <regular identifier> the case-normal form is considered for comparison. This form consists of the upper-case equivalent of all the letters. When a database object is created with one of the CREATE statements or renamed with the ALTER statement, if the name is enclosed in double quotes, the exact name is used as the case-normal form. But if it is not enclosed in double quotes, the name is converted to uppercase and this uppercase version is stored in the database as the case-normal form.

The character sequence length of all identifiers must be between 1 and 128 characters.

A reserved word is one that is used by the SQL Standard for special purposes. It is similar to a <regular identifier> but it cannot be used as an identifier for user objects. If a reserved word is enclosed in double quote characters, it becomes a quoted identifier and can be used for database objects.

Case sensitivity rules for identifiers can be described simply as follows:

  • all parts of SQL statements are converted to upper case before processing, except identifiers in double quotes and strings in single quotes

  • identifiers, both unquoted and double quoted, are then treated as case-sensitive

  • most database engines follow the same rule, except, in some respects, MySQL and MS SQLServer.

CASCADE or RESTRICT

drop behavior

<drop behavior> ::= CASCADE | RESTRICT

The <drop behavior> is a required element of statements that drop a SCHEMA or a schema object. If <drop behavior> is not specified then RESTRICT is implicit. It determines the effect of the statement if there are other objects in the catalog that reference the SCHEMA or the schema object. If RESTRICT is specified, the statement fails if there are referencing objects. If CASCADE is specified, all the referencing objects are modified or dropped with cascading effect. Whether a referencing object is modified or dropped, depends on the kind of schema object that is dropped.

IF EXISTS

drop condition (HyperSQL)

<if exists clause> ::= IF EXISTS

This clause is not part of the SQL standard and is a HyperSQL extension to some commands that drop objects (schemas, tables, views, sequences and indexes). If it is specified, then the statement does not return an error if the drop statement is issued on a non-existent object.

SPECIFIC

specific routine designator

<specific routine designator> ::= SPECIFIC <routine type> <specific name>

<routine type> ::= ROUTINE | FUNCTION | PROCEDURE

This clause is used in statements that need to specify one of the multiple versions of an overloaded routine. The <specific name> is the one specified in the <routine definition> statement.

Renaming Objects

RENAME

rename statement (HyperSQL)

<rename statement> ::= ALTER <object type> <name> RENAME TO <new name>

<object type> ::= CATALOG | SCHEMA | DOMAIN | TYPE | TABLE | CONSTRAINT | INDEX | ROUTINE | SPECIFIC ROUTINE

<column rename statement> ::= ALTER TABLE <table name> ALTER COLUMN <name> RENAME TO <new name>

This statement is used to rename an existing object. It is not part of the SQL Standard. The specified <name> is the existing name, which can be qualified with a schema name, while the <new name> is the new name for the object.

Commenting Objects

COMMENT

comment statement (HyperSQL)

<comment statement> ::= COMMENT ON { TABLE | COLUMN | ROUTINE } <name> IS <character string literal>

Adds a comment to the object metadata, which can later be read from an INFORMATION_SCHEMA view. This command is not part of the SQL Standard. The strange syntax is due to compatibility with other database engines that support the statement. The <name> is the name of a table, view, column or routine. The name of the column consists of dot-separated <table name> . <column name>. The name of the table, view or routine can be a simple name. All names can be qualified with a schema name. If there is already a comment on the object, the new comment will replace it.

The comments appear in the results returned by JDBC DatabaseMetaData methods, getTables() and getColumns(). The INFORMATION_SCHEMA.SYSTEM_COMMENTS view contains the comments. You can query this view using the schema, table, and column names to retrieve the comments.

Schema Creation

CREATE SCHEMA

schema definition

The CREATE_SCHEMA or DBA role is required in order to create a schema. A schema can be created with or without schema objects. Schema objects can always be added after creating the schema, or existing ones can be dropped. Within the <schema definition> statement, all schema object creation takes place inside the newly created schema. Therefore, if a schema name is specified for the schema objects, the name must match that of the new schema. In addition to statements for creating schema objects, the statement can include instances of <grant statement> and <role definition>. This is a curious aspect of the SQL standard, as these elements do not really belong to schema creation.

<schema definition> ::= CREATE SCHEMA <schema name clause> [ <schema character set specification> ] [ <schema element>... ]

<schema name clause> ::= <schema name> | AUTHORIZATION <authorization identifier> | <schema name> AUTHORIZATION <authorization identifier>

If the name of the schema is specified simply as <schema name>, then the AUTHORIZATION is the current user. Otherwise, the specified <authorization identifier> is used as the AUTHORIZATION for the schema. If <schema name> is omitted, then the name of the schema is the same as the specified <authorization identifier>.

<schema element> ::= <table definition> | <view definition> | <domain definition> | <character set definition> | <collation definition> | <transliteration definition> | <assertion definition> | <trigger definition> | <user-defined type definition> | <user-defined cast definition> | <user-defined ordering definition> | <transform definition> | <schema routine> | <sequence generator definition> | <grant statement> | <role definition>

An example of the command is given below. Note that a single semicolon appears at the end. There should be no semicolon between the statements:

 CREATE SCHEMA ACCOUNTS AUTHORIZATION DBA
   CREATE TABLE AB(A INTEGER, ...)
   CREATE TABLE CD(C CHAR(10), ...)
   CREATE VIEW VI AS SELECT ...
   GRANT SELECT ON AB TO PUBLIC
   GRANT SELECT ON CD TO JOE;

It is not really necessary to create a schema and all its objects as one command. The schema can be created first, and its objects can be created one by one.

DROP SCHEMA

drop schema statement

<drop schema statement> ::= DROP SCHEMA [ IF EXISTS ] <schema name> [ IF EXISTS ] <drop behavior>

This command destroys an existing schema. If <drop behavior> is RESTRICT, the schema must be empty, otherwise an error is raised. If CASCADE is specified, then all the objects contained in the schema are destroyed with a CASCADE option.

Table Creation

CREATE TABLE

table definition

<table definition> ::= CREATE [ { <table scope> | <table type> } ] TABLE <table name> <table contents source> [ ON COMMIT { PRESERVE | DELETE } ROWS ]

<table scope> ::= { GLOBAL | LOCAL } TEMPORARY

<table type> :: = MEMORY | CACHED

<table contents source> ::= <table element list> | <as subquery clause>

<table element list> ::= <left paren> <table element> [ { <comma> <table element> }... ] <right paren>

<table element> ::= <column definition> | <table constraint definition> | <like clause>

like clause

A <like clause> copies all column definitions from another table into the newly created table. Its three options indicate if the <default clause>, <identity column specification> and <generation clause> associated with the column definitions are copied or not. If an option is not specified, it defaults to EXCLUDING. The <generation clause> refers to columns that are generated by an expression but not to identity columns. All NOT NULL constraints are copied with the original columns, other constraints are not. The <like clause> can be used multiple times, allowing the new table to have copies of the column definitions of one or more other tables.

 CREATE TABLE t (id INTEGER PRIMARY KEY, LIKE atable INCLUDING DEFAULTS EXCLUDING IDENTITY)

<like clause> ::= LIKE <table name> [ <like options> ]

<like options> ::= <like option>...

<like option> ::= <identity option> | <column default option> | <generation option>

<identity option> ::= INCLUDING IDENTITY | EXCLUDING IDENTITY

<column default option> ::= INCLUDING DEFAULTS | EXCLUDING DEFAULTS

<generation option> ::= INCLUDING GENERATED | EXCLUDING GENERATED

as subquery clause

<as subquery clause> ::= [ <left paren> <column name list> <right paren> ] AS <table subquery> { WITH NO DATA | WITH DATA }

An <as subquery clause> used in table definition creates a table based on a <table subquery>. This kind of table definition is similar to a view definition. If WITH DATA is specified, then the new table will contain the rows of data returned by the <table subquery>.

 CREATE TABLE t (a, b, c) AS (SELECT * FROM atable) WITH DATA

column definition

A column definition consists of a <column name> and in most cases a <data type> or <domain name> as minimum. The other elements of <column definition> are optional. Each <column name> in a table is unique.

<column definition> ::= <column name> [ <data type or domain name> ] [ <default clause> | <identity column specification> | <identity column sequence specification> | <generation clause> ] [ <column constraint definition>... ] [ <collate clause> ]

<data type or domain name> ::= <data type> | <domain name>

<column constraint definition> ::= [ <constraint name definition> ] <column constraint> [ <constraint characteristics> ]

<column constraint> ::= NOT NULL | <unique specification> | <references specification> | <check constraint definition>

A <column constraint definition> is a shortcut for a <table constraint definition>. A constraint that is defined in this way is automatically turned into a table constraint. A name is automatically generated for the constraint and assigned to it.

If a <collate clause> is specified, then a UNIQUE or PRIMARY KEY constraint or an INDEX on the column will use the specified collation. Otherwise the default collation for the database is used.

generated columns

The value of a column can be autogenerated in two ways.

One way is specific to columns of integral types (INTEGER, BIGINT, etc.) and associates a sequence generator with the column. When a new row is inserted into the table, the value of the column is generated as the next available value in the sequence.

The SQL Standard supports the use of unnamed sequences with the IDENTITY keyword. In addition, HyperSQL supports the use of a named SEQUENCE object, which must be in the same schema as the table.

<identity column specification> ::= GENERATED { ALWAYS | BY DEFAULT } AS IDENTITY [ <left paren> <common sequence generator options> <right paren> ]

<identity column sequence specification ::= GENERATED BY DEFAULT AS SEQUENCE <sequence name>

The <identity column specification> or <identity column sequence specification> can be specified for only a single column of the table.

The <identity column specification> is used for columns which represent values based on an unnamed sequence generator. It is possible to insert a row into the table without specifying a value for the column. The value is then generated by the sequence generators according to its rules. An identity column may or may not be the primary key. Example below:

 CREATE TABLE t (id INTEGER GENERATED ALWAYS AS IDENTITY(START WITH 100), name VARCHAR(20) PRIMARY KEY)

The <identity column sequence specification> is used when the column values are based on a named SEQUENCE object (which must already exist). Example below:

 CREATE TABLE t (id INTEGER GENERATED BY DEFAULT AS SEQUENCE s, name VARCHAR(20) PRIMARY KEY)

Inserting rows is done in the same way for a named or unnamed sequence generator. In both cases, if no value is specified to be inserted, or the DEFAULT keyword is used for the column, the value is generated by the sequence generator. If a value is specified, this value is used if the column definition has the BY DEFAULT specification. If the column definition has the ALWAYS specification, a value can be specified but the OVERRIDING SYSTEM VALUES must be specified in the INSERT statement.

The other way in which the column value is autogenerated is by using the values of other columns in the same row. This method is often used to create an index on a value that is derived from other column values.

<generation clause> ::= GENERATED ALWAYS AS <generation expression>

<generation expression> ::= <left paren> <value expression> <right paren>

The <generation clause> is used for special columns which represent values based on the values held in other columns in the same row. The <value expression> must reference only other, non-generated, columns of the table in the same row. Any function used in the expression must be deterministic and must not access SQL-data. No <query expression> is allowed. When <generation clause> is used, <data type> must be specified.

A generated column can be part of a foreign key or unique constraints or a column of an index. This capability is the main reason for using generated columns. A generated column may contain a formula that computes a value based on the values of other columns. Fast searches of the computed value can be performed when an index is declared on the generated column. Or the computed values can be declared to be unique, using a UNIQUE constraint on the table. The computed column cannot be overridden by user supplied values. When a row is updated and the column values change, the generated columns are computed with the new values.

When a row is inserted into a table, or an existing row is updated, no value except DEFAULT can be specified for a generated column. In the example below, data is inserted into the non-generated columns and the generated column will contain 'Felix the Cat' or 'Pink Panther'.

 CREATE TABLE t (id INTEGER PRIMARY KEY, 
   firstname VARCHAR(20),
   lastname VARCHAR(20), 
   fullname VARCHAR(40) GENERATED ALWAYS AS (firstname || ' ' || lastname)) 
 INSERT INTO t (id, firstname, lastname) VALUES (1, 'Felix', 'the Cat')
 INSERT INTO t (id, firstname, lastname, fullname) VALUES (2, 'Pink', 'Panther', DEFAULT)

DEFAULT

default clause

A default clause can be used if GENERATED is not specified. If a column has a <default clause> then it is possible to insert a row into the table without specifying a value for the column.

<default clause> ::= DEFAULT <default option>

<default option> ::= <literal> | <datetime value function> | USER | CURRENT_USER | CURRENT_ROLE | SESSION_USER | SYSTEM_USER | CURRENT_CATALOG | CURRENT_SCHEMA | CURRENT_PATH | NULL

The type of the <default option> must match the type of the column.

In PGS (PostgreSQL) compatibility mode, a NEXTVAL function can be used. Also, in MSS compatibility mode, the default value can be enclosed in parentheses.

CONSTRAINT

constraint name and characteristics

<constraint name definition> ::= CONSTRAINT <constraint name>

<constraint characteristics> ::= <constraint check time> [ [ NOT ] DEFERRABLE [ <constraint check time> ] ]

<constraint check time> ::= INITIALLY DEFERRED | INITIALLY IMMEDIATE

Specify the name of a constraint and its characteristics. By default the constraint is NOT DEFERRABLE and INITIALLY IMMEDIATE. This means the constraint is enforced as soon as a data change statement is executed. If INITIALLY DEFERRED is specified, then the constraint is enforced when the session commits. The characteristics must be compatible. The constraint check time can be changed temporarily for an SQL session. HyperSQL does not support deferring constraint enforcement. This feature of the SQL Standard has been criticised because it allows a session to read uncommitted data that violates database integrity constraints but has not yet been checked.

CONSTRAINT

table constraint definition

<table constraint definition> ::= [ <constraint name definition> ] <table constraint> [ <constraint characteristics> ]

<table constraint> ::= <unique constraint definition> | <referential constraint definition> | <check constraint definition>

Three kinds of constraint can be defined on a table: UNIQUE (including PRIMARY KEY), FOREIGN KEY and CHECK. Each kind has its own rules to limit the values that can be specified for different columns in each row of the table.

UNIQUE

unique constraint definition

<unique constraint definition> ::= <unique specification> <left paren> <unique column list> <right paren> | UNIQUE ( VALUE )

<unique specification> ::= UNIQUE | PRIMARY KEY

<unique column list> ::= <column name list>

A unique constraint is specified on a single column or on multiple columns. On each set of columns taken together, only one UNIQUE constraint can be specified. Each column of a PRIMARY KEY constraint has an implicit NOT NULL constraint.

If UNIQUE( VALUE ) is specified, the constraint created on all columns of the table.

FOREIGN KEY

referential constraint definition

<referential constraint definition> ::= FOREIGN KEY <left paren> <referencing columns> <right paren> <references specification>

<references specification> ::= REFERENCES <referenced table and columns> [ MATCH <match type> ] [ <referential triggered action> ]

<match type> ::= FULL | PARTIAL | SIMPLE

<referencing columns> ::= <reference column list>

<referenced table and columns> ::= <table name> [ <left paren> <reference column list> <right paren> ]

<reference column list> ::= <column name list>

<referential triggered action> ::= <update rule> [ <delete rule> ] | <delete rule> [ <update rule> ]

<update rule> ::= ON UPDATE <referential action>

<delete rule> ::= ON DELETE <referential action>

<referential action> ::= CASCADE | SET NULL | SET DEFAULT | RESTRICT | NO ACTION

A referential constraint allows links to be established between the rows of two tables. The specified list of <referencing columns> corresponds one by one to the columns of the specified list of <referenced columns> in another table (or sometimes in the same table). For each row in the table, a row must exist in the referenced table with equivalent values in the two column lists. There must exist a single unique constraint in the referenced table on all the <referenced columns>.

The [ MATCH match type ] clause is optional and has an effect only on multi-column foreign keys and only on rows containing at least a NULL in one of the <referencing columns>. If the clause is not specified, MATCH SIMPLE is the default. If MATCH SIMPLE is specified, then any NULL means the row can exist (without a corresponding row in the referenced table). If MATCH FULL is specified then either all the column values must be NULL or none of them. MATCH PARTIAL allows any NULL but the non NULL values must match those of a row in the referenced table. HyperSQL does not support MATCH PARTIAL.

Referential actions are specified with ON UPDATE and ON DELETE clauses. These actions take place when a row in the referenced table (the parent table) has referencing rows in the referencing table and it is deleted or modified with any SQL statement. The default is NO ACTION. This means the SQL statement that causes the DELETE or UPDATE is terminated with an exception. The RESTRICT option is similar and works exactly the same without deferrable constraints (which are not allowed by HyperSQL). The other three options, CASCADE, SET NULL and SET DEFAULT all allow the DELETE or UPDATE statement to complete. With DELETE statements the CASCADE option results in the referencing rows to be deleted. With UPDATE statements, the changes to the values of the referenced columns are copied to the referencing rows. With both DELETE or UPDATE statement, the SET NULL option results in the columns of the referencing rows to be set to NULL. Similarly, the SET DEFAULT option results in the columns of the referencing rows to be set to their default values.

CHECK

check constraint definition

<check constraint definition> ::= CHECK <left paren> <search condition> <right paren>

A CHECK constraint can exist for a TABLE or for a DOMAIN. The <search condition> evaluates to an SQL BOOLEAN value for each row of the table. Within the <search condition> all columns of the table row can be referenced. For all rows of the table, the <search condition> evaluates to TRUE or UNKNOWN. When a new row is inserted, or an existing row is updated, the <search condition> is evaluated and if it is FALSE, the insert or update fails.

A CHECK constraint for a DOMAIN is similar. In its <search condition>, the term VALUE is used to represents the value to which the DOMAIN applies.

 CREATE TABLE t (a VARCHAR(20) CHECK (a IS NOT NULL AND CHARACTER_LENGTH(a) > 2))

The search condition of a CHECK constraint cannot contain any function that is not deterministic. A check constraint is a data integrity constraint, therefore it must hold with respect to the rest of the data in the database. It cannot use values that are temporal or ephemeral. For example CURRENT_USER is a function that returns different values depending on who is using the database, or CURRENT_DATE changes day-to-day. Some temporal expressions are retrospectively deterministic and are allowed in check constraints. For example, (CHECK VALUE < CURRENT_DATE) is valid, because CURRENT_DATE will not move backwards in time, but (CHECK VALUE > CURRENT_DATE) is not acceptable.

If you want to enforce the condition that a date value that is inserted into the database belongs to the future (at the time of insertion), or any similar constraint, then use a TRIGGER with the desired condition.

DROP TABLE

drop table statement

<drop table statement> ::= DROP TABLE [ IF EXISTS ] <table name> [ IF EXISTS ] <drop behavior>

Destroy a table. The default drop behaviour is RESTRICT and will cause the statement to fail if there is any view, routine or foreign key constraint that references the table. If <drop behavior> is CASCADE, it causes all schema objects that reference the table to drop. Referencing views are dropped. In the case of foreign key constraints that reference the table, the constraint is dropped, rather than the TABLE or DOMAIN that contains it.

Table Manipulation

Table manipulation statements change the attributes of tables or modify the objects such as columns and constraints.

SET TABLE CLUSTERED

set table clustered property

<set table clustered statement> ::= SET TABLE <table name> CLUSTERED ON <left paren> <column name list> <right paren>

Set the row clustering property of a table. The <column name list> is a list of column names that must correspond to the columns of an existing PRIMARY KEY, UNIQUE or FOREIGN KEY index, or to the columns of a user defined index. This statement is only valid for CACHED or TEXT tables.

Tables rows are stored in the database files as they are created, sometimes at the end of the file, sometimes in the middle of the file. After a CHECKPOINT DEFRAG or SHUTDOWN COMPACT, the rows are reordered according to the primary key of the table, or if there is no primary key, in no particular order.

When several consecutive rows of a table are retrieved during query execution it is more efficient to retrieve rows that are stored adjacent to one another. After executing this command, nothing changes until a CHECKPOINT DEFRAG or SHUTDOWN COMPACT or SHUTDOWN SCRIPT is performed. After these operations, the rows are stored in the specified clustered order. The property is stored in the database and applies to all future reordering of rows. Note that if extensive inserts or updates are performed on the tables, the rows will get out of order until the next reordering.

SET TABLE TYPE

set table type

<set table type statement> ::= SET TABLE <table name> TYPE { MEMORY | CACHED }

Changes the storage type of an existing table between CACHED and MEMORY types.

Only a user with the DBA role can execute this statement.

SET TABLE writeability

set table write property

<set table read only statement> ::= SET TABLE <table name> { READ ONLY | READ WRITE }

Set the writeability property of a table. Tables are writeable by default. This statement can be used to change the property between READ ONLY and READ WRITE. This is a feature of HyperSQL.

SET TABLE SOURCE

set table source statement

<set table source statement> ::= SET TABLE <table name> SOURCE <file and options> [DESC]

<file and options>::= <doublequote> <file path> [<semicolon> <property>...] <doublequote>

Set the text source for a text table. This statement cannot be used for tables that are not defined as TEXT TABLE.

Supported Properties

quoted = { true | false }

default is true. If false, treats double quotes as normal characters

all_quoted = { true | false }

default is false. If true, adds double quotes around all fields.

encoding = <encoding name>

character encoding for text and character fields, for example, encoding=UTF-8. UTF-16 cannot be used.

ignore_first = { true | false }

default is false. If true ignores the first line of the file

cache_rows= <numeric value>

rows of the text file in the cache. Default is 1000 rows

cache_size = <numeric value>r

total size of the row in the cache. Default is 100 KB.

cache_scale= <numeric value> and cache_size_scale = <numeric value>

deprecated properties, replaced by cached_rows and cache_size properties above.

fs = <unquoted character>

field separator

vs = <unquoted character>

varchar separator

Special indicators for HyperSQL Text Table separators

\semi

semicolon

\quote

quote

\space

space character

\apos

apostrophe

\n

newline - Used as an end anchor (like $ in regular expressions)

\r

carriage return

\t

tab

\\

backslash

\u####

a Unicode character specified in hexadecimal

In the example below, the text source of the table is set to "myfile", the field separator to the pipe symbol, and the varchar separator to the tilde symbol.

 SET TABLE mytable SOURCE 'myfile;fs=|;vs=.;vs=~'

Only a user with the DBA role can execute this statement.

SET TABLE SOURCE HEADER

set table source header statement

<set table source header statement> ::= SET TABLE <table name> SOURCE HEADER <header string>

Set the header for the text source for a text table. If this command is used, the <header string> is used as the first line of the source file of the text table. This line is not part of the table data. Only a user with the DBA role can execute this statement.

SET TABLE SOURCE on-off

set table source on-off statement

<set table source on-off statement> ::= SET TABLE <table name> SOURCE { ON | OFF }

Attach or detach a text table from its text source. This command does not change the properties or the name of the file that is the source of a text table. When OFF is specified, the command detaches the table from its source and closes the file for the source. In this state, it is not possible to read or write to the table. This allows the user to replace the file with a different file, or delete it. When ON is specified, the source file is read. Only a user with the DBA role can execute this statement

ALTER TABLE

alter table statement

<alter table statement> ::= ALTER TABLE <table name> <alter table action>

<alter table action> ::= <add column definition> | <alter column definition> | <drop column definition> | <add table constraint definition> | <drop table constraint definition>

Change the definition of a table. Specific types of this statement are covered below.

ADD COLUMN

add column definition

<add column definition> ::= ADD [ COLUMN ] <column definition> [ BEFORE <other column name> ]

Add a column to an existing table. The <column definition> is specified the same way as it is used in <table definition>. HyperSQL allows the use of [ BEFORE <other column name> ] to specify at which position the new column is added to the table.

If the table contains rows, the new column must have a <default clause> or use one of the forms of GENERATED. The column values for each row is then filled with the result of the <default clause> or the generated value.

DROP COLUMN

drop column definition

<drop column definition> ::= DROP [ COLUMN ] <column name> <drop behavior>

Destroy a column of a base table. The <drop behavior> is either RESTRICT or CASCADE. If the column is referenced in a table constraint that references other columns as well as this column, or if the column is referenced in a VIEW, or the column is referenced in a TRIGGER, then the statement will fail if RESTRICT is specified. If CASCADE is specified, then any CONSTRAINT, VIEW or TRIGGER object that references the column is dropped with a cascading effect.

ADD CONSTRAINT

add table constraint definition

<add table constraint definition> ::= ADD <table constraint definition>

Add a constraint to a table. The existing rows of the table must conform to the added constraint, otherwise the statement will not succeed.

DROP CONSTRAINT

drop table constraint definition

<drop table constraint definition> ::= DROP CONSTRAINT <constraint name> <drop behavior>

Destroy a constraint on a table. The <drop behavior> has an effect only on UNIQUE and PRIMARY KEY constraints. If such a constraint is referenced by a FOREIGN KEY constraint, the FOREIGN KEY constraint will be dropped if CASCADE is specified. If the columns of such a constraint are used in a GROUP BY clause in the query expression of a VIEW or another kind of schema object, and a functional dependency relationship exists between these columns and the other columns in that query expression, then the VIEW or other schema object will be dropped when CASCADE is specified.

ALTER COLUMN

alter column definition

<alter column definition> ::= ALTER [ COLUMN ] <column name> <alter column action>

<alter column action> ::= <set column default clause> | <drop column default clause> | <alter column data type clause> | <alter identity column specification> | <alter column nullability> | <alter column name> | <add column identity specification> | <drop column identity specification>

Change a column and its definition. Specific types of this statement are covered below. See also the RENAME statement above.

SET DEFAULT

set column default clause

<set column default clause> ::= SET <default clause>

Set the default clause for a column. This can be used if the column is not defined as GENERATED.

DROP DEFAULT

drop column default clause

<drop column default clause> ::= DROP DEFAULT

Drop the default clause from a column.

SET DATA TYPE

alter column data type clause

<alter column data type clause> ::= SET DATA TYPE <data type>

Change the declared type of a column. The latest SQL Standard allows only changes to type properties such as maximum length, precision, or scale, and only changes that cause the property to enlarge. HyperSQL allows changing the type if all the existing values can be cast into the new type without string truncation or loss of significant digits.

alter column add identity generator

alter column add identity generator

<add column identity generator> ::= <identity column specification>

Adds an identity specification to the column. The type of the column must be an integral type and the existing values must not include nulls. This option is specific to HyperSQL

 ALTER TABLE mytable ALTER COLUMN id GENERATED ALWAYS AS IDENTITY (START WITH 20000)

alter column identity generator

alter identity column specification

<alter identity column specification> ::= <alter identity column option>...

<alter identity column option> ::= <alter sequence generator restart option> | SET <basic sequence generator option>

Change the properties of an identity column. This command is similar to the commands used for changing the properties of named SEQUENCE objects discussed earlier and can use the same options.

 ALTER TABLE mytable ALTER COLUMN id RESTART WITH 1000
 ALTER TABLE mytable ALTER COLUMN id SET INCREMENT BY 5

DROP GENERATED

drop column identity generator

<drop column identity specification> ::= DROP GENERATED

Removes the identity generator from a column. After executing this statement, the column values are no longer generated automatically. This option is specific to HyperSQL

  ALTER TABLE mytable ALTER COLUMN id DROP GENERATED

SET [ NOT ] NULL

alter column nullability

<alter column nullability> ::= SET [ NOT ] NULL

Adds or removes a NOT NULL constraint from a column. This option is specific to HyperSQL

View Creation and Manipulation

CREATE VIEW

view definition

<view definition> ::= CREATE VIEW <table name> <view specification> AS <query expression> [ WITH [ CASCADED | LOCAL ] CHECK OPTION ]

<view specification> ::= [ <left paren> <view column list> <right paren> ]

<view column list> ::= <column name list>

Define a view. The <query expression> is a SELECT or similar statement. The <view column list> is the list of unique names for the columns of the view. The number of columns in the <view column list> must match the number of columns returned by the <query expression>. If <view column list> is not specified, then the columns of the <query expression> should have unique names and are used as the names of the view column.

Some views are updatable. As covered elsewhere, an updatable view is based on a single table or updatable view. For updatable views, the optional CHECK OPTION clause can be specified. If this option is specified, then if a row of the view is updated or a new row is inserted into the view, then it should contain such values that the row would be included in the view after the change. If WITH CASCADED CHECK OPTION is specified, then if the <query expression> of the view references another view, then the search condition of the underlying view should also be satisfied by the update or insert operation.

DROP VIEW

drop view statement

<drop view statement> ::= DROP VIEW [ IF EXISTS ] <table name> [ IF EXISTS ] <drop behavior>

Destroy a view. The <drop behavior> is similar to dropping a table.

ALTER VIEW

alter view statement

<alter view statement> ::= ALTER VIEW <table name> <view specification> AS <query expression> [ WITH [ CASCADED | LOCAL ] CHECK OPTION ]

Alter a view. The statement is otherwise identical to CREATE VIEW. The new definition replaces the old. If there are database objects such as routines or views that reference the view, then these objects are recompiled with the new view definition. If the new definition is not compatible, the statement fails.

Domain Creation and Manipulation

CREATE DOMAIN

domain definition

<domain definition> ::= CREATE DOMAIN <domain name> [ AS ] <predefined type> [ <default clause> ] [ <domain constraint>... ] [ <collate clause> ]

<domain constraint> ::= [ <constraint name definition> ] <check constraint definition> [ <constraint characteristics> ]

Define a domain. Although a DOMAIN is not strictly a type in the SQL Standard, it can be informally considered as a type. A DOMAIN is based on a <predefined type>, which is a base type defined by the Standard. It can have a <default clause>, similar to a column default clause. It can also have one or more CHECK constraints which limit the values that can be assigned to a column or variable that has the DOMAIN as its type.

 CREATE DOMAIN valid_string AS VARCHAR(20) DEFAULT 'NO VALUE' CHECK (value IS NOT NULL AND CHARACTER_LENGTH(value) > 2) 

ALTER DOMAIN

alter domain statement

<alter domain statement> ::= ALTER DOMAIN <domain name> <alter domain action>

<alter domain action> ::= <set domain default clause> | <drop domain default clause> | <add domain constraint definition> | <drop domain constraint definition>

Change a domain and its definition.

SET DEFAULT

set domain default clause

<set domain default clause> ::= SET <default clause>

Set the default value in a domain.

DROP DEFAULT

drop domain default clause

<drop domain default clause> ::= DROP DEFAULT

Remove the default clause of a domain.

ADD CONSTRAINT

add domain constraint definition

<add domain constraint definition> ::= ADD <domain constraint>

Add a constraint to a domain.

DROP CONSTRAINT

drop domain constraint definition

<drop domain constraint definition> ::= DROP CONSTRAINT <constraint name>

Destroy a constraint on a domain. If the <drop behavior> is CASCADE, and the constraint is a UNIQUE constraint which is referenced by a FOREIGN KEY constraint on another table, then the FOREIGN KEY constraint is also dropped.

DROP DOMAIN

drop domain statement

<drop domain statement> ::= DROP DOMAIN <domain name> <drop behavior>

Destroy a domain. If <drop behavior> is CASCADE, it works differently from most other objects. If a table features a column of the specified DOMAIN, the column survives and inherits the DEFAULT CLAUSE, and the CHECK CONSTRAINT of the DOMAIN.

Trigger Creation

CREATE TRIGGER

trigger definition

<trigger definition> ::= CREATE TRIGGER <trigger name> <trigger action time> <trigger event> ON <table name> [ REFERENCING <transition table or variable list> ] <triggered action>

<trigger action time> ::= BEFORE | AFTER | INSTEAD OF

<trigger event> ::= INSERT | DELETE | UPDATE [ OF <trigger column list> ]

<trigger column list> ::= <column name list>

<triggered action> ::= [ FOR EACH { ROW | STATEMENT } ] [ <triggered when clause> ] <triggered SQL statement>

<triggered when clause> ::= WHEN <left paren> <search condition> <right paren>

<triggered SQL statement> ::= <SQL procedure statement> | BEGIN ATOMIC { <SQL procedure statement> <semicolon> }... END | [QUEUE <integer literal>] [NOWAIT] CALL <HSQLDB trigger class FQN>

<transition table or variable list> ::= <transition table or variable>...

<transition table or variable> ::= OLD [ ROW ] [ AS ] <old transition variable name> | NEW [ ROW ] [ AS ] <new transition variable name> | OLD TABLE [ AS ] <old transition table name> | NEW TABLE [ AS ] <new transition table name>

<old transition table name> ::= <transition table name>

<new transition table name> ::= <transition table name>

<transition table name> ::= <identifier>

<old transition variable name> ::= <correlation name>

<new transition variable name> ::= <correlation name>

Trigger definition is a relatively complex statement. The combination of <trigger action time> and <trigger event> determines the type of the trigger. Examples include BEFORE DELETE, AFTER UPDATE, INSTEAD OF INSERT. If the optional [ OF <trigger column list> ] is specified for an UPDATE trigger, then the trigger is activated only if one of the columns that is in the <trigger column list> is specified in the UPDATE statement that activates the trigger.

If a trigger is FOR EACH ROW, which is the default option, then the trigger is activated for each row of the table that is affected by the execution of an SQL statement. Otherwise, it is activated once only per statement execution. In the first case, there is a before and after state for each row. For UPDATE triggers, both before and after states exist, representing the row before the update, and after the update. For DELETE, triggers, there is only a before state. For INSERT triggers, there is only an after state. If a trigger is FOR EACH STATEMENT, then a transient table is created containing all the rows for the before state and another transient table is created for the after state.

The [ REFERENCING <transition table or variable> ] is used to give a name to the before and after data row or table. This name can be referenced in the <SQL procedure statement> to access the data.

The optional <triggered when clause> is a search condition, similar to the search condition of a DELETE or UPDATE statement. If the search condition is not TRUE for a row, then the trigger is not activated for that row.

The <SQL procedure statement> is limited to INSERT, DELETE, UPDATE and MERGE statements.

The <HSQLDB trigger class FQN> is a delimited identifier that contains the fully qualified name of a Java class that implements the org.hsqldb.Trigger interface.

HyperSQL do not yet allow the use of OLD TABLE or NEW TABLE in statement level trigger definitions.

DROP TRIGGER

drop trigger statement

<drop trigger statement> ::= DROP TRIGGER <trigger name>

Destroy a trigger.

Routine Creation

schema routine

SQL-invoked routine

<SQL-invoked routine> ::= <schema routine>

<schema routine> ::= <schema procedure> | <schema function>

<schema procedure> ::= CREATE <SQL-invoked procedure>

<schema function> ::= CREATE <SQL-invoked function>

<SQL-invoked procedure> ::= PROCEDURE <schema qualified routine name> <SQL parameter declaration list> <routine characteristics> <routine body>

<SQL-invoked function> ::= { <function specification> | <method specification designator> } <routine body>

<SQL parameter declaration list> ::= <left paren> [ <SQL parameter declaration> [ { <comma> <SQL parameter declaration> }... ] ] <right paren>

<SQL parameter declaration> ::= [ <parameter mode> ] [ <SQL parameter name> ] <parameter type> [ RESULT ]

<parameter mode> ::= IN | OUT | INOUT

<parameter type> ::= <data type>

<function specification> ::= FUNCTION <schema qualified routine name> <SQL parameter declaration list> <returns clause> <routine characteristics> [ <dispatch clause> ]

<method specification designator> ::= SPECIFIC METHOD <specific method name> | [ INSTANCE | STATIC | CONSTRUCTOR ] METHOD <method name> <SQL parameter declaration list> [ <returns clause> ] FOR <schema-resolved user-defined type name>

<routine characteristics> ::= [ <routine characteristic>... ]

<routine characteristic> ::= <language clause> | <parameter style clause> | SPECIFIC <specific name> | <deterministic characteristic> | <SQL-data access indication> | <null-call clause> | <returned result sets characteristic> | <savepoint level indication>

<savepoint level indication> ::= NEW SAVEPOINT LEVEL | OLD SAVEPOINT LEVEL

<returned result sets characteristic> ::= DYNAMIC RESULT SETS <maximum returned result sets>

<parameter style clause> ::= PARAMETER STYLE <parameter style>

<dispatch clause> ::= STATIC DISPATCH

<returns clause> ::= RETURNS <returns type>

<returns type> ::= <returns data type> [ <result cast> ] | <returns table type>

<returns table type> ::= TABLE <table function column list>

<table function column list> ::= <left paren> <table function column list element> [ { <comma> <table function column list element> }... ] <right paren>

<table function column list element> ::= <column name> <data type>

<result cast> ::= CAST FROM <result cast from type>

<result cast from type> ::= <data type> [ <locator indication> ]

<returns data type> ::= <data type> [ <locator indication> ]

<routine body> ::= <SQL routine spec> | <external body reference>

<SQL routine spec> ::= [ <rights clause> ] <SQL routine body>

<rights clause> ::= SQL SECURITY INVOKER | SQL SECURITY DEFINER

<SQL routine body> ::= <SQL procedure statement>

<external body reference> ::= EXTERNAL [ NAME <external routine name> ] [ <parameter style clause> ]

<parameter style> ::= SQL | GENERAL

<deterministic characteristic> ::= DETERMINISTIC | NOT DETERMINISTIC

<SQL-data access indication> ::= NO SQL | CONTAINS SQL | READS SQL DATA | MODIFIES SQL DATA

<null-call clause> ::= RETURNS NULL ON NULL INPUT | CALLED ON NULL INPUT

<maximum returned result sets> ::= <unsigned integer>

Define an SQL-invoked routine. A few of the options are not used by HyperSQL and have default behaviours. See the SQL-Invoked Routines chapter for more details of various options and examples.

ALTER routine

alter routine statement

<alter routine statement> ::= ALTER <specific routine designator> [ <alter routine characteristics> ] [ RESTRICT ] <routine body>

<alter routine characteristics> ::= <alter routine characteristic>...

<alter routine characteristic> ::= <language clause> | <parameter style clause> | <SQL-data access indication> | <null-call clause> | <returned result sets characteristic>

<alter routine body> ::= <SQL routine body>

Alter the characteristic and the body of an SQL-invoked routine. If RESTRICT is specified and the routine is already used in a a different routine or view definition, an exception is raised. Altering the routine changes the implementation without changing the parameters. Defining recursive SQL/PSM SQL functions is only possible by altering a non-recursive routine body. An example is given in the SQL-Invoked Routines chapter.

An example is given below for a function defined as a Java method, then redefined as an SQL function.

 CREATE FUNCTION zero_pad(x BIGINT, digits INT, maxsize INT)
  RETURNS CHAR VARYING(100)
  SPECIFIC zero_pad_01
  NO SQL DETERMINISTIC
  LANGUAGE JAVA
  EXTERNAL NAME 'CLASSPATH:org.hsqldb.lib.StringUtil.toZeroPaddedString';

 ALTER SPECIFIC ROUTINE zero_pad_01
  LANGUAGE SQL
  BEGIN ATOMIC
  DECLARE str VARCHAR(128);
  SET str = CAST(x AS VARCHAR(128));
  SET str = SUBSTRING('0000000000000' FROM 1 FOR digits - CHAR_LENGTH(str)) + str;
  return str;
  END

DROP

drop routine statement

<drop routine statement> ::= DROP <specific routine designator> <drop behavior>

Destroy an SQL-invoked routine.

Sequence Creation

CREATE SEQUENCE

sequence generator definition

<sequence generator definition> ::= CREATE SEQUENCE <sequence generator name> [ <sequence generator options> ]

<sequence generator options> ::= <sequence generator option> ...

<sequence generator option> ::= <sequence generator data type option> | <common sequence generator options>

<common sequence generator options> ::= <common sequence generator option> ...

<common sequence generator option> ::= <sequence generator start with option> | <basic sequence generator option>

<basic sequence generator option> ::= <sequence generator increment by option> | <sequence generator maxvalue option> | <sequence generator minvalue option> | <sequence generator cycle option>

<sequence generator data type option> ::= AS <data type>

<sequence generator start with option> ::= START WITH <sequence generator start value>

<sequence generator start value> ::= <signed numeric literal>

<sequence generator increment by option> ::= INCREMENT BY <sequence generator increment>

<sequence generator increment> ::= <signed numeric literal>

<sequence generator maxvalue option> ::= MAXVALUE <sequence generator max value> | NO MAXVALUE

<sequence generator max value> ::= <signed numeric literal>

<sequence generator minvalue option> ::= MINVALUE <sequence generator min value> | NO MINVALUE

<sequence generator min value> ::= <signed numeric literal>

<sequence generator cycle option> ::= CYCLE | NO CYCLE

Define a named sequence generator. A SEQUENCE object generates a sequence of integers according to the specified rules. The simple definition without the options defines a sequence of numbers in INTEGER type starting at 1 and incrementing by 1. By default the CYCLE property is set and the minimum and maximum limits are the minimum and maximum limits of the type of returned values. There are self-explanatory options for changing various properties of the sequence. The MAXVALUE and MINVALUE specify the upper and lower limits. If CYCLE is specified, after the sequence returns the highest or lowest value in range, the next value will respectively be the lowest or highest value in range. If NO CYCLE is specified, the use of the sequence generator results in an error once the limit has been reached.

The integer types: SMALLINT, INTEGER, BIGINT, DECIMAL and NUMERIC can be used as the type of the sequence. DECIMAL and NUMERIC types must have a scale of 0 and a precision not exceeding 18.

ALTER SEQUENCE

alter sequence generator statement

<alter sequence generator statement> ::= ALTER SEQUENCE <sequence generator name> <alter sequence generator options>

<alter sequence generator options> ::= <alter sequence generator option>...

<alter sequence generator option> ::= <alter sequence generator restart option> | <basic sequence generator option>

<alter sequence generator restart option> ::= RESTART [ WITH <sequence generator restart value> ]

<sequence generator restart value> ::= <signed numeric literal>

Change the definition of a named sequence generator. The same options that are used in the definition of the SEQUENCE can be used to alter it. The exception is the option for the start value which is RESTART WITH for the ALTER SEQUENCE statement.

If RESTART is used by itself (without a value), then the current value of the sequence is reset to the start value. Otherwise, the current value is reset to the given restart value.

DROP SEQUENCE

drop sequence generator statement

<drop sequence generator statement> ::= DROP SEQUENCE [ IF EXISTS ] <sequence generator name> [ IF EXISTS ] <drop behavior>

Destroy an external sequence generator. If the <drop behavior> is CASCADE, then all objects that reference the sequence are dropped. These objects can be VIEW, ROUTINE or TRIGGER objects.

SQL Procedure Statement

SQL procedure statement

SQL procedure statement

The definition of CREATE TRIGGER and CREATE PROCEDURE statements refers to <SQL procedure statement>. The definition of this element is given below. However, only a subset of these statements are allowed in trigger or routine definition.

<SQL procedure statement> ::= <SQL executable statement>

<SQL executable statement> ::= <SQL schema statement> | <SQL data statement> | <SQL control statement> | <SQL transaction statement> | <SQL connection statement> | <SQL session statement> | <SQL diagnostics statement> | <SQL dynamic statement>

<SQL schema statement> ::= <SQL schema definition statement> | <SQL schema manipulation statement>

<SQL schema definition statement> ::= <schema definition> | <table definition> | <view definition> | <SQL-invoked routine> | <grant statement> | <role definition> | <domain definition> | <character set definition> | <collation definition> | <transliteration definition> | <assertion definition> | <trigger definition> | <user-defined type definition> | <user-defined cast definition> | <user-defined ordering definition> | <transform definition> | <sequence generator definition>

<SQL schema manipulation statement> ::= <drop schema statement> | <alter table statement> | <drop table statement> | <drop view statement> | <alter routine statement> | <drop routine statement> | <drop user-defined cast statement> | <revoke statement> | <drop role statement> | <alter domain statement> | <drop domain statement> | <drop character set statement> | <drop collation statement> | <drop transliteration statement> | <drop assertion statement> | <drop trigger statement> | <alter type statement> | <drop data type statement> | <alter sequence generator statement> | <drop sequence generator statement>

Other Schema Object Creation

CREATE INDEX

create index statement

<create index statement> ::= CREATE INDEX <index name> ON <table name> <left paren> {<column name> [ASC | DESC]}, ... <right paren>

Creates an index on a group of columns of a table. The optional [ASC | DESC] specifies if the column is indexed in the ascending or descending order, but has no effect on how the index is created (it is allowed for compatibility with other database engines). HyperSQL can use all indexes in ascending or descending order as needed. Indexes should not duplicate the columns of PRIMARY KEY, UNIQUE or FOREIGN key constraints as each of these constraints creates an index automatically.

DROP INDEX

drop index statement

<drop index statement> ::= DROP INDEX [ IF EXISTS ] <index name> [ IF EXISTS ]

Destroy an index.

ALTER INDEX

change the columns of an index

<alter index statement> ::= ALTER INDEX <index name> <left paren> {<column name>} , ... <right paren>

Redefine an index with a new column list. This statement is more efficient than dropping an existing index and creating a new one.

CREATE TYPE

user-defined type definition

<user-defined type definition> ::= CREATE TYPE <user-defined type body>

<user-defined type body> ::= <schema-resolved user-defined type name> [ AS <representation> ]

<representation> ::= <predefined type>

Define a user-defined type. Currently only simple distinct types can be defined without further attributes.

CREATE CAST

user-defined cast definition

<user-defined cast definition> ::= CREATE CAST <left paren> <source data type> AS <target data type> <right paren> WITH <cast function> [ AS ASSIGNMENT ]

<cast function> ::= <specific routine designator>

<source data type> ::= <data type>

<target data type> ::= <data type>

Define a user-defined cast. This feature may be supported in a future versions of HyperSQL.

DROP CAST

drop user-defined cast statement

<drop user-defined cast statement> ::= DROP CAST <left paren> <source data type> AS <target data type> <right paren> <drop behavior>

Destroy a user-defined cast. This feature may be supported in a future versions of HyperSQL.

CREATE CHARACTER SET

character set definition

<character set definition> ::= CREATE CHARACTER SET <character set name> [ AS ] <character set source> [ <collate clause> ]

<character set source> ::= GET <character set specification>

Define a character set. A new CHARACTER SET is based on an existing CHARACTER SET. The optional <collate clause> specifies the collation to be used, otherwise the collation is inherited from the default collation for the source CHARACTER SET. Currently this statement has no effect, as the character set used by HSQLDB is Unicode and there is no need for subset character sets.

DROP CHARACTER SET

drop character set statement

<drop character set statement> ::= DROP CHARACTER SET <character set name>

Destroy a character set. If the character set name is referenced in any database object, the command fails. Note that CASCADE or RESTRICT cannot be specified for this command.

CREATE COLLATION

collation definition

<collation definition> ::= CREATE COLLATION <collation name> FOR <character set specification> FROM <existing collation name> [ <pad characteristic> ]

<existing collation name> ::= <collation name>

<pad characteristic> ::= NO PAD | PAD SPACE

Define a collation. A new collation is based on an existing COLLATION and applies to an existing CHARACTER SET. The <character set specification> is always SQL_TEXT. The <existing collation name> is either SQL_TEXT or one of the language collations supported by HSQLDB. The <pad characteristic> specifies whether strings are padded with spaces for comparison.

This statement is typically used when a collation is required that does not pad spaces before comparing two strings. For example, CREATE COLLATION FRENCH_NOPAD FOR INFORMATION_SCHEMA.SQL_TEXT FROM "French" NO PAD, results in a French collation without padding. This collation can be used for sorting or for individual columns of tables.

DROP COLLATION

drop collation statement

<drop collation statement> ::= DROP COLLATION <collation name> <drop behavior>

Destroy a collation. If the <drop behavior> is CASCADE, then all references to the collation revert to the default collation that would be in force if the dropped collation was not specified.

CREATE TRANSLATION

transliteration definition

<transliteration definition> ::= CREATE TRANSLATION <transliteration name> FOR <source character set specification> TO <target character set specification> FROM <transliteration source>

<source character set specification> ::= <character set specification>

<target character set specification> ::= <character set specification>

<transliteration source> ::= <existing transliteration name> | <transliteration routine>

<existing transliteration name> ::= <transliteration name>

<transliteration routine> ::= <specific routine designator>

Define a character transliteration. This feature may be supported in a future versions of HyperSQL.

DROP TRANSLATION

drop transliteration statement

<drop transliteration statement> ::= DROP TRANSLATION <transliteration name>

Destroy a character transliteration. This feature may be supported in a future versions of HyperSQL.

CREATE ASSERTION

assertion definition

<assertion definition> ::= CREATE ASSERTION <constraint name> CHECK <left paren> <search condition> <right paren> [ <constraint characteristics> ]

Specify an integrity constraint. This feature may be supported in a future versions of HyperSQL.

DROP ASSERTION

drop assertion statement

<drop assertion statement> ::= DROP ASSERTION <constraint name> [ <drop behavior> ]

Destroy an assertion. This feature may be supported in a future versions of HyperSQL.

The Information Schema

The Information Schema is a special schema in each catalog. The SQL Standard defines a number of character sets and domains in this schema. In addition, all the implementation-defined collations belong to the Information Schema.

The SQL Standard defines many views in the Information Schema. These views show the properties of the database objects that currently exist in the database. When a user accesses one these views, only the properties of database objects that the user can access are included.

HyperSQL supports all the views defined by the Standard, apart from a few views that report on extended user-defined types and other optional features of the Standard that are not supported by HyperSQL.

HyperSQL also adds some views to the Information Schema. These views are for features that are not reported in any of the views defined by the Standard, or for use by JDBC DatabaseMetaData.

Predefined Character Sets, Collations and Domains

The SQL Standard defines a number of character sets and domains in the INFORMATION SCHEMA.

These domains are used in the INFORMATION SCHEMA views:

CARDINAL_NUMBER, YES_OR_NO, CHARACTER_DATA, SQL_IDENTIFIER, TIME_STAMP

All available collations are in the INFORMATION SCHEMA.

Views in INFORMATION SCHEMA

HyperSQL supports a vast range of views in the INFORMATION_SCHEMA. These include views specified by the SQL Standard, SQL/Schemata part, plus views that are specific to HyperSQL and are used for JDBC DatabaseMetaData queries, which are based on SQL/CLI part, or other information that is not covered by the SQL Standard. The names of views that are not part of SQL/Schemata start with SYSTEM_.

The views cover different types of information. These are covered in the next sections.

Visibility of Information

Users with the special ADMIN role can see the full information on all database objects. Ordinary, non-admin users can see information on the objects for which they have some privileges.

The rows returned to a non-admin user exclude objects on which the user has no privilege. The extent of the information in visible rows varies with the user's privilege. For example, the owner of a VIEW can see the text of the view query, but a user of the view cannot see this text. When a user cannot see the contents of some column, null is returned for that column.

Name Information

The names of database objects are stored in hierarchical views. The top level view is INFORMATION_SCHEMA_CATALOG_NAME.

Below this level, there is a group of views that covers authorizations and roles, without referencing schema objects. These are AUTHORIZATIONS and ADMINSTRABLE_ROLE_AUTHORIZATIONS.

Also below the top level, there is the SCHEMATA view, which lists the schemas in the catalog.

The views that refer to top-level schema objects are divided by object type. These includes ASSERTIONS, CHARACTER_SETS, COLLATIONS, DOMAINS, ROUTINES, SEQUENCES, TABLES, USER_DEFINED_TYPES and VIEWS.

There are views that refer to objects that are dependent on the top-level schema objects. These include COLUMNS and PARAMETERS, views for constraints, including CHECK_CONSTRAINTS, REFERENTIAL_CONSTRAINTS and TABLE_CONSTRAINTS, and finally the TRIGGERS view.

The usage of each type of top-level object by another is covered by several views. For example TRIGGER_SEQUENCE_USAGE or ROUTINE_TABLE_USAGE.

Several other views list the individual privileges owned or granted to each AUTHORIZATION. For example ROLE_ROUTINE_GRANTS or TABLE_PRIVILEGES.

Data Type Information

The INFORMATION_SCHEMA contains comprehensive information on the data types of each schema object and its elements. For example, the ROUTINES view includes the return data type for each FUNCTION definition. The columns for this information contain nulls for rows that cover PROCEDURE definitions.

The COLUMNS, PARAMETERS and SEQUENCES views contain the type information in columns with similar names.

The type information for ARRAY types is returned in the ELEMENT_TYPES view. When a row of the COLUMNS or other view indicates that the type of the object is an ARRAY type, then there is a corresponding entry for this row in the ELEMENT_TYPES view. The following columns in the ELEMENTS_TYPES view identify the database object whose data type is being described: OBJECT_CATALOG, OBJECT_SCHEMA, OBJECT_NAME, OBJECT_TYPE, COLLECTION_TYPE_IDENTIFIER. The last column's counterpart in the COLUMNS view is named differently as DTD_IDENTIFIER. So in order to determine the array element type of a column, an equi-join between the COLUMNS and ELEMENT_TYPES tables on the six listed columns in the ELEMENT_TYPES view and their counterparts in the COLUMNS view is needed.

Product Information

A group of views, including SQL_IMPLEMENTATION_INFO, SQL_FEATURES, SQL_SIZING and others cover the capabilities of HyperSQL in detail. These views hold static data and can be explored even when the database is empty.

Operations Information

There are some HyperSQL custom views cover the current state of operation of the database. These include SYSTEM_CACHEINFO, SYSTEM_SESSIONINFO and SYSTEM_SESSIONS views.

SQL Standard Views

The following views are defined by the SQL Standard and supported by HyperSQL. The columns and contents exactly match the Standard requirements.

ADMINISTRABLE_ROLE_AUTHORIZATIONS

Information on ROLE authorizations, all granted by the admin role.

APPLICABLE_ROLES

Information on ROLE authorizations for the current user

ASSERTIONS

Empty view as ASSERTION objects are not yet supported.

AUTHORIZATIONS

Top level information on USER and ROLE objects in the database

CHARACTER_SETS

List of supported CHARACTER SET objects

CHECK_CONSTRAINTS

Additional information specific to each CHECK constraint, including the search condition

CHECK_CONSTRAINT_ROUTINE_USAGE

Information on FUNCTION objects referenced in CHECK constraints search conditions

COLLATIONS

Information on collations supported by the database.

COLUMNS

Information on COLUMN objects in TABLE and VIEW definitions

COLUMN_COLUMN_USAGE

Information on references to COLUMN objects from other, GENERATED, COLUMN objects

COLUMN_DOMAIN_USAGE

Information on DOMAIN objects used in type definition of COLUMN objects

COLUMN_PRIVILEGES

Information on privileges on each COLUMN object, granted to different ROLE and USER authorizations

COLUMN_UDT_USAGE

Information on distinct TYPE objects used in type definition of COLUMN objects

CONSTRAINT_COLUMN_USAGE

Information on COLUMN objects referenced by CONSTRAINT objects in the database

CONSTRAINT_TABLE_USAGE

Information on TABLE and VIEW objects referenced by CONSTRAINT objects in the database

DATA_TYPE_PRIVILEGES

Information on top level schema objects of various kinds that reference TYPE objects

DOMAINS

Top level information on DOMAIN objects in the database.

DOMAIN_CONSTRAINTS

Information on CONSTRAINT definitions used for DOMAIN objects

ELEMENT_TYPES

Information on the type of elements of ARRAY used in database columns or routine parameters and return values

ENABLED_ROLES

Information on ROLE privileges enabled for the current session

INFORMATION_SCHEMA_CATALOG_NAME

Information on the single CATALOG object of the database

KEY_COLUMN_USAGE

Information on COLUMN objects of tables that are used by PRIMARY KEY, UNIQUE and FOREIGN KEY constraints

PARAMETERS

Information on parameters of each FUNCTION or PROCEDURE

REFERENTIAL_CONSTRAINTS

Additional information on FOREIGN KEY constraints, including triggered action and name of UNIQUE constraint they refer to

ROLE_AUTHORIZATION_DESCRIPTORS

ROLE_COLUMN_GRANTS

Information on privileges on COLUMN objects granted to or by the current session roles

ROLE_ROUTINE_GRANTS

Information on privileges on FUNCTION and PROCEDURE objects granted to or by the current session roles

ROLE_TABLE_GRANTS

Information on privileges on TABLE and VIEW objects granted to or by the current session roles

ROLE_UDT_GRANTS

Information on privileges on TYPE objects granted to or by the current session roles

ROLE_USAGE_GRANTS

Information on privileges on USAGE privileges granted to or by the current session roles

ROUTINE_COLUMN_USAGE

Information on COLUMN objects of different tables that are referenced in FUNCTION and PROCEDURE definitions

ROUTINE_JAR_USAGE

Information on JAR usage by Java language FUNCTION and PROCEDURE objects.

ROUTINE_PRIVILEGES

Information on EXECUTE privileges granted on PROCEDURE and FUNCTION objects

ROUTINE_ROUTINE_USAGE

Information on PROCEDURE and FUNCTION objects that are referenced in FUNCTION and PROCEDURE definitions

ROUTINE_SEQUENCE_USAGE

Information on SEQUENCE objects that are referenced in FUNCTION and PROCEDURE definitions

ROUTINE_TABLE_USAGE

Information on TABLE and VIEW objects that are referenced in FUNCTION and PROCEDURE definitions

ROUTINES

Top level information on all PROCEDURE and FUNCTION objects in the database

SCHEMATA

Information on all the SCHEMA objects in the database

SEQUENCES

Information on SEQUENCE objects

SQL_FEATURES

List of all SQL:2008 standard features, including information on whether they are supported or not supported by HyperSQL

SQL_IMPLEMENTATION_INFO

Information on name, capabilities and defaults of the database engine software.

SQL_PACKAGES

List of the SQL:2008 Standard packages, including information on whether they are supported or not supported by HyperSQL

SQL_PARTS

List of the SQL:2008 Standard parts, including information on whether they are supported or not supported by HyperSQL

SQL_SIZING

List of the SQL:2008 Standard maximum supported sizes for different features as supported by HyperSQL

SQL_SIZING_PROFILES

TABLES

Information on all TABLE and VIEW object, including the INFORMATION_SCHEMA views themselves

TABLE_CONSTRAINTS

Information on all table level constraints, including PRIMARY KEY, UNIQUE, FOREIGN KEY and CHECK constraints

TABLE_PRIVILEGES

Information on privileges on TABLE and VIEW objects owned or given to the current user

TRANSLATIONS

TRIGGERED_UPDATE_COLUMNS

Information on columns that have been used in TRIGGER definitions in the ON UPDATE clause

TRIGGERS

Top level information on the TRIGGER definitions in the databases

TRIGGER_COLUMN_USAGE

Information on COLUMN objects that have been referenced in the body of TRIGGER definitions

TRIGGER_ROUTINE_USAGE

Information on FUNCTION and PROCEDURE objects that have been used in TRIGGER definitions

TRIGGER_SEQUENCE_USAGE

Information on SEQUENCE objects that been referenced in TRIGGER definitions

TRIGGER_TABLE_USAGE

Information on TABLE and VIEW objects that have been referenced in TRIGGER definitions

USAGE_PRIVILEGES

Information on USAGE privileges granted to or owned by the current user

USER_DEFINED_TYPES

Top level information on TYPE objects in the database

VIEWS

Top Level information on VIEW objects in the database

VIEW_COLUMN_USAGE

Information on COLUMN objects referenced in the query expressions of the VIEW objects

VIEW_ROUTINE_USAGE

Information on FUNCTION and PROCEDURE objects that have been used in the query expressions of the VIEW objects

VIEW_TABLE_USAGE

Information on TABLE and VIEW objects that have been referenced in the query expressions of the VIEW objects

HyperSQL Custom Views

The following views are specific to HyperSQL. Most of these views are used directly by JDBC DatabaseMetaData method calls and are indicated as such. Some views contain information that is specific to HyperSQL and is not covered by the SQL Standard views.

SYSTEM_BESTROWIDENTIFIER

For DatabaseMetaData.getBestRowIdentifier

SYSTEM_CACHEINFO

Contains the current settings and variables of the data cache used for all CACHED tables, and the data cache of each TEXT table.

SYSTEM_COLUMN_SEQUENCE_USAGE

Contains a row for each column that is defined as GENERATED BY DEFAULT AS SEQUENCE with the column name and sequence name

SYSTEM_COLUMNS

For DatabaseMetaData.getColumns, contains a row for each column

SYSTEM_COMMENTS

Contains the user-defined comments added to tables and their columns.

SYSTEM_CONNECTION_PROPERTIES

For DatabaseMetaData.getClientInfoProperties

SYSTEM_CROSSREFERENCE

Full list of all columns referenced by FOREIGN KEY constraints. For DatabaseMetaData.getCrossReference, getExportedKeys and getImportedKeys.

SYSTEM_INDEXINFO

For DatabaseMetaData.getIndexInfo

SYSTEM_PRIMARYKEYS

For DatabaseMetaData.getPrimaryKeys

SYSTEM_PROCEDURECOLUMNS

For DatabaseMetaData.getProcedureColumns

SYSTEM_PROCEDURES

For DatabaseMetaData.getFunctionColumns, getFunctions and getProcedures

SYSTEM_PROPERTIES

Contains the current values of all the database level properties. Settings such as SQL rule enforcement, database transaction model and default transaction level are all reported in this view. The names of the properties are listed in the Properties chapter together with the corresponding SQL statements used to change the properties.

SYSTEM_SCHEMAS

For DatabaseMetaData.getSchemas

SYSTEM_SEQUENCES

SYSTEM_SESSIONINFO

Information on the settings and properties of the current session.

SYSTEM_SESSIONS

Information on all open sessions in the database (when used by a DBA user), or just the current session. Includes the current transaction state of each session.

SYSTEM_TABLES

Information on tables and views for DatabaseMetaData.getTables

SYSTEM_TABLESTATS

Information on table spaces and cardinality for each table

SYSTEM_TABLETYPES

For DatabaseMetaData.getTableTypes

SYSTEM_TEXTTABLES

Information on the settings of each text table.

SYSTEM_TYPEINFO

For DatabaseMetaData.getTypeInfo

SYSTEM_UDTS

For DatabaseMetaData.getUDTs

SYSTEM_USERS

Contains the list of all users in the database (when used by a DBA user), or just the current user.

SYSTEM_VERSIONCOLUMNS

For DatabaseMetaData.getVersionColumns

Chapter 5. Text Tables

Text Tables as a Standard Feature of Hsqldb

Bob Preston

The HSQL Development Group

Fred Toussi

The HSQL Development Group

$Revision: 5212 $

Copyright 2002-2012 Bob Preston and Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Overview

Text Table support for HSQLDB was originally developed by Bob Preston independently from the Project. Subsequently Bob joined the Project and incorporated this feature into version 1.7.0, with a number of enhancements, especially the use of conventional SQL commands for specifying the files used for Text Tables.

In a nutshell, Text Tables are CSV or other delimited files treated as SQL tables. Any ordinary CSV or other delimited file can be used. The full range of SQL queries can be performed on these files, including SELECT, INSERT, UPDATE and DELETE. Indexes and unique constraints can be set up, and foreign key constraints can be used to enforce referential integrity between Text Tables themselves or with conventional tables.

The delimited file can be created by the engine, or an existing file can be used.

HyperSQL with Text Table support is the only comprehensive solution that employs the power of SQL and the universal reach of JDBC to handle data stored in text files.

The Implementation

Definition of Tables

Text Tables are defined similarly to conventional tables with the added TEXT keyword.

 CREATE TEXT TABLE <tablename> (<column definition> [<constraint definition>])

The table is at first empty and cannot be written to. An additional SET command specifies the file and the separator character that the Text table uses. It assigns the file to the table.

 SET TABLE <tablename> SOURCE <quoted_filename_and_options> [DESC]

Scope and Reassignment

  • A Text table without a file assigned to it is READ ONLY and EMPTY.

  • Reassigning a Text Table definition to a new file has implications in the following areas:

    1. The user is required to be an administrator.

    2. Existing transactions are committed at this point.

    3. Constraints, including foreign keys referencing this table, are kept intact but not checked. It is the responsibility of the administrator to ensure their integrity.

    The new source file is scanned and indexes are built when it is assigned to the table. At this point any violation of NOT NULL, UNIQUE or PRIMARY KEY constraints are caught and the assignment is aborted. However, foreign key constraints are not checked at the time of assignment or reassignment of the source file.

Null Values in Columns of Text Tables

  • Empty fields are treated as NULL. These are fields where there is nothing or just spaces between the separators.

  • Quoted empty strings are treated as empty strings.

Configuration

The default field separator is a comma (,). A different field separator can be specified within the SET TABLE SOURCE statement. For example, to change the field separator for the table mytable to a vertical bar, place the following in the SET TABLE SOURCE statement, for example:

 SET TABLE mytable SOURCE "myfile;fs=|"

Since HSQLDB treats CHAR and VARCHAR strings the same, the ability to assign a different separator to the latter is provided. When a different separator is assigned to a VARCHAR, it will terminate any CSV field of that type. For example, if the first field is CHAR, and the second field VARCHAR, and the separator fs has been defined as the pipe (|) and vs as the period (.) then the data in the CSV file for a row will look like:

 First field data|Second field data.Third field data

This facility in effect offers an extra, special separator which can be used in addition to the global separator. The following example shows how to change the default separator to the pipe (|), VARCHAR separator to the period (.) within a SET TABLE SOURCE statement:

 SET TABLE mytable SOURCE "myfile;fs=|;vs=."

HSQLDB also recognises the following special indicators for separators:

special indicators for separators

\semi

semicolon

\quote

single-quote

\space

space character

\apos

apostrophe

\n

newline - Used as an end anchor (like $ in regular expressions)

\r

carriage return

\t

tab

\\

backslash

\u####

a Unicode character specified in hexadecimal

Furthermore, HSQLDB provides csv file support with three additional boolean options: ignore_first, quoted and all_quoted. The ignore_first option (default false) tells HSQLDB to ignore the first line in a file. This option is used when the first line of the file contains column headings. The all_quoted option (default false) tells the program that it should use quotes around all character fields when writing to the source file. The quoted option (default true) uses quotes only when necessary to distinguish a field that contains the separator character. It can be set to false to prevent the use of quoting altogether and treat quote characters as normal characters. These options may be specified within the SET TABLE SOURCE statement:

 SET TABLE mytable SOURCE "myfile;ignore_first=true;all_quoted=true"

When the default options all_quoted= false and quoted=true are in force, fields that are written to a line of the csv file will be quoted only if they contain the separator or the quote character. The quote character is doubled when used inside a string. When all_quoted=false and quoted=false the quote character is not doubled. With this option, it is not possible to insert any string containing the separator into the table, as it would become impossible to distinguish from a separator. While reading an existing data source file, the program treats each individual field separately. It determines that a field is quoted only if the first character is the quote character. It interprets the rest of the field on this basis.

The character encoding for the source file is ASCII by default. To support UNICODE or source files prepared with different encodings this can be changed to UTF-8 or any other encoding. The default is encoding=ASCII and the option encoding=UTF-8 or other supported encodings can be used.

Finally, HSQLDB provides the ability to read a text file as READ ONLY, by placing the keyword "DESC" at the end of the SET TABLE SOURCE statement:

 SET TABLE mytable SOURCE "myfile" DESC

Text table source files are cached in memory. The maximum number of rows of data that are in memory at any time is controlled by the cache_rows property. The default value for cache_rows is 1000 and can be changed by setting the default database property .The cache_size property sets the maximum amount of memory used for each text table. The default is 100 KB. The properties can be set for individual text tables. These properties do not control the maximum size of each text table, which can be much larger. An example is given below:

 SET TABLE mytable SOURCE "myfile;ignore_first=true;all_quoted=true;cache_rows=10000;cache_size=1000"

The properties used in earlier versions, namely the textdb.cache_scale and the textdb.cache_size_scale can still be used for backward compatibility, but the new properties are preferred.

Supported Properties

quoted = { true | false }

default is true. If false, treats double quotes as normal characters

all_quoted = { true | false }

default is false. If true, adds double quotes around all fields.

encoding = <encoding name>

character encoding for text and character fields, for example, encoding=UTF-8. UTF-16 cannot be used.

ignore_first = { true | false }

default is false. If true ignores the first line of the file

cache_rows= <numeric value>

rows of the text file in the cache. Default is 1000 rows

cache_size = <numeric value>r

total size of the rows in the cache. Default is 100 KB.

cache_scale= <numeric value> and cache_size_scale = <numeric value>

deprecated properties, replaced by cached_rows and cache_size properties above.

fs = <unquoted character>

field separator

vs = <unquoted character>

varchar separator

Disconnecting Text Tables

Text tables may be disconnected from their underlying data source, i.e. the text file.

You can explicitly disconnect a text table from its file by issuing the following statement:

 SET TABLE mytable SOURCE OFF

Subsequently, mytable will be empty and read-only. However, the data source description will be preserved, and the table can be re-connected to it with

 SET TABLE mytable SOURCE ON

When a database is opened, if the source file for an existing text table is missing, the table remains disconnected from its data source but the source description is preserved. This allows the missing source file to be added to the directory and the table re-connected to it with the above command.

Disconnecting text tables from their source has several uses. While disconnected, the text source can be edited outside HSQLDB, provided data integrity is respected. When large text sources are used, and several constraints or indexes need to be created on the table, it is possible to disconnect the source during the creation of constraints and indexes and reduce the time it takes to perform the operation.

Text File Usage

The following information applies to the usage of text tables.

Text File Issues

  • With file databases, text fiile locations are restricted to below the directory that contains the database, unless the textdb.allow_full_path property is set true as a Java system property. This feature is for security, otherwise an admin database user may be able to open random files. The specified text source path is interpreted differently according to this property. By default, the path is interpreted as a relative path to the directory path of database files, it therefore cannot contain the double dot notation for parent directory. This path is then appended by the engine to the directory path to form a full path.

    When the property is true, and the path starts with the forward slash or back slash, or the path contains a semicolon, the path is not appended to the directory path and is used as it is to open the file. In this usage the path is absolute.

  • By default, all-in-memory databases cannot use text tables. To enable this capability the textdb.allow_full_path property must be set true as a Java system property. The text file path is used as submitted and interpreted as an absolute path as described above, or a path relative to the Java process execute path. These text tables are always read-only.

  • Databases store in jars or as files on the classpath and opened with the res: protocol can reference read-only text files. These files are opened as resources. The file path is an absolute path beginning with a forward slash.

  • Blank lines are allowed anywhere in the text file, and are ignored.

  • It is possible to define a primary key, identity column, unique, foreign key and check constraints for text tables.

  • When a table source file is used with the ignore_first=true option, the first, ignored line is replaced with a blank line after a SHUTDOWN COMPACT, unless the SOURCE HEADER statement has been used.

  • An existing table source file may include CHARACTER fields that do not begin with the quote character but contain instances of the quote character. These fields are read as literal strings. Alternatively, if any field begins with the quote character, then it is interpreted as a quoted string that should end with the quote character and any instances of the quote character within the string is doubled. When any field containing the quote character or the separator is written out to the source file by the program, the field is enclosed in quote character and any instance of the quote character inside the field is doubled.

  • Inserts or updates of CHARACTER type field values are allowed with strings that contains the linefeed or the carriage return character. This feature is disabled when both quoted and all_quoted properties are false.

  • ALTER TABLE commands that add or drop columns or constraints (apart from check constraints) are not supported with text tables that are connected to a source. First use the SET TABLE <name> SOURCE OFF, make the changes, then turn the source ON.

  • Use the default setting (quoted=true) for selective quoting of fields. Those fields that need quoting are quoted, other not.

  • Use the quoted=false setting to avoid quoting of fields completely. With this setting any quote character is considered part of the text.

  • Use the all_quoted=true setting to force all fields to be quoted.

  • SHUTDOWN COMPACT results in a complete rewrite of text table sources that are open at the time. The settings for quoted and all_quoted are applied for the rewrite.

Text File Global Properties

The database engine uses a set of defaults for text table properties. Each table's data source may override these defaults. It is also possible to override the defaults globally, so they apply to all text tables. The statement SET DATABASE TEXT TABLE DEFAULTS <properties string> can be used to override the default global properties. An example is given below:

 SET DATABASE TEXT TABLE DEFAULTS 'all_quoted=true;encoding=UTF-8;cache_rows=10000;cache_size=2000'

List of supported global properties

  • fs=,

  • vs=,

  • quoted=true

  • all_quoted=false

  • ignore_first=false

  • encoding=ASCII

  • cache_rows=1000

  • cache_size=100

  • textdb.allow_full_path=false (a system property)

Transactions

Text tables fully support transactions. New or changed rows that have not been committed are not updated in the source file. Therefore the source file always contains committed rows.

However, text tables are not as resilient to machine crashes as other types of tables. If the crash happens while the text source is being written to, the text source may contain only some of the changes made during a committed transaction. With other types of tables, additional mechanisms ensure the integrity of the data and this situation will not arise.

Chapter 6. Access Control

Fred Toussi

The HSQL Development Group

$Revision: 3096 $

Copyright 2010-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Overview

This chapter is about access control to database objects such as tables, inside the database engine. Other issues related to security include user authentication, password complexity and secure connections are covered in the System Management chapter and the HyperSQL Network Listeners (Servers) chapter.

Apart from schemas and their object, each HyperSQL catalog has USER and ROLE objects. These objects are collectively called authorizations. Each AUTHORIZATION has some access rights on some of the schemas or the objects they contain. The persistent elements of an SQL environment are database objects

Authorizations names are stored in the database in the case-normal form. When connecting to a database via JDBC, the user name and password must match the case of this case-normal form.

When a user is created with the CREATE USER statement, if the user name is enclosed in double quotes, the exact name is used as the case-normal form. But if it is not enclosed in double quotes, the name is converted to uppercase and this uppercase version is stored in the database as the case-normal form.

Authorizations and Access Control

In general, ROLE and USER objects simply control access to schema objects. This is the scope of the SQL Standard. However, there are special roles that allow the creation of USER and ROLE objects and also allow some special operations on the database as a whole. These roles are not defined by the Standard, which has left it to implementers to define such roles as they are needed for the particular SQL implementation.

A ROLE has a name a collection of zero or more other roles, plus some privileges (access rights). A USER has a name and a password. It similarly has a collection of zero or more roles plus some privileges.

USER objects existed in the SQL-92, but ROLE objects were introduced in SQL:1999. Originally it was intended that USER objects would normally be the same as the operating system USER objects and their authentication would be handled outside the SQL environment. The co-existence of ROLE and USER objects results in complexity. With the addition of ROLE objects, there is no rationale, other than legacy support, for granting privileges to USER objects directly. It is better to create roles and grant privileges to them, then grant the roles to USER objects.

The Standard effectively defines a special ROLE, named PUBLIC. All authorization have the PUBLIC role, which cannot be removed from them. Therefore any access right assigned to the PUBLIC role applies to all authorizations in the database. For many simple databases, it is adequate to create a single, non-admin user, then assign access rights to the pre-existing PUBLIC role. Access to INFORMATION_SCHEMA views is granted to PUBLIC, therefore these views are accessible to all. However, the contents of each view depends on the ROLE or USER (AUTHORIZATION) that is in force while accessing the view.

Each schema has a single AUTHORIZATION. This is commonly known as the owner of the schema. All the objects in the schema inherit the schema owner. The schema owner can add objects to the schema, drop them or alter them.

By default, the objects in a schema can only be accessed by the schema owner. The schema owner can grant access rights on the objects to other users or roles.

authorization identifier

authorization identifier

<authorization identifier> ::= <role name> | <user name>

Authorization identifiers share the same name-space within the database. The same name cannot be used for a USER and a ROLE.

Built-In Roles and Users

There are some pre-defined roles in each database; some defined by the SQL Standard, some by HyperSQL. These roles can be assigned to users (directly or via other, user-defined roles). In addition, there is the initial SYS user created with each new database. The initial user name and password is defined in the connection properties when the first connection to the database is made. In older versions of HSQLDB, this name was always SA. But in the latest version, the name can be defined as a different string.

PUBLIC

the PUBLIC role

The role that is assigned to all authorizations (roles and users) in the database. This role has access rights to all objects in the INFORMATION_SCHEMA. Any roles or rights granted to this role, are in effect granted to all users of the database.

_SYSTEM

the _SYSTEM role

This role is the authorization for the pre-defined (system) objects in the database, including the INFORMATION_SCHEMA. This role cannot be assigned to any authorization.

DBA

the DBA role (HyperSQL-specific)

This is a special role in HyperSQL. A user that has this role can perform all possible administrative tasks on the database. The DBA role can also act as a proxy for all the roles and users in the database. This means it can do everything the authorization for a schema can do, including dropping the schema or its objects, or granting rights on the schema objects to a grantee.

CREATE_SCHEMA

the CREATE_SCHEMA role (HyperSQL-specific)

An authorization that has this role, can create schemas. The DBA authorization has this role and can grant it to other authorizations.

CHANGE_AUTHORIZATION

the CHANGE_AUTHORIZATION role (HyperSQL-specific)

A user that has this role, can change the authorization for the current session to another user. The other user cannot have the DBA role (otherwise, the original user would gain DBA privileges). The DBA authorization has this role and can grant it to other authorizations.

SYS User

the SYS user (HyperSQL-specific)

This user is automatically created with a new database and has the DBA role. This user name and its password are defined in the connection properties when connecting to the new database to create the database. As this user, it is possible to change the password, create other users and created new schema objects. The initial SYS user can be dropped by another user that has the DBA role. As a result, there is always at least one SYS user in the database.

Listing Users and Roles

Tables in the INFORMATION_SCHEMA contain the list of users and roles for the database.

The SYSTEM_USERS tables contains the list of users, with some extra settings for each user. The AUTHORIZATIONS table contains a list of both users and roles.

Several other INFORMATION_SCHEMA tables list the privileges granted to users and roles on different database objects. Refer to the Schemas and Database Objects chapter for a list and description of the tables. Example below:

 SELECT * FROM INFORMATION_SCHEMA.SYSTEM_USERS 
 SELECT * FROM INFORMATION_SCHEMA.TABLE_PRIVILEGES

Access Rights

By default, the objects in a schema can only be accessed by the schema owner. But the schema owner can grant privileges (access rights) on the objects to other users or roles.

Things can get far more complex, because the grant of privileges can be made WITH GRANT OPTION. In this case, the role or user that has been granted the privilege can grant the privilege to other roles and users.

Privileges can also be revoked from users or roles.

The statements for granting and revoking privileges normally specify which privileges are granted or revoked. However, there is a shortcut, ALL PRIVILEGES, which means all the privileges that the <grantor> has on the schema object. The <grantor> is normally the CURRENT_USER of the session that issues the statement.

The user or role that is granted privileges is referred to as <grantee> for the granted privileges.

Table

For tables, including views, privileges can be granted with different degrees of granularity. It is possible to grant a privilege on all columns of a table, or on specific columns of the table.

The DELETE privilege applies to the table, rather than its columns. It applies to all DELETE statements.

The SELECT, INSERT and UPDATE privileges may apply to all columns or to individual columns. These privileges determine whether the <grantee> can execute SQL data statements on the table.

The SELECT privilege designates the columns that can be referenced in SELECT statements, as well as the columns that are read in a DELETE or UPDATE statement, including the search condition.

The INSERT privilege designates the columns into which explicit values can be inserted. To be able to insert a row into the table, the user must therefore have the INSERT privilege on the table, or at least all the columns that do not have a default value.

The UPDATE privilege simply designates the table or the specific columns that can be updated.

The REFERENCES privilege allows the <grantee> to define a FOREIGN KEY constraint on a different table, which references the table or the specific columns designated for the REFERENCES privilege.

The TRIGGER privilege allows adding a trigger to the table.

Sequence, Type, Domain, Character Set, Collation, Transliteration,

For these objects, only USAGE can be granted. The USAGE privilege is needed when object is referenced directly in an SQL statement.

Routine

For routines, including procedures or functions, only EXECUTE privilege can be granted. This privilege is needed when the routine is used directly in an SQL statement.

Other Objects

Other objects such as constraints and assertions are not used directly and there is no grantable privilege that refers to them.

Statements for Authorization and Access Control

The statements listed below allow creation and destruction of USER and ROLE objects. The GRANT and REVOKE statements allow roles to be assigned to other roles or to users. The same statements are also used in a different form to assign privileges on schema objects to users and roles.

CREATE USER

user definition (HyperSQL)

<user definition> ::= CREATE USER <user name> PASSWORD <password> [ ADMIN ]

Define a new user and its password. <user name> is an SQL identifier. If it is double-quoted it is case-sensitive, otherwise it is turned to uppercase. <password> is a string enclosed with single quote characters and is case-sensitive. If ADMIN is specified, the DBA role is granted to the new user. Only a user with the DBA role can execute this statement.

DROP USER

drop user statement (HyperSQL)

<drop user statement> ::= DROP USER <user name>

Drop (destroy) an existing user. If the specified user is the authorization for a schema, the schema is destroyed.

Only a user with the DBA role can execute this statement.

ALTER USER ... SET PASSWORD

set the password for a user (HyperSQL)

<alter user set password statement> ::= ALTER USER <user name> SET PASSWORD <password>

Change the password of an existing user. <user name> is an SQL identifier. If it is double-quoted it is case-sensitive, otherwise it is turned to uppercase. <password> is a string enclosed with single quote characters and is case-sensitive.

Only a user with the DBA role can execute this command.

ALTER USER ... SET INITIAL SCHEMA

set the initial schema for a user (HyperSQL)

<alter user set initial schema statement> ::= ALTER USER <user name> SET INITIAL SCHEMA <schema name> | DEFAULT

Change the initial schema for a user. The initial schema is the schema used by default for SQL statements issued during a session. If DEFAULT is used, the default initial schema for all users is used as the initial schema for the user. The SET SCHEMA command allows the user to change the schema for the duration of the session.

Only a user with the DBA role can execute this statement.

ALTER USER ... SET LOCAL

set the user authentication as local (HyperSQL)

<alter user set local> ::= ALTER USER <user name> SET LOCAL { TRUE | FALSE }

Sets the authentication method for the user as local. This statement has an effect only when external authentication with role names is enabled. In this method of authentication, users created in the database are ignored and an external authentication mechanism, such as LDAP is used. This statement is used if you want to use local, password authentication for a specific user.

Only a user with the DBA role can execute this statement.

SET PASSWORD

set password statement (HyperSQL)

<set password statement> ::= SET PASSWORD <password>

Set the password for the current user. <password> is a string enclosed with single quote characters and is case-sensitive.

SET INITIAL SCHEMA

set the initial schema for the current user (HyperSQL)

<set initial schema statement> ::= SET INITIAL SCHEMA <schema name> | DEFAULT

Change the initial schema for the current user. The initial schema is the schema used by default for SQL statements issued during a session. If DEFAULT is used, the default initial schema for all users is used as the initial schema for the current user. The separate SET SCHEMA command allows the user to change the schema for the duration of the session. See also the Sessions and Transactions chapter.

SET DATABASE DEFAULT INITIAL SCHEMA

set the default initial schema for all users (HyperSQL)

<set database default initial schema statement> ::= SET DATABASE DEFAULT INITIAL SCHEMA <schema name>

Sets the initial schema for new users. This schema can later be changed with the <set initial schema statement> command.

CREATE ROLE

role definition

<role definition> ::= CREATE ROLE <role name> [ WITH ADMIN <grantor> ]

Defines a new role. Initially the role has no rights, except those of the PUBLIC role. Only a user with the DBA role can execute this command.

DROP ROLE

drop role statement

<drop role statement> ::= DROP ROLE <role name>

Drop (destroy) a role. If the specified role is the authorization for a schema, the schema is destroyed. Only a user with the DBA role can execute this statement.

GRANTED BY

grantor determination

GRANTED BY <grantor>

<grantor> ::= CURRENT_USER | CURRENT_ROLE

The authorization that is granting or revoking a role or privileges. The optional GRANTED BY <grantor> clause can be used in various statements that perform GRANT or REVOKE actions. If the clause is not used, the authorization is CURRENT_USER. Otherwise, it is the specified authorization.

GRANT

grant privilege statement

<grant privilege statement> ::= GRANT <privileges> TO <grantee> [ { <comma> <grantee> }... ] [ WITH GRANT OPTION ] [ GRANTED BY <grantor> ]

Assign privileges on schema objects to roles or users. Each <grantee> is a role or a user. If [ WITH GRANT OPTION ] is specified, then the <grantee> can assign the privileges to other <grantee> objects.

<privileges> ::= <object privileges> ON <object name>

<object name> ::= [ TABLE ] <table name> | DOMAIN <domain name> | COLLATION <collation name> | CHARACTER SET <character set name> | TRANSLATION <transliteration name> | TYPE <user-defined type name> | SEQUENCE <sequence generator name> | <specific routine designator> | ROUTINE <routine name> | FUNCTION <function name> | PROCEDURE <procedure name>

<object privileges> ::= ALL PRIVILEGES | <action> [ { <comma> <action> }... ]

<action> ::= SELECT | SELECT <left paren> <privilege column list> <right paren> | DELETE | INSERT [ <left paren> <privilege column list> <right paren> ] | UPDATE [ <left paren> <privilege column list> <right paren> ] | REFERENCES [ <left paren> <privilege column list> <right paren> ] | USAGE | TRIGGER | EXECUTE

<privilege column list> ::= <column name list>

<grantee> ::= PUBLIC | <authorization identifier>

The <object privileges> that can be used depend on the type of the <object name>. These are discussed in the previous section. For a table, if <privilege column list> is not specified, then the privilege is granted on the table, which includes all of its columns and any column that may be added to it in the future. For routines, the name of the routine can be specified in two ways, either as the generic name as the specific name. HyperSQL allows referencing all overloaded versions of a routine at the same time, using its name. This differs from the SQL Standard which requires the use of <specific routine designator> to grant privileges separately on each different signature of the routine.

Each <grantee> is the name of a role or a user. Examples of GRANT statement are given below:

 GRANT ALL ON SEQUENCE aSequence TO roleOrUser 
 GRANT SELECT ON aTable TO roleOrUser  
 GRANT SELECT, UPDATE ON aTABLE TO roleOrUser1, roleOrUser2
 GRANT SELECT(columnA, columnB), UPDATE(columnA, columnB) ON TABLE aTable TO roleOrUser
 GRANT EXECUTE ON SPECIFIC ROUTINE aroutine_1234 TO rolOrUser

As mentioned in the general discussion, it is better to define a role for the collection of all the privileges required by an application. This role is then granted to any user. If further changes are made to the privileges of this role, they are automatically reflected in all the users that have the role.

GRANT

grant role statement

<grant role statement> ::= GRANT <role name> [ { <comma> <role name> }... ] TO <grantee> [ { <comma> <grantee> }... ] [ WITH ADMIN OPTION ] [ GRANTED BY <grantor> ]

Assign roles to roles or users. One or more roles can be assigned to one or more <grantee> objects. A <grantee> is a user or a role. If the [ WITH ADMIN OPTION ] is specified, then each <grantee> can grant the newly assigned roles to other grantees. An example of user and role creation with grants is given below:

 CREATE USER appuser
 CREATE ROLE approle
 GRANT approle TO appuser
 GRANT SELECT, UPDATE ON TABLE atable TO approle
 GRANT USAGE ON SEQUENCE asequence to approle
 GRANT EXECUTE ON ROUTINE aroutine TO approle

REVOKE privilege

revoke statement

<revoke privilege statement> ::= REVOKE [ GRANT OPTION FOR ] <privileges> FROM <grantee> [ { <comma> <grantee> }... ] [ GRANTED BY <grantor> ] RESTRICT | CASCADE

Revoke privileges from a user or role.

REVOKE role

revoke role statement

<revoke role statement> ::= REVOKE [ ADMIN OPTION FOR ] <role revoked> [ { <comma> <role revoked> }... ] FROM <grantee> [ { <comma> <grantee> }... ] [ GRANTED BY <grantor> ] RESTRICT | CASCADE

<role revoked> ::= <role name>

Revoke a role from users or roles.

Chapter 7. Data Access and Change

Fred Toussi

The HSQL Development Group

$Revision: 5240 $

Copyright 2010-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Overview

HyperSQL data access and data change statements are fully compatible with the latest SQL:2008 Standard. There are a few extensions and some relaxation of rules, but these do not affect statements that are written to the Standard syntax. There is full support for classic SQL, as specified by SQL-92, and many enhancements added in later versions of the standard.

Cursors And Result Sets

An SQL statement can executed in two ways. One way is to use the java.sql.Statement interface. The Statement object can be reused to execute completely different SQL statements. Alternatively a PreparedStatment can be used to execute an SQL statement repeatedly, and the statements can be parameterized. Using either form, if the SQL statement is a query expression, a ResultSet is returned.

In SQL, when a query expression (SELECT or similar SQL statement) is executed, an ephemeral table is created. When this table is returned to the application program, it is returned as a result set, which is accessed row-by-row by a cursor. A JDBC ResultSet represents an SQL result set and its cursor.

The minimal definition of a cursor is a list of rows with a position that can move forward. Some cursors also allow the position to move backwards or jump to any position in the list.

An SQL cursor has several attributes. These attributes depend on the query expression. Some of these attributes can be overridden by specifying qualifiers in the SQL statement or by specifying values for the parameters of the JDBC Statement or PreparedStatement.

Columns and Rows

The columns of the rows of the result set are determined by the query expression. The number of columns and the type and name characteristics of each column are known when the query expression is compiled and before its execution. This metadata information remains constant regardless of changes to the contents of the tables used in the query expression. The metadata for the JDBC ResultSet is in the form of a ResultSetMetaData object. Various methods of the ResultSetMetaData interface return different properties of each column of the ResultSet.

A result set may contain 0 or more rows. The rows are determined by the execution of the query expression.

The setMaxRows(int) method of JDBC Statement allows limiting the number of rows returned by the statement. This limit is conceptually applied after the result has been built, and the excess rows are discarded.

Navigation

A cursor is either scrollable or not. Scrollable cursors allow accessing rows by absolute or relative positioning. No-scroll cursors only allow moving to the next row. The cursor can be optionally declared with the SQL qualifiers SCROLL, or NO SCROLL. The JDBC statement parameter can be specified as: TYPE_FORWARD_ONLY and TYPE_SCROLL_INSENSITIVE. The JDBC type TYPE_SCROLL_SENSITIVE is not supported by HSQLDB.

The default is NO SCROLL or TYPE_FORWARD_ONLY.

When a JDBC ResultSet is opened, it is positioned before the first row. Using the next() method the position is moved to the first row. While the ResultSet is positioned on a row, various getter methods can be used to access the columns of the row.

Updatability

The result returned by some query expressions is updatable. HSQLDB supports core SQL updatability features, plus some enhancements from the SQL optional features.

A query expression is updatable if it is a SELECT from a single underlying base table (or updatable view) either directly or indirectly. A SELECT statement featuring DISTINCT or GROUP BY or FETCH, LIMIT, OFFSET is not updatable. In an updatable query expression, one or more columns are updatable. An updatable column is a column that can be traced directly to the underlying table. Therefore, columns that contain expressions are not updatable. Examples of updatable query expressions are given below. The view V is updatable when its query expression is updatable. The SELECT statement from this view is also updatable:

 SELECT A, B FROM T WHERE C > 5
 SELECT A, B FROM (SELECT * FROM T WHERE C > 10) AS TT WHERE TT.B <10
 CREATE VIEW V(X,Y) AS SELECT A, B FROM T WHERE C > 0 AND B < 10
 SELECT X FROM V WHERE Y = 5

If a cursor is declared with the SQL qualifier, FOR UPDATE OF <column name list>, then only the stated columns in the result set become updatable. If any of the stated columns is not actually updatable, then the cursor declaration will not succeed.

If the SQL qualifier, FOR UPDATE is used, then all the updatable columns of the result set become updatable.

If a cursor is declared with FOR READ ONLY, then it is not updatable.

In HSQLDB, if FOR READ ONLY or FOR UPDATE is not used then all the updatable columns of the result set become updatable. This relaxes the SQL standard rule that in this case limits updatability to only simply updatable SELECT statements (where all columns are updatable).

In JDBC, CONCUR_READ_ONLY or CONCUR_UPDATABLE can be specified for the Statement parameter. CONCUR_UPDATABLE is required if the returning ResultSet is to be updatable. If CONCUR_READ_ONLY, which is the default, is used, then even an updatable ResultSet becomes read-only.

When a ResultSet is updatable, various setter methods can be used to modify the column values. The names of the setter methods begin with "update". After all the updates on a row are done, the updateRow() method must be called to finalise the row update.

An updatable ResultSet may or may not be insertable-into. In an insertable ResultSet, all columns of the result are updatable and any column of the base table that is not in the result must be a generated column or have a default value.

In the ResultSet object, a special pseudo-row, called the insert row, is used to populate values for insertion into the ResultSet (and consequently, into the base table). The setter methods must be used on all the columns, followed by a call to insertRow().

Individual rows from all updatable result sets can be deleted one at a time. The deleteRow() is called when the ResultSet is positioned on a row.

While using an updatable ResultSet to modify data, it is recommended not to change the same data using another ResultSet and not to execute SQL data change statements that modify the same data.

Sensitivity

The sensitivity of the cursor relates to visibility of changes made to the data by the same transaction but without using the given cursor. While the result set is open, the same transaction may use statements such as INSERT or UPDATE, and change the data of the tables from which the result set data is derived. A cursor is SENSITIVE if it reflects those changes. It is INSENSITIVE if it ignores such changes. It is ASENSITIVE if behaviour is implementation dependent.

The SQL default is ASENSITIVE, i.e., implantation dependent.

In HSQLDB all cursors are INSENSITIVE. They do not reflect changes to the data made by other statements.

Holdability

A cursor is holdable if the result set is not automatically closed when the current transaction is committed. Holdability can be specified in the cursor declaration using the SQL qualifiers WITH HOLD or WITHOUT HOLD.

In JDBC, holdability is specified using either of the following values for the Statement parameter: HOLD_CURSORS_OVER_COMMIT, or CLOSE_CURSORS_AT_COMMIT.

The SQL default is WITHOUT HOLD.

The JDBC default for HSQLDB result sets is WITH HOLD for read-only result sets and WITHOUT HOLD for updatable result sets.

If the holdability of a ResultSet is specified in a conflicting manner in the SQL statement and the JDBC Statement object, the JDBC setting takes precedence.

Autocommit

The autocommit property of a connection is a feature of JDBC and ODBC and is not part of the SQL Standard. In autocommit mode, all transactional statements are followed by an implicit commit. In autocommit mode, all ResultSet objects are read-only and holdable.

JDBC Overview

The JDBC settings, ResultSet.CONCUR_READONLY and ResultSet.CONCUR_UPDATABLE are the available alternatives for read-only or updatability. The default is ResultSet.CONCUR_READONLY.

The JDBC settings, ResultSet.TYPE_FORWARD_ONLY, ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.TYPE_SCROLL_SENSITIVE are the available alternatives for both scrollability (navigation) and sensitivity. HyperSQL does not support ResultSet.TYPE_SCROLL_SENSITIVE. The two other alternatives can be used for both updatable and read-only result sets.

The JDBC settings ResultSet.CLOSE_CURSORS_AT_COMMIT and ResultSet.HOLD_CURSORS_OVER_COMMIT are the alternatives for the lifetime of the result set. The default is ResultSet.CLOSE_CURSORS_AT_COMMIT. The other setting can only be used for read-only result sets.

Examples of creating statements for updatable result sets are given below:

 Connection c = newConnection();
 Statement st;
 c.setAutoCommit(false);
 st = c.createStatement(ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_UPDATABLE);
 st = c.createStatement(ResultSet.TYPE_SCROLL_INSENSITIVE, ResultSet.CONCUR_UPDATABLE);

JDBC Parameters

When a JDBC PreparedStatement or CallableStatement is used with an SQL statement that contains dynamic parameters, the data types of the parameters are resolved and determined by the engine when the statement is prepared. The SQL Standard has detailed rules to determine the data types and imposes limits on the maximum length or precision of the parameter. HyperSQL applies the standard rules with two exceptions for parameters with String and BigDecimal Java types. HyperSQL ignores the limits when the parameter value is set, and only enforces the necessary limits when the PreparedStatement is executed. In all other cases, parameter type limits are checked and enforce when the parameter is set.

In the example below the setString() calls do not raise an exception, but one of the execute() statements does.

 // table definition: CREATE TABLE T (NAME VARCHAR(12), ...)
 Connection c = newConnection();
 PreparedStatement st = c.prepareStatement("SELECT * FROM T WHERE NAME = ?");
 // type of the parameter is VARCHAR(12), which limits length to 12 characters
 st.setString(1, "Eyjafjallajokull"); // string is longer than type, but no exception is raised here
 set.execute(); // executes with no exception and does not find any rows

 // but if an UPDATE is attempted, an exception is raised
 st = c.prepareStatement("UPDATE T SET NAME = ? WHERE ID = 10");
 st.setString(1, "Eyjafjallajokull"); // string is longer than type, but no exception is raised here
 st.execute(); // exception is thrown when HyperSQL checks the value for update

All of the above also applies to setting the values in new and updated rows in updatable ResultSet objects.

JDBC parameters can be set with any compatible type, as supported by the JDBC specification. For CLOB and BLOB types, you can use streams, or create instances of BLOB or CLOB before assigning them to the parameters. You can even use CLOB or BLOB objects returned from connections to other RDBMS servers. The Connection.createBlob() and createClob() methods can be used to create the new LOBs. For very large LOB's the stream methods are preferable as they use less memory.

For array parameters, you must use a java.sql.Array object that contains the array elements before assigning to JDBC parameters. The Connection.createArrayOf(...) method can be used to create a new object, or you can use an Array returned from connections to other RDBMS servers.

JDBC and Data Change Statements

Data change statements, also called data manipulation statements (DML) such as INSERT, UPDATE, MERGE can be called with different executeUpdate() methods of java.sql.Statement and java.sql.PreparedStatement. Some of these methods allow you to specify how values for generated columns of the table are returned. These methods are documented in the JavaDoc for org.hsqldb.jdbc.JDBCStatement and org.hsqldb.jdbc.JDBCPreparedStatement. HyperSQL can return not just the generated columns, but any set of columns of the table. You can use this to retrieve the columns values that may be modified by a BEFORE TRIGGER on the table.

JDBC Callable Statement

The JDBC CallableStatement interface is used to call Java or SQL procedures that have been defined in the database. The SQL statement in the form of CALL procedureName ( ... ) with constant value arguments or with parameter markers. Note that you must use a parameter marker for OUT and INOUT arguments of the procedure you are calling. The OUT arguments should not be set before executing the callable statement.

After executing the statement, you can retrieve the OUT and INOUT parameters with the appropriate getXXX() method.

Procedures can also return one or more result sets. You should call the getResultSet() and getMoreResults() methods to retrieve the result sets one by one.

SQL functions can also return a table. You can call such functions the same way as procedures and retrieve the table as a ResultSet.

JDBC Returned Values

The methods of the JDBC ResultSet interface can be used to return values and to convert value to different types as supported by the JDBC specification.

When a CLOB and BLOB object is returned from a ResultSet, no data is transferred until the data is read by various methods of java.sql.CLOB and java.sql.BLOB. Data is streamed in large blocks to avoid excessive memory use.

Array objects are returned as instances of java.sql.Array.

Cursor Declaration

The DECLARE CURSOR statement is used within an SQL PROCEDURE body. In the early releases of HyperSQL 2.0, the cursor is used only to return a result set from the procedure. Therefore the cursor must be declared WITH RETURN and can only be READ ONLY.

DECLARE CURSOR

declare cursor statement

<declare cursor> ::= DECLARE <cursor name>

[ { SENSITIVE | INSENSITIVE | ASENSITIVE } ] [ { SCROLL | NO SCROLL } ]

CURSOR [ { WITH HOLD | WITHOUT HOLD } ] [ { WITH RETURN | WITHOUT RETURN } ]

FOR <query expression>

[ FOR { READ ONLY | UPDATE [ OF <column name list> ] } ]

The query expression is a SELECT statement or similar, and is discussed in the rest of this chapter. In the example below a cursor is declared for a SELECT statement. It is later opened to create the result set. The cursor is specified WITHOUT HOLD, so the result set is not kept after a commit. Use WITH HOLD to keep the result set. Note that you need to declare the cursor WITH RETURN as it is returned by the CallableStatement.

 DECLARE thiscursor SCROLL CURSOR WITHOUT HOLD WITH RETURN FOR SELECT * FROM INFORMATION_SCHEMA.TABLES;
 --
 OPEN thiscursor;

Syntax Elements

The syntax elements that can be used in data access and data change statements are described in this section. The SQL Standard has a very extensive set of definitions for these elements. The BNF definitions given here are sometimes simplified.

Literals

Literals are used to express constant values. The general type of a literal is known by its format. The specific type is based on conventions.

unicode escape elements

unicode escape elements

<Unicode escape specifier> ::= [ UESCAPE <quote><Unicode escape character><quote> ]

<Unicode escape value> ::= <Unicode 4 digit escape value> | <Unicode 6 digit escape value> | <Unicode character escape value>

<Unicode 4 digit escape value> ::= <Unicode escape character><hexit><hexit><hexit><hexit>

<Unicode 6 digit escape value> ::= <Unicode escape character><plus sign> <hexit><hexit><hexit><hexit><hexit><hexit>

<Unicode character escape value> ::= <Unicode escape character><Unicode escape character>

<Unicode escape character> ::= a single character than a <hexit> (a-f, A-F, 0-9), <plus sign>, <quote>, <double quote>, or <white space>

character literal

character literal

<character string literal> ::= [ <introducer><character set specification> ] <quote> [ <character representation>... ] <quote> [ { <separator> <quote> [ <character representation>... ] <quote> }... ]

<introducer> ::= <underscore>

<character representation> ::= <nonquote character> | <quote symbol>

<nonquote character> ::= any character apart from the quote symbol.

<quote symbol> ::= <quote><quote>

<national character string literal> ::= N <quote> [ <character representation>... ] <quote> [ { <separator> <quote> [ <character representation>... ] <quote> }... ]

<Unicode character string literal> ::= [ <introducer><character set specification> ] U<ampersand><quote> [ <Unicode representation>... ] <quote> [ { <separator> <quote> [ <Unicode representation>... ] <quote> }... ] <Unicode escape specifier>

<Unicode representation> ::= <character representation> | <Unicode escape value>

The type of a character literal is CHARACTER. The length of the string literal is the character length of the type. If the quote character is used in a string, it is represented with two quote characters. Long literals can be divided into multiple quoted strings, separated with a space or end-of-line character.

Unicode literals start with U& and can contain ordinary characters and unicode escapes. A unicode escape begins with the backslash ( \ ) character and is followed by four hexadecimal characters which specify the character code.

Example of character literals are given below:

 'a literal'  ' string seperated'  ' into parts'
 'a string''s literal form with quote character'
 U&'Unicode string with Greek delta \0394 and phi \03a6 letters'

binary literal

binary literal

<binary string literal> ::= X <quote> [ <space>... ] [ { <hexit> [ <space>... ] <hexit> [ <space>... ] }... ] <quote> [ { <separator> <quote> [ <space>... ] [ { <hexit> [ <space>... ] <hexit> [ <space>... ] }... ] <quote> }... ]

<hexit> ::= <digit> | A | B | C | D | E | F | a | b | c | d | e | f

The type of a binary literal is BINARY. The octet length of the binary literal is the length of the type. Case-insensitive hexadecimal characters are used in the binary string. Each pair of characters in the literal represents a byte in the binary string. Long literals can be divided into multiple quoted strings, separated with a space or end-of-line character.

 X'1abACD34' 'Af'

bit literal

bit literal

<bit string literal> ::= B <quote> [ <bit> ... ] <quote> [ { <separator> <quote> [ <bit>... ] <quote> }... ]

<bit> ::= 0 | 1

The type of a binary literal is BIT. The bit length of the bit literal is the length of the type. Digits 0 and 1 are used to represent the bits. Long literals can be divided into multiple quoted strings, separated with a space or end-of-line character.

 B'10001001' '00010'

numeric literal

numeric literal

<signed numeric literal> ::= [ <sign> ] <unsigned numeric literal>

<unsigned numeric literal> ::= <exact numeric literal> | <approximate numeric literal>

<exact numeric literal> ::= <unsigned integer> [ <period> [ <unsigned integer> ] ] | <period> <unsigned integer>

<sign> ::= <plus sign> | <minus sign>

<approximate numeric literal> ::= <mantissa> E <exponent>

<mantissa> ::= <exact numeric literal>

<exponent> ::= <signed integer>

<signed integer> ::= [ <sign> ] <unsigned integer>

<unsigned integer> ::= <digit>...

The type of an exact numeric literal without a decimal point is INTEGER, BIGINT, or DECIMAL, depending on the value of the literal (the smallest type that can represent the value is the type).

The type of an exact numeric literal with a decimal point is DECIMAL. The precision of a decimal literal is the total number of digits of the literal. The scale of the literal is the total number of digits to the right of the decimal point.

The type of an approximate numeric literal is DOUBLE. An approximate numeric literal always includes the mantissa and exponent, separated by E.

 12
 34.35
 +12E-2

boolean literal

boolean literal

<boolean literal> ::= TRUE | FALSE | UNKNOWN

The boolean literal is one of the specified keywords.

datetime and interval literal

datetime and interval literal

<datetime literal> ::= <date literal> | <time literal> | <timestamp literal>

<date literal> ::= DATE <date string>

<time literal> ::= TIME <time string>

<timestamp literal> ::= TIMESTAMP <timestamp string>

<date string> ::= <quote> <unquoted date string> <quote>

<time string> ::= <quote> <unquoted time string> <quote>

<timestamp string> ::= <quote> <unquoted timestamp string> <quote>

<time zone interval> ::= <sign> <hours value> <colon> <minutes value>

<date value> ::= <years value> <minus sign> <months value> <minus sign> <days value>

<time value> ::= <hours value> <colon> <minutes value> <colon> <seconds value>

<interval literal> ::= INTERVAL [ <sign> ] <interval string> <interval qualifier>

<interval string> ::= <quote> <unquoted interval string> <quote>

<unquoted date string> ::= <date value>

<unquoted time string> ::= <time value> [ <time zone interval> ]

<unquoted timestamp string> ::= <unquoted date string> <space> <unquoted time string>

<unquoted interval string> ::= [ <sign> ] { <year-month literal> | <day-time literal> }

<year-month literal> ::= <years value> [ <minus sign> <months value> ] | <months value>

<day-time literal> ::= <day-time interval> | <time interval>

<day-time interval> ::= <days value> [ <space> <hours value> [ <colon> <minutes value> [ <colon> <seconds value> ] ] ]

<time interval> ::= <hours value> [ <colon> <minutes value> [ <colon> <seconds value> ] ] | <minutes value> [ <colon> <seconds value> ] | <seconds value>

<years value> ::= <datetime value>

<months value> ::= <datetime value>

<days value> ::= <datetime value>

<hours value> ::= <datetime value>

<minutes value> ::= <datetime value>

<seconds value> ::= <seconds integer value> [ <period> [ <seconds fraction> ] ]

<seconds integer value> ::= <unsigned integer>

<seconds fraction> ::= <unsigned integer>

<datetime value> ::= <unsigned integer>

The type of a datetime or interval type is specified in the literal. The fractional second precision is the number of digits in the fractional part of the literal. Details are described in the SQL Language chapter

 DATE '2008-08-08'
 TIME '20:08:08'
 TIMESTAMP '2008-08-08 20:08:08.235'

 INTERVAL '10' DAY
 INTERVAL -'08:08' MINUTE TO SECOND

References, etc.

References are identifier chains, which can be a single identifiers or identifiers chains composed of single identifiers chained together with the period symbol.

identifier chain

identifier chain

<identifier chain> ::= <identifier> [ { <period> <identifier> }... ]

<basic identifier chain> ::= <identifier chain>

A period-separated chain of identifiers. The identifiers in an identifier chain can refer to database objects in a hierarchy. The possible hierarchies are as follows. In each hierarchy, elements from the start or the end can be missing, but the order of elements cannot be changed.

catalog, schema, database object

catalog, schema, table, column

correlation name, column

Examples of identifier chain are given below:

 SELECT MYCAT.MYSCHEMA.MYTABLE.MYCOL FROM MYCAT.MYSCHEMA.MYTABLE
 DROP TABLE MYCAT.MYSCHEMA.MYTABLE CASCADE
 ALTER SEQUENCE MYCAT.MYSCHEMA.MYSEQUENCE RESTART WITH 100

column reference

column reference

<column reference> ::= <basic identifier chain> | MODULE <period> <qualified identifier> <period> <column name>

Reference a column or a routine variable.

SQL parameter reference

SQL parameter reference

<SQL parameter reference> ::= <basic identifier chain>

Reference an SQL routine parameter.

contextually typed value specification

contextually typed value specification

<contextually typed value specification> ::= <null specification> | <default specification>

<null specification> ::= NULL

<default specification> ::= DEFAULT

Specify a value whose data type or value is inferred from its context.

DEFAULT is used for assignments to table columns that have a default value, or to table columns that are generated either as an IDENTITY value or as an expression.

NULL can be used only in a context where the type of the value is known. For example, a NULL can be assigned to a column of the table in an INSERT or UPDATE statement, because the type of the column is known. But if NULL is used in a SELECT list, it must be used in a CAST statement.

Value Expression

Value expression is a general name for all expressions that return a value. Different types of expressions are allowed in different contexts.

value expression primary

value expression primary

<value expression primary> ::= <parenthesized value expression> | <nonparenthesized value expression primary>

<parenthesized value expression> ::= <left paren> <value expression> <right paren>

<nonparenthesized value expression primary> ::= <unsigned value specification> | <column reference> | <set function specification> | <scalar subquery> | <case expression> | <cast specification> | <next value expression> | <current value expression> | <routine invocation>

Specify a value that is syntactically self-delimited.

value specification

value specification

<value specification> ::= <literal> | <general value specification>

<unsigned value specification> ::= <unsigned literal> | <general value specification>

<target specification> ::= <host parameter specification> | <SQL parameter reference> | <column reference> | <dynamic parameter specification>

<simple target specification> ::= <host parameter specification> | <SQL parameter reference> | <column reference> | <embedded variable name>

<host parameter specification> ::= <host parameter name> [ <indicator parameter> ]

<dynamic parameter specification> ::= <question mark>

Specify one or more values, host parameters, SQL parameters, dynamic parameters, or host variables.

row value expression

row value expression

<row value expression> ::= <row value special case> | <explicit row value constructor>

<row value predicand> ::= <row value special case> | <row value constructor predicand>

<row value special case> ::= <nonparenthesized value expression primary>

<explicit row value constructor> ::= <left paren> <row value constructor element> <comma> <row value constructor element list> <right paren> |

ROW <left paren> <row value constructor element list> <right paren> | <row subquery>

Specify a row consisting of one or more elements. A comma separated list of expressions, enclosed in brackets, with the optional keyword ROW. In SQL, a row containing a single element can often be used where a single value is expected.

set function specification

set function specification

<set function specification> ::= <aggregate function> | <grouping operation>

<grouping operation> ::= GROUPING <left paren> <column reference> [ { <comma> <column reference> }... ] <right paren>

Specify a value derived by the application of a function to an argument. HyperSQL does not yet support <grouping operation> .

COALESCE

coalesce expression

<coalesce expression> := COALESCE <left paren> <value expression> { <comma> <value expression> }... <right paren>

Replace null values with another value. The coalesce expression has two or more instances of <value expression>. If the first <value expression> evaluates to a non-null value, it is returned as the result of the coalesce expression. If it is null, the next <value expression> is evaluated and if it evaluates to a non-non value, it is returned, and so on.

The type of the return value of a COALESCE expression is the aggregate type of the types of all the <value expression> instances. Therefore, any value returned is implicitly cast to this type. HyperSQL also features built-in functions with similar functionality.

NULLIF

nullif expression

<nullif expression> := NULLIF <left paren> <value expression> <comma> <value expression> <right paren>

Return NULL if two values are equal. If the result of the first <value expression> is not equal to the result of the second, then it is returned, otherwise NULL is returned. The type of the return value is the type of the first <value expression>.

 SELECT i, NULLIF(n, 'not defined') FROM t

CASE

case specification

<case specification> ::= <simple case> | <searched case>

<simple case> ::= CASE <case operand> <simple when clause>... [ <else clause> ] END

<searched case> ::= CASE <searched when clause>... [ <else clause> ] END

<simple when clause> ::= WHEN <when operand list> THEN <result>

<searched when clause> ::= WHEN <search condition> THEN <result>

<else clause> ::= ELSE <result>

<case operand> ::= <row value predicand> | <overlaps predicate part 1>

<when operand list> ::= <when operand> [ { <comma> <when operand> }... ]

<when operand> ::= <row value predicand> | <comparison predicate part 2> | <between predicate part 2> | <in predicate part 2> | <character like predicate part 2> | <octet like predicate part 2> | <similar predicate part 2> | <regex like predicate part 2> | <null predicate part 2> | <quantified comparison predicate part 2> | <match predicate part 2> | <overlaps predicate part 2> | <distinct predicate part 2>

<result> ::= <result expression> | NULL

<result expression> ::= <value expression>

Specify a conditional value. The result of a case expression is always a value. All the values introduced with THEN must be of the same type.

Some simple examples of the CASE expression are given below. The first two examples return 'Britain', 'Germany', or 'Other country' depending on the value of dialcode. The third example uses IN and smaller-than predicates.

 CASE dialcode WHEN 44 THEN 'Britain' WHEN 49 THEN 'Germany' ELSE 'Other country' END
 CASE WHEN dialcode=44 THEN 'Britain' WHEN dialcode=49 THEN 'Germany' WHEN dialcode < 0 THEN 'bad dial code' ELSE 'Other country' END
 CASE dialcode WHEN IN (44, 49,30) THEN 'Europe' WHEN IN (86,91,91) THEN 'Asia' WHEN < 0 THEN 'bad dial code' ELSE 'Other continent' END

The case statement can be far more complex and involve several conditions.

CAST

cast specification

<cast specification> ::= CAST <left paren> <cast operand> AS <cast target> <right paren>

<cast operand> ::= <value expression> | <implicitly typed value specification>

<cast target> ::= <domain name> | <data type>

Specify a data conversion. Data conversion takes place automatically among variants of a general type. For example numeric values are freely converted from one type to another in expressions.

Explicit type conversion is necessary in two cases. One case is to determine the type of a NULL value. The other case is to force conversion for special purposes. Values of data types can be cast to a character type. The exception is BINARY and OTHER types. The result of the cast is the literal expression of the value. Conversely, a value of a character type can be converted to another type if the character value is a literal representation of the value in the target type. Special conversions are possible between numeric and interval types, which are described in the section covering interval types.

The examples below show examples of cast with their result:

 CAST (NULL AS TIMESTAMP)
 CAST ('   199  ' AS INTEGER) = 199
 CAST ('tRue ' AS BOOLEAN) = TRUE
 CAST (INTERVAL '2' DAY AS INTEGER) = 2
 CAST ('1992-04-21' AS DATE) = DATE '1992-04-21'

NEXT VALUE FOR

next value expression

<next value expression> ::= NEXT VALUE FOR <sequence generator name>

Return the next value of a sequence generator. This expression can be used as a select list element in queries, or in assignments to table columns in data change statements. If the expression is used more than once in a single row that is being evaluated, the same value is returned for each invocation. After evaluation of the particular row is complete, the sequence generator will return a different value from the old value. The new value is generated by the sequence generator by adding the increment to the last value it generated. In the example below the expression is used in an insert statement:

INSERT INTO MYTABLE(COL1, COL2) VALUES 2, NEXT VALUE FOR MYSEQUENCE

CURRENT VALUE FOR

current value expression

<current value expression> ::= CURRENT VALUE FOR <sequence generator name>

Return the latest value that was returned by the NEXT VALUE FOR expression for a sequence generator. In the example below, the value that was generated by the sequence for the first insert, is reused for the second insert:

 INSERT INTO MYTABLE(COL1, COL2) VALUES 2, NEXT VALUE FOR MYSEQUENCE;
 INSERT INTO CHILDTABLE(COL1, COL2) VALUES 10, CURRENT VALUE FOR MYSEQUENCE;

value expression

value expression

<value expression> ::= <numeric value expression> | <string value expression> | <datetime value expression> | <interval value expression> | <boolean value expression> | <row value expression>

An expression that returns a value. The value can be a single value, or a row consisting more than one value.

numeric value expression

numeric value expression

<numeric value expression> ::= <term> | <numeric value expression> <plus sign> <term> | <numeric value expression> <minus sign> <term>

<term> ::= <factor> | <term> <asterisk> <factor> | <term> <solidus> <factor>

<factor> ::= [ <sign> ] <numeric primary>

<numeric primary> ::= <value expression primary> | <numeric value function>

Specify a numeric value. The BNF indicates that <asterisk> and <solidus> (the operators for multiplication and division) have precedence over <minus sign> and <plus sign>.

numeric value function

numeric value function

<numeric value function> ::= <position expression> | <extract expression> | <length expression> ...

Specify a function yielding a value of type numeric. The supported numeric value functions are listed and described in the Built In Functions chapter.

string value expression

string value expression

<string value expression> ::= <string concatenation> | <string factor>

<string factor> ::= <value expression primary> | <string value function>

<string concatenation> ::= <string value expression> <concatenation operator> <string factor>

<concatenation operator> ::= ||

Specify a character string value, a binary string value, or a bit string value. The BNF indicates that a string value expression can be formed by concatenation of two or more <value expression primary>. The types of the <value expression primary> elements must be compatible, that is, all must be string, or binary or bit string values.

character value function

string value function

<string value function> ::= ...

Specify a function that returns a character string or binary string. The supported character value functions are listed and described in the Built In Functions chapter.

datetime value expression

datetime value expression

<datetime value expression> ::= <datetime term> | <interval value expression> <plus sign> <datetime term> | <datetime value expression> <plus sign> <interval term> | <datetime value expression> <minus sign> <interval term>

<datetime term> ::= <datetime factor>

<datetime factor> ::= <datetime primary> [ <time zone> ]

<datetime primary> ::= <value expression primary> | <datetime value function>

<time zone> ::= AT <time zone specifier>

<time zone specifier> ::= LOCAL | TIME ZONE <interval primary>

Specify a datetime value. Details are described in the SQL Language chapter.

datetime value function

datetime value function

<datetime value function> ::= ...

Specify a function that returns a datetime value. The supported datetime value functions are listed and described in the Built In Functions chapter.

interval term

interval value expression

<interval value expression> ::= <interval term> | <interval value expression 1> <plus sign> <interval term 1> | <interval value expression 1> <minus sign> <interval term 1> | <left paren> <datetime value expression> <minus sign> <datetime term> <right paren> <interval qualifier>

<interval term> ::= <interval factor> | <interval term 2> <asterisk> <factor> | <interval term 2> <solidus> <factor> | <term> <asterisk> <interval factor>

<interval factor> ::= [ <sign> ] <interval primary>

<interval primary> ::= <value expression primary> [ <interval qualifier> ] | <interval value function>

<interval value expression 1> ::= <interval value expression>

<interval term 1> ::= <interval term>

<interval term 2> ::= <interval term>

Specify an interval value. Details are described in the SQL Language chapter.

interval absolute value function

interval value function

<interval value function> ::= <interval absolute value function>

<interval absolute value function> ::= ABS <left paren> <interval value expression> <right paren>

Specify a function that returns the absolute value of an interval. If the interval is negative, it is negated, otherwise the original value is returned.

boolean value expression

boolean value expression

<boolean value expression> ::= <boolean term> | <boolean value expression> OR <boolean term>

<boolean term> ::= <boolean factor> | <boolean term> AND <boolean factor>

<boolean factor> ::= [ NOT ] <boolean test>

<boolean test> ::= <boolean primary> [ IS [ NOT ] <truth value> ]

<truth value> ::= TRUE | FALSE | UNKNOWN

<boolean primary> ::= <predicate> | <boolean predicand>

<boolean predicand> ::= <parenthesized boolean value expression> | <nonparenthesized value expression primary>

<parenthesized boolean value expression> ::= <left paren> <boolean value expression> <right paren>

Specify a boolean value.

Predicates

Predicates are conditions with two sides and evaluate to a boolean value. The left side of the predicate, the <row value predicand>, is the common element of all predicates. This element is a generalisation of both <value expression>, which is a scalar, and of <explicit row value constructor>, which is a row. The two sides of a predicate can be split in CASE statements where the <row value predicand> is part of multiple predicates.

The number of fields in all <row value predicand> used in predicates must be the same and the types of the fields in the same position must be compatible for comparison. If either of these conditions does not hold, an exception is raised. The number of fields in a row is called the degree.

In many types of predicates (but not all of them), if the <row value predicand> evaluates to NULL, the result of the predicate is UNKNOWN. If the <row value predicand> has more than one element, and one or more of the fields evaluate to NULL, the result depends on the particular predicate.

comparison predicand

comparison predicate

<comparison predicate> ::= <row value predicand> <comp op> <row value predicand>

<comp op> ::= <equals operator> | <not equals operator> | <less than operator> | <greater than operator> | <less than or equals operator> | <greater than or equals operator>

Specify a comparison of two row values. If either <row value predicand> evaluates to NULL, the result of <comparison predicate> is UNKNOWN. Otherwise, the result is TRUE, FALSE or UNKNOWN.

If the degree of <row value predicand> is larger than one, comparison is performed between each field and the corresponding field in the other <row value predicand> from left to right, one by one.

When comparing two elements, if either field is NULL then the result is UNKNOWN.

For <equals operator>, if the result of comparison is TRUE for all field, the result of the predicate is TRUE. If the result of comparison is FALSE for one field, the result of predicate is FALSE. Otherwise the result is UNKNOWN.

The <not equals operator> is translated to NOT (<row value predicand> = <row value predicand>).

The <less than or equals operator> is translated to (<row value predicand> = <row value predicand>) OR (<row value predicand> < <row value predicand>). The <greater than or equals operator> is translated similarly.

For the <less than operator> and <greater than operator>, if two fields at a given position are equal, then comparison continues to the next field. Otherwise, the result of the last performed comparison is returned as the result of the predicate. This means that if the first field is NULL, the result is always UNKNOWN.

The logic that governs NULL values and UNKNOWN result is as follows: Suppose the NULL values were substituted by arbitrary real values. If substitution cannot change the result of the predicate, then the result is TRUE or FALSE, based on the existing non-NULL values, otherwise the result of the predicate is UNKNOWN.

The examples of comparison given below use literals, but the literals actually represent the result of evaluation of some expression.

 ((1, 2, 3, 4) = (1, 2, 3, 4)) IS TRUE
 ((1, 2, 3, 4) = (1, 2, 3, 5)) IS FALSE
 ((1, 2, 3, 4) < (1, 2, 3, 4)) IS FALSE
 ((1, 2, 3, 4) < (1, 2, 3, 5)) IS TRUE
 ((NULL, 1, NULL) = (NULL, 1, NULL)) IS UNKNOWN  
 ((NULL, 1, NULL) = (NULL, 2, NULL)) IS FALSE  
 ((NULL, 1, NULL) <> (NULL, 2, NULL)) IS TRUE  
 ((NULL, 1, 2) <all operators> (NULL, 1, 2)) IS UNKNOWN
 ((1, NULL, ...) < (1, 2, ...)) IS UNKNOWN  
 ((1, NULL, ...) < (2, NULL, ...)) IS TRUE
 ((2, NULL, ...) < (1, NULL, ...)) IS FALSE

BETWEEN

between predicate

<between predicate> ::= <row value predicand> <between predicate part 2>

<between predicate part 2> ::= [ NOT ] BETWEEN [ ASYMMETRIC | SYMMETRIC ] <row value predicand> AND <row value predicand>

Specify a range comparison. The default is ASYMMETRIC. The expression X BETWEEN Y AND Z is equivalent to (X >= Y AND X <= Z). Therefore if Y > Z, the BETWEEN expression is never true. The expression X BETWEEN SYMMETRIC Y AND Z is equivalent to (X >= Y AND X <= Z) OR (X >= Z AND X <= Y). The expression Z NOT BETWEEN ... is equivalent to NOT (Z BETWEEN ...). If any of the three <row value predicand> evaluates to NULL, the result is UNKNOWN.

IN

in predicate

<in predicate> ::= <row value predicand> [ NOT ] IN <in predicate value>

<in predicate value> ::= <table subquery> | <left paren> <in value list> <right paren>

| <left paren> UNNEST <left paren> <array value expression> <right paren> <right paren>

<in value list> ::= <row value expression> [ { <comma> <row value expression> }... ]

Specify a quantified comparison. The expression X NOT IN Y is equivalent to NOT (X IN Y). The ( <in value list> ) is converted into a table with one or more rows. The expression X IN Y is equivalent to X = ANY Y, which is a <quantified comparison predicate>.

If the <table subquery> returns no rows, the result is FALSE. Otherwise the <row value predicand> is compared one by one with each row of the <table subquery>.

If the comparison is TRUE for at least one row, the result is TRUE. If the comparison is FALSE for all rows, the result is FALSE. Otherwise the result is UNKNOWN.

HyperSQL supports an extension to the SQL Standard to allow an array to be used in the <in predicate value>. This is intended to be used with prepared statements where a variable length array of values can be used as the parameter value for each call. The example below shows how this is used in SQL. The JDBC code must create a new java.sql.Array object that contains the values and set the parameter with this array.

 SELECT * FROM customer WHERE firstname IN ( UNNEST(?) )

 Connection conn;
 PreparedStatement ps;
 // conn and ps are instantiated here
 Array arr = conn.createArrayOf("INTEGER", new Integer[] {1, 2, 3});
 ps.setArray(1, arr);
 ResultSet rs = ps.executeQuery();

LIKE

like predicate

<like predicate> ::= <character like predicate> | <octet like predicate>

<character like predicate> ::= <row value predicand> [ NOT ] LIKE <character pattern> [ ESCAPE <escape character> ]

<character pattern> ::= <character value expression>

<escape character> ::= <character value expression>

<octet like predicate> ::= <row value predicand> [ NOT ] LIKE <octet pattern> [ ESCAPE <escape octet> ]

<octet pattern> ::= <binary value expression>

<escape octet> ::= <binary value expression>

Specify a pattern-match comparison for character or binary strings. The <row value predicand> is always a <string value expression> of character or binary type. The <character pattern> or <octet pattern> is a <string value expression> in which the underscore and percent characters have special meanings. The underscore means match any one character, while the percent means match a sequence of zero or more characters. The <escape character> or <escape octet> is also a <string value expression> that evaluates to a string of exactly one character length. If the underscore or the percent is required as normal characters in the pattern, the specified <escape character> or <escape octet> can be used in the pattern before the underscore or the percent. The <row value predicand> is compared with the <character pattern> and the result of comparison is returned. If any of the expressions in the predicate evaluates to NULL, the result of the predicate is UNKNOWN. The expression A NOT LIKE B is equivalent to NOT (A LIKE B). If the length of the escape is not 1 or it is used in the pattern not immediately before an underscore or a percent character, an exception is raised.

IS NULL

null predicate

<null predicate> ::= <row value predicand> IS [ NOT ] NULL

Specify a test for a null value. The expression X IS NOT NULL is NOT equivalent to NOT (X IS NULL)if the degree of the <row value predicand> is larger than 1. The rules are: If all fields are null, X IS NULL is TRUE and X IS NOT NULL is FALSE. If only some fields are null, both X IS NULL and X IS NOT NULL are FALSE. If all fields are not null, X IS NULL is FALSE and X IS NOT NULL is TRUE.

ALL and ANY

quantified comparison predicate

<quantified comparison predicate> ::= <row value predicand> <comp op> <quantifier> <table subquery>

<quantifier> ::= <all> | <some>

<all> ::= ALL

<some> ::= SOME | ANY

Specify a quantified comparison. For a quantified comparison, the <row value predicand> is compared one by one with each row of the <table sub query>.

If the <table subquery> returns no rows, then if ALL is specified the result is TRUE, but if SOME or ANY is specified the result is FALSE.

If ALL is specified, if the comparison is TRUE for all rows, the result of the predicate is TRUE. If the comparison is FALSE for at least one row, the result is FALSE. Otherwise the result is UNKNOWN.

If SOME or ANY is specified, if the comparison is TRUE for at least one row, the result is TRUE. If the comparison is FALSE for all rows, the result is FALSE. Otherwise the result is UNKNOWN. Note that the IN predicate is equivalent to the SOME or ANY predicate using the <equals operator>.

In the examples below, the date of an invoice is compared to holidays in a given year. In the first example the invoice date must equal one of the holidays, in the second example it must be later than all holidays (later than the last holiday), in the third example it must be on or after some holiday (on or after the first holiday), and in the fourth example, it must be before all holidays (before the first holiday).

 invoice_date = SOME (SELECT holiday_date FROM holidays)
 invoice_date > ALL (SELECT holiday_date FROM holidays)
 invoice_date >= ANY (SELECT holiday_date FROM holidays)
 invoice_date < ALL (SELECT holiday_date FROM holidays)

EXISTS

exists predicate

<exists predicate> ::= EXISTS <table subquery>

Specify a test for a non-empty set. If the evaluation of <table subquery> results in one or more rows, then the expression is TRUE, otherwise FALSE.

UNIQUE

unique predicate

<unique predicate> ::= UNIQUE <table subquery>

Specify a test for the absence of duplicate rows. The result of the test is either TRUE or FALSE (never UNKNOWN). The rows of the <table subquery> that contain one or more NULL values are not considered for this test. If the rest of the rows are distinct from each other, the result of the test is TRUE, otherwise it is FALSE. The distinctness of rows X and Y is tested with the predicate X IS DISTINCT FROM Y.

MATCH

match predicate

<match predicate> ::= <row value predicand> MATCH [ UNIQUE ] [ SIMPLE | PARTIAL | FULL ] <table subquery>

Specify a test for matching rows. The default is MATCH SIMPLE without UNIQUE. The result of the test is either TRUE or FALSE (never UNKNOWN).

The interpretation of NULL values is different from other predicates and quite counter-intuitive. If the <row value predicand> is NULL, or all of its fields are NULL, the result is TRUE.

Otherwise, the <row value predicand> is compared with each row of the <table subquery>.

If SIMPLE is specified, if some field of <row value predicate> is NULL, the result is TRUE. Otherwise if <row value predicate> is equal to one or more rows of <table subquery> the result is TRUE if UNIQUE is not specified, or if UNIQUE is specified and only one row matches. Otherwise the result is FALSE.

If PARTIAL is specified, if the non-null values <row value predicate> are equal to those in one or more rows of <table subquery> the result is TRUE if UNIQUE is not specified, or if UNIQUE is specified and only one row matches. Otherwise the result is FALSE.

If FULL is specified, if some field of <row value predicate> is NULL, the result is FALSE. Otherwise if <row value predicate> is equal to one or more rows of <table subquery> the result is TRUE if UNIQUE is not specified, or if UNIQUE is specified and only one row matches.

Note that MATCH can also used be used in FOREIGN KEY constraint definitions. The exact meaning is described in the Schemas and Database Objects chapter.

OVERLAPS

overlaps predicate

<overlaps predicate> ::= <row value predicand> OVERLAPS <row value predicand>

Specify a test for an overlap between two datetime periods. Each <row value predicand> must have two fields and the fields together represent a datetime period. So the predicates is always in the form (X1, X2) OVERLAPS (Y1, Y2). The first field is always a datetime value, while the second field is either a datetime value or an interval value.

If the second value is an interval value, it is replaced with the sum of the datetime value and itself, for example (X1, X1 + X2) OVERLAPS (Y1, Y1 + Y 2).

If any of the values is NULL, the result is UNKNOWN.

The expression is true if there is there is any overlap between the two datetime periods. In the example below, the period is compared with a week long period ending yesterday.

 (startdate, enddate) OVERLAPS (CURRENT_DATE - 7 DAY, CURRENT_DATE - 1 DAY)

IS DISTINCT

is distinct predicate

<distinct predicate> ::= <row value predicand> IS [ NOT ] DISTINCT FROM <row value predicand>

Specify a test of whether two row values are distinct. The result of the test is either TRUE or FALSE (never UNKNOWN). The degree the two <row value predicand> must be the same. Each field of the first <row value predicand> is compared to the field of the second <row value predicand> at the same position. If one field is NULL and the other is not NULL, or if the elements are NOT equal, then the result of the expression is TRUE. If no comparison result is TRUE, then the result of the predicate is FALSE. The expression X IS NOT DISTINCT FROM Y is equivalent to NOT (X IS DISTINCT FORM Y). The following check returns true if startdate is not equal to enddate. It also returns true if either startdate or enddate is NULL. It returns false in other cases.

 startdate IS DISTINCT FROM enddate

Aggregate Functions

aggregate function

aggregate function

<aggregate function> ::= COUNT <left paren> <asterisk> <right paren> [ <filter clause> ] | <general set function> [ <filter clause> ] | <array aggregate function> [ <filter clause> ]

<general set function> ::= <set function type> <left paren> [ <set quantifier> ] <value expression> <right paren>

<set function type> ::= <computational operation>

<computational operation> ::= AVG | MAX | MIN | SUM | EVERY | ANY | SOME | COUNT | STDDEV_POP | STDDEV_SAMP | VAR_SAMP | VAR_POP | MEDIAN

<set quantifier> ::= DISTINCT | ALL

<filter clause> ::= FILTER <left paren> WHERE <search condition> <right paren>

<array aggregate function> ::= ARRAY_AGG <left paren> [ <set quantifier> ] <value expression> [ <order by clause> ] <right paren>

<group concat function> ::= GROUP_CONCAT <left paren> [ <set quantifier> ] <value expression> [ <order by clause> ] [ SEPARATOR <separator> ] <right paren>

<separator> ::= <character string literal>

Specify a value computed from a collection of rows.

An aggregate function is used exclusively in a <query specification> and its use transforms a normal query into an aggregate query returning a single row instead of the multiple rows that the original query returns. For example, SELECT acolumn <table expression> is a query that returns the value of acolumn for all the rows the satisfy the given condition. But SELECT MAX(acolumn) <table expression> returns only one row, containing the largest value in that column. The query SELECT COUNT(*) <table expression> returns the count of rows, while SELECT COUNT(acolumn) <table expression> returns the count of rows where acolumn IS NOT NULL.

If the <table expression> is a grouped table (has a GROUP BY clause), the aggregate function returns the result of the COUNT or <computational operation> for each group. In this case the result has the same number of rows as the original grouped query. For example SELECT SUM(acolumn) <table expression> when <table expression> has a GROUP BY clause, returns the sum of values for acolumn in each group.

If all values are NULL, the aggregate function returns NULL.

The SUM operations can be performed on numeric and interval expressions only. AVG and MEDIAN can be performed on numeric, interval or datetime expressions. AVG returns the average value, while SUM returns the sum of all values. MEDIAN returns the middle value in the sorted list of values.

MAX and MIN can be performed on all types of expressions and return the minimum or the maximum value.

COUNT(*) returns the count of all values, including nulls, while COUNT(<value expression>) returns the count of non-NULL values. COUNT with DISTINCT also accepts multiple arguments. In this usage the distinct combinations of the arguments are counted. Examples below:

 SELECT COUNT(DISTINCT firstname, lastname) FROM customer
 SELECT COUNT(DISTINCT (firstname, lastname)) FROM customer

The EVERY, ANY and SOME operations can be performed on boolean expressions only. EVERY returns TRUE if all the values are TRUE, otherwise FALSE. ANY and SOME are the same operation and return TRUE if one of the values is TRUE, otherwise it returns FALSE.

The other operations perform the statistical functions STDDEV_POP, STDDEV_SAMP, VAR_SAMP, VAR_POP on numeric values. NULL values are ignored in calculations.

User defined aggregate functions can be defined and used instead of the built-in aggregate functions. Syntax and examples are given in the SQL-Invoked Routines chapter.

The <filter clause> allows you to add a search condition. When the search condition evaluates to TRUE for a row, the row is included in aggregation. Otherwise the row is not included. In the example below a single query returns two different filtered counts:

 SELECT COUNT(ITEM) FILTER (WHERE GENDER = 'F') AS "FEMALE COUNT", COUNT(ITEM) FILTER (WHERE GENDER = 'M') AS "MALE COUNT" FROM PEOPLE

ARRAY_AGG is different from all other aggregate functions, as it does not ignore the NULL values. This set function returns an array that contains all the values, for different rows, for the <value expression>. For example, if the <value expression> is a column reference, the SUM function adds the values for all the row together, while the ARRAY_AGG function adds the value for each row as a separate element of the array. ARRAY_AGG can include an optional <order by clause>. If this is used, the elements of the returned array are sorted according to the <order by clause>, which can reference all the available columns of the query, not just the <value expression> that is used as the ARRAY_AGG argument. The <order by clause> can have multiple elements (columns) and each element can include NULLS LAST or DESC qualifiers. No <separator> is used with this function.

GROUP_CONCAT is a specialised function derived from ARRAY_AGG. This function computes the array in the same way as ARRAY_AGG, removes all the NULL elements, then returns a string that is a concatenation of the elements of the array. If <separator> has been specified, it is used to separate the elements of the array. Otherwise the comma is used to separate the elements.

The example below shows a grouped query with ARRAY_AGG and GROUP_CONCAT. The CUSTOMER table that is included for tests in the DatabaseManager GUI app is the source of the data.

 SELECT LASTNAME, ARRAY_AGG(FIRSTNAME ORDER BY FIRSTNAME) FROM Customer  GROUP BY LASTNAME

 LASTNAME  C2                                                         
 --------- ---------------------------------------------------------- 
 Steel     ARRAY['John','John','Laura','Robert']                      
 King      ARRAY['George','George','James','Julia','Robert','Robert'] 
 Sommer    ARRAY['Janet','Robert']                                    

 SELECT LASTNAME, GROUP_CONCAT(DISTINCT FIRSTNAME ORDER BY FIRSTNAME DESC SEPARATOR ' * ') FROM Customer  GROUP BY LASTNAME

 LASTNAME  C2                                                
 --------- ------------------------------------------------- 
 Steel     Robert * Laura * John                   
 King      Robert * Julia * James * George         
 Sommer    Robert * Janet

Other Syntax Elements

search condition

search condition

<search condition> ::= <boolean value expression>

Specify a condition that is TRUE, FALSE, or UNKNOWN. A search condition is often a predicate.

PATH

path specification

<path specification> ::= PATH <schema name list>

<schema name list> ::= <schema name> [ { <comma> <schema name> }... ]

Specify an order for searching for a user-defined SQL-invoked routine. This is not currently supported by HyperSQL.

routine invocation

routine invocation

<routine invocation> ::= <routine name> <SQL argument list>

<routine name> ::= [ <schema name> <period> ] <qualified identifier>

<SQL argument list> ::= <left paren> [ <SQL argument> [ { <comma> <SQL argument> }... ] ] <right paren>

<SQL argument> ::= <value expression> | <target specification>

Invoke an SQL-invoked routine. Examples are given in the SQL-Invoked Routines chapter.

COLLATE

collate clause

<collate clause> ::= COLLATE <collation name>

Specify a collation for a column or for an ORDER BY expression. This collation is used for comparing the values of the column in different rows. Comparison can happens during the execution of SELECT, UPDATE or DELETE statements, when a UNIQUE constraint or index is defined on the column, or when the the rows are sorted by an ORDER BY clause.

CONSTRAINT

constraint name definition

<constraint name definition> ::= CONSTRAINT <constraint name>

<constraint characteristics> ::= <constraint check time> [ [ NOT ] DEFERRABLE ] | [ NOT ] DEFERRABLE [ <constraint check time> ]

<constraint check time> ::= INITIALLY DEFERRED | INITIALLY IMMEDIATE

Specify the name of a constraint and its characteristics. This is an optional element of CONSTRAINT definition, not yet supported by HyperSQL.

Data Access Statements

HyperSQL fully supports all of SQL-92 data access statements, plus some additions from SQL:2008. Due to time constraints, the current version of this Guide does not cover the subject fully. You are advised to consult an SQL book such as the recent O'Reilly title "SQL and Relational Theory" by C. J. Date.

Database queries are data access statements. The most commonly used data access statement is the SELECT statement, but there are other statements that perform a similar role. Data access statements access tables and return result tables. The returned result tables are falsely called result sets, as they are not necessarily sets of rows, but multisets of rows.

Result tables are formed by performing the following operations on base tables and views. These operations are loosely based on Relational Algebra.

JOIN operations

SET and MULTISET operations

SELECTION

PROJECTION

COMPUTING

COLUMN NAMING

GROUPING and AGGREGATION

SELECTION AFTER GROUPING OR AGGREGATION

SET and MULTISET (COLLECTION) OPERATIONS

ORDERING

SLICING

Conceptually, the operations are performed one by one in the above order if they apply to the given data access statement. In the example below a simple select statement is made more complex by adding various operations.

 CREATE TABLE atable (a INT, b INT, c INT, d INT, e INT, f INT);
 /* in the next SELECT, no join is performed and no further operation takes place */
 SELECT * FROM atable
 /* in the next SELECT, selection is performed by the WHERE clause, with no further action */
 SELECT * FROM atable WHERE a + b = c
 /* in the next SELECT, projection is performed after the other operations */
 SELECT d, e, f FROM atable WHERE a + b = c
 /* in the next SELECT, computation is performed after projection */
 SELECT (d + e) / f FROM atable WHERE a + b = c
 /* in the next two SELECT statements, column naming is performed in different ways*/
 SELECT (a + e) / f AS calc, f AS div FROM atable WHERE a + b = c
 SELECT dcol, ecol, fcol FROM atable(acol, bcol, ccol, dcol, ecol, fcol) WHERE acol + bcol = ccol
 /* in the next SELECT, both grouping and aggregation is performed */
 SELECT d, e, SUM(f) FROM atable GROUP BY d, e
 /* in the next SELECT, selection is performed after grouping and aggregation is performed */
 SELECT d, e, SUM(f) FROM atable GROUP BY d, e HAVING SUM(f) > 10
 /* in the next SELECT, a UNION is performed on two selects from the same table */
 SELECT d, e, f FROM atable WHERE d = 3 UNION SELECT a, b, c FROM atable WHERE a = 30
 /* in the next SELECT, ordering is performed */
 SELECT (a + e) / f AS calc, f AS div FROM atable WHERE a + b = c ORDER BY calc DESC, div NULLS LAST
 /* in the next SELECT, slicing is performed after ordering */
 SELECT * FROM atable WHERE a + b = c ORDER BY a FETCH 5 ROWS ONLY

The next sections discuss various types of tables and operations involved in data access statements.

Select Statement

The SELECT statement itself does not cover all types of data access statements, which may combine multiple SELECT statements. The <query specification> is the most common data access statement and begins with the SELECT keyword.

SELECT STATEMENT

select statement (general)

Users generally refer to the SELECT statement when they mean a <query specification> or <query expression>. If a statement begins with SELECT and has no UNION or other set operations, then it is a <query specification>. Otherwise it is a <query expression>.

Table

In data access statements, a table can be a database table (or view) or an ephemeral table formed for the duration of the query. Some types of table are <table primary> and can participate in joins without the use of extra parentheses. The BNF in the Table Primary section below lists different types of <table primary>:

Tables can also be formed by specifying the values that are contained in them:

<table value constructor> ::= VALUES <row value expression list>

<row value expression list> ::= <table row value expression> [ { <comma> <table row value expression> }... ]

In the example below a table with two rows and 3 columns is constructed out of some values:

 VALUES (12, 14, null), (10, 11, CURRENT_DATE)

When a table is used directly in a UNION or similar operation, the keyword TABLE is used with the name:

<explicit table> ::= TABLE <table or query name>

In the examples below, all rows of the two tables are included in the union. The keyword TABLE is used in the first example. The two examples below are equivalent.

 TABLE atable UNION TABLE anothertable
 SELECT * FROM atable UNION SELECT * FROM anothertable

Subquery

A subquery is simply a query expression in brackets. A query expression is usually a complete SELECT statement and is discussed in the rest of this chapter. A scalar subquery returns one row with one column. A row subquery returns one row with one or more columns. A table subquery returns zero or more rows with one or more columns. The distinction between different forms of subquery is syntactic. Different forms are allowed in different contexts. If a scalar subquery or a row subquery return more than one row, an exception is raised. If a scalar or row subquery returns no row, it is usually treated as returning a NULL. Depending on the context, this has different consequences.

<scalar subquery> ::= <subquery>

<row subquery> ::= <subquery>

<table subquery> ::= <subquery>

<subquery> ::= <left paren> <query expression> <right paren>

Query Specification

A query specification is also known as a SELECT statement. It is the most common form of <derived table> . A <table expression> is a base table, a view or any form of allowed derived table. The SELECT statement performs projection, naming, computing or aggregation on the rows of the <table expression> .

<query specification> ::= SELECT [ DISTINCT | ALL ] <select list> <table expression>

<select list> ::= <asterisk> | <select sublist> [ { <comma> <select sublist> }... ]

<select sublist> ::= <derived column> | <qualified asterisk>

<qualified asterisk> ::= <asterisked identifier chain> <period> <asterisk>

<asterisked identifier chain> ::= <asterisked identifier> [ { <period> <asterisked identifier> }... ]

<asterisked identifier> ::= <identifier>

<derived column> ::= <value expression> [ <as clause> ]

<as clause> ::= [ AS ] <column name>

The qualifier DISTINCT or ALL apply to the results of the SELECT statement after all other operations have been performed. ALL simply returns the rows, while DISTINCT compares the rows and removes the duplicate ones.

Projection is performed by the <select list>.

A single <asterisk> means all columns of the <table expression> are included, in the same order as they appear in the <table expression>. An asterisk qualified by a table name means all the columns of the qualifier table name are included. If an unqualified asterisk is used, then no other items are allowed in the <select list>. When the the <table expression> is the direct result of NATURAL or USING joins, the use of <asterisk> includes the columns used for the join before the other columns. A qualified asterisk does not cover the join columns.

A derived column is a <value expression>, optionally named with the <as clause>. A <value expression> can be many things. Common types include: the name of a column in the <table expression>; an expression based on different columns or constant values; a function call; an aggregate function; a CASE WHEN expression.

Table Expression

A table expression is part of the SELECT statement and consists of the FROM clause with optional other clauses that performs selection (of rows) and grouping from the table(s) in the FROM clause.

<table expression> ::= <from clause> [ <where clause> ] [ <group by clause> ] [ <having clause> ]

<from clause> ::= FROM <table reference> [ { <comma> <table reference> }... ]

<table reference> ::= <table primary> | <joined table>

<table primary> ::= <table or query name> [ [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ] ]

| <derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]

| <lateral derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]

| <collection derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]

| <table function derived table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]

| <parenthesized joined table> [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ]

<where clause> ::= WHERE <boolean value expression>

<group by clause> ::= GROUP BY [ <set quantifier> ] <grouping element> [ { <comma> <grouping element> }... ]

<having clause> ::= HAVING <boolean value expression>

The <from clause> contains one or more <table reference> separated by commas. A table reference is often a table or view name or a joined table.

The <where clause> filters the rows of the table in the <from clause> and removes the rows for which the search condition is not TRUE.

Table primary refers to different forms of table reference in the FROM clause.

Table or Query Name

The simplest form of reference is simply a name. This is the name of a table, a view, a transition table in a trigger definition, or a query name specified in the WITH clause of a query expression.

<table or query name> ::= <table name> | <transition table name> | <query name>

Derived Table

derived table

A query expression that is enclosed in parentheses and returns from zero to many rows is a <table subquery>. In a <derived table> the query expression is self contained and cannot reference the columns of other table references. This is the traditional and most common form of use of a <table subquery>.

<derived table> ::= <table subquery>

Lateral

LATERAL

When the word LATERAL is used before a <table subquery>, it means the query expression can reference the columns of other table references that go before it.

<lateral derived table> ::= LATERAL <table subquery>

The use of <lateral derived table> completely transforms the way a query is written. For example, the two queries below are equivalent, but with different forms. The query with LATERAL is evaluated separately for each row of the first table that satisfies the WHERE condition. The example below uses the tables that are created and populated in DatabaseManagerSwing with the "Insert test data" menu option. The first query uses a scalar subquery to compute the sum of invoice values for each customer. The second query is equivalent and uses a join with a LATERAL table.

SELECT firstname, lastname, (SELECT SUM(total) FROM invoice WHERE customerid = customer.id) s FROM customer

SELECT firstname, lastname, a.c FROM customer, LATERAL(SELECT SUM(total) FROM invoice WHERE customerid = customer.id) a (c)

UNNEST

UNNEST

UNNEST is similar to LATERAL, but instead of a query expression, one or more expressions that return an array are used. These expressions are converted into a table which has one column for each expression and contains the elements of the array. If WITH ORDINALITY is used, an extra column that contains the index of each element is added to this table. The number or rows in the table equals the length of the largest arrays. The smaller arrays are padded with NULL values. If an <array value expression> evaluates to NULL, an empty array is used in its place. The array expression can contain references to any column of the table references preceding the current table reference.

<collection derived table> ::= UNNEST <left paren> <array value expression>, ... <right paren> [ WITH ORDINALITY ]

The <array value expression> can be the result of a function call. If the arguments of the function call are values from the tables on the left of the UNNEST, then the function is called for each row of table.

In the first example below, UNNEST is used with the built in-function SEQUENCE_ARRAY to build a table containing dates for the last seven days and their ordinal position. In the second example, a select statement returns costs for the last seven days . In the third example, the WITH clause turns the two selects into named subqueries which are used in a SELECT statement that uses a LEFT join.

 SELECT * FROM UNNEST(SEQUENCE_ARRAY(CURRENT_DATE - 7 DAY, CURRENT_DATE - 1 DAY, 1 DAY)) WITH ORDINALITY AS T(D, I)  

 D          I 
 ---------- - 
 2010-07-25 1 
 2010-07-26 2 
 2010-07-27 3 
 2010-07-28 4 
 2010-07-29 5 
 2010-07-30 6 
 2010-07-31 7 

 CREATE TABLE expenses (item_date DATE, cost DECIMAL(8,2))
 --
 SELECT item_date, SUM(cost) AS s FROM expenses WHERE item_date >= CURRENT_DATE - 7 DAY GROUP BY item_date

 ITEM_DATE  S      
 ---------- ------ 
 2010-07-27 100.12 
 2010-07-29 50.45  

 WITH costs(i_d, s) AS (SELECT item_date, SUM(cost) AS s FROM expenses WHERE item_date >= CURRENT_DATE - 7 DAY GROUP BY item_date),
 dates(d, i) AS (SELECT * FROM UNNEST(SEQUENCE_ARRAY(CURRENT_DATE - 7 DAY, CURRENT_DATE - 1 DAY, 1 DAY)) WITH ORDINALITY)   
 SELECT i, d, s FROM dates LEFT OUTER JOIN costs ON dates.d = costs.i_d

 I D          S      
 - ---------- ------ 
 1 2010-07-25 (null) 
 2 2010-07-26 (null) 
 3 2010-07-27 100.12 
 4 2010-07-28 (null) 
 5 2010-07-29 50.45  
 6 2010-07-30 (null) 
 7 2010-07-31 (null) 

Table Function Derived Table

Table Function Derived Table

When TABLE is used in this context, the <collection value expression> must be the result of a function call to a built-in function or user-defined function that returns an array or a table. When the function returns an array, this array is converted into a table, similar to the way UNNEST operates. When the function returns a table, the result is a MULTISET and is used as is.

<table function derived table> ::= TABLE <left paren> <collection value expression> <right paren>

Parenthesized Joined Table

A parenthesized joined table is simply a joined table contained in parentheses. Joined tables are discussed below.

<parenthesized joined table> ::= <left paren> <parenthesized joined table> <right paren> | <left paren> <joined table> <right paren>

Column Name List

column name list

The column list that is specified for the table reference must contain names that are unique within the list

<derived column list> ::= <column name list>

<column name list> ::= <column name> [ { <comma> <column name> }... ]

Joined Table

Joins are operators with two table as the operands, resulting in a third table, called joined table. All join operators are evaluated left to right, therefore, with multiple joins, the table resulting from the first join operator becomes an operand of the next join operator. Parentheses can be used to group sequences of joined tables and change the evaluation order. So if more than two tables are joined together with join operators, the end result is also a joined table. There are different types of join, each producing the result table in a different way.

<joined table> ::= <cross join> | <qualified join> | <natural join>

<cross join> ::= <table reference> CROSS JOIN <table factor>

<qualified join> ::= <table reference> | [ <join type> ] JOIN <table reference> <join specification>

<natural join> ::= <table reference> NATURAL [ <join type> ] JOIN <table factor>

<join specification> ::= <join condition> | <named columns join>

<join condition> ::= ON <search condition>

<named columns join> ::= USING <left paren> <join column list> <right paren>

<join type> ::= INNER | <outer join type> [ OUTER ]

<outer join type> ::= LEFT | RIGHT | FULL

<join column list> ::= <column name list>

CROSS JOIN

The simplest form of join is CROSS JOIN. The CROSS JOIN of two tables is a table that has all the columns of the first table, followed by all the columns of the second table, in the original order. Each row of the first table is combined with each row of the second table to fill the rows of the new table. If the rows of each table form a set, then the rows of the CROSS JOIN table form the Cartesian product of the rows of the two table operands.

Conditions are not allowed as part of a cross join, which is simply A CROSS JOIN B. Any conditions in a WHERE clause are later applied to the table resulting from the cross join.

Tables in the FROM CLAUSE separated with commas, are equivalent to cross joins between the tables. Two joined tables separated with a comma, such as A, B, is equivalent to (A) CROSS JOIN (B), which means the joined tables A and B are populated separately before they are joined.

CROSS JOIN is not is not generally very useful, as it returns large result tables unless WHERE conditions are used.

UNION JOIN

The UNION JOIN has limited use in queries. The result table has the same columns as that of CROSS JOIN. Each row of the first table is extended to the right with nulls and added to the new table. Each row of the second table is extended to the left with nulls and added to the new table. The UNION JOIN is expressed as A UNION JOIN B. This should not be confused with A UNION B, which is a set operation. Union join is for special applications and is not commonly used.

JOIN ... ON

The condition join is similar to CROSS JOIN, but a condition is tested for each row of the new table and the row is created only if the condition is true. This form of join is expressed as A JOIN B ON (<search condition>).

Equijoin is a condition join in which the search condition is an equality condition between on or more pairs of columns from the two table. Equijoin is the most commonly used type of join.

SELECT a.*, b.* FROM a INNER JOIN b ON a.col_one = b.col_two

JOIN ... USING

NATURAL JOIN

Joins with USING or NATURAL keywords joins are similar to an equijoin but they cannot be replaced simply with an equijoin. The new table is formed with the specified or implied shared columns of the two tables, followed by the rest of the columns from each table. In NATURAL JOIN, the shared columns are all the column pairs that have the same name in the first and second table. In JOIN USING, only columns names that are specified by the USING clause are shared. The joins are expressed as A NATURAL JOIN B, and A JOIN B USING (<comma separated column name list>).

The columns of the joined table are formed by the following procedures: In JOIN ... USING the shared columns are added to the joined table in the same order as they appear in the column name list. In NATURAL JOIN the shared columns are added to the joined table in the same order as they appear in the first table. In both forms of join, the non-shared columns of the first table are added in the order they appear in the first table, finally the non-shared columns of the second table are added in the order they appear in the second table.

The type of each shared column of the joined table is based on the type of the columns in the original tables. If the original types are not exactly the same, the type of the shared column is formed by type aggregation. Type aggregations selects a type that can represent values of both aggregated types. Simple type aggregation picks one of the types. For example SMALLINT and INTEGER, results in INTEGER, or VARCHAR(10) and VARCHAR(20) results in VARCHAR(20). More complex type aggregation inherits properties from both types. For example DECIMAL(8) and DECIMAL (6,2) results in DECIMAL (8,2).

In the examples below, the rows are joined exactly the same way, but the first query contains a.col_two and b.col_two together with all the rest of the columns of both tables, while the second query returns only one copy of col_two.

 SELECT * FROM a INNER JOIN b ON a.col_two = b.col_two
 SELECT * FROM a INNER JOIN b USING (col_two)

OUTER JOIN

LEFT, RIGHT and FULL OUTER JOIN

The three qualifiers can be added to all types of JOIN except CROSS JOIN and UNION JOIN. First the new table is populated with the rows from the original join. If LEFT is specified, all the rows from the first table that did not make it into the new table are extended to the right with nulls and added to the table. If RIGHT is specified, all the rows from the second table that did not make it into the new table are extended to the left with nulls and added to the table. If FULL is specified, the addition of leftover rows is performed from both the first and the second table. These forms are expressed by prefixing the join specification with the given keyword. For example A LEFT OUTER JOIN B ON (<search condition>) or A NATURAL FULL OUTER JOIN B or A FULL OUTER JOIN B USING (<comma separated column name list>).

 SELECT a.*, b.* FROM a LEFT OUTER JOIN b ON a.col_one = b.col_two

Selection

Despite the name, selection has nothing to do with the list of columns in a SELECT statement. In fact, it refers to the search condition used to limit the rows that from a result table (selection of rows, not columns). In SQL, simple selection is expressed with a WHERE condition appended to a single table or a joined table. In some cases, this method of selection is the only method available. For example in DELETE and UPDATE statements. But when it is possible to perform the selection with join conditions, this is the better method, as it results in a clearer expression of the query.

Projection

Projection is selection of the columns from a simple or joined table to form a result table. Explicit projection is performed in the SELECT statement by specifying the select column list. Some form of projection is also performed in JOIN ... USING and NATURAL JOIN.

The joined table has columns that are formed according to the rules mentioned above. But in many cases, not all the columns are necessary for the intended operation. If the statement is in the form, SELECT * FROM <joined table>, then all the columns of <joined table> are returned. But normally, the columns to be returned are specified after the SELECT keyword, separated from each other with commas.

Computed Columns

In the select list, it is possible to use expressions that reference any columns of <joined table>. Each of these expressions forms a computed column. It is computed for each row of the result table, using the values of the columns of the <joined table> for that row.

Naming

Naming is used to hide the original names of tables or table columns and to replace them with new names in the scope of the query. Naming is also used for defining names for computed columns.

Without explicit naming, the name of a column is a predefined name. If the column is a column of a table, or is a named parameter, the name is of the table column or parameter is used. Otherwise it is generated by the database engine. HyperSQL generates column names such as C1, C2, etc. As generated naming is implementation defined according to the SQL Standard, it is better to explicitly name the computed and derived columns in your applications.

Naming in Joined Table

Naming is performed by adding a new name after a table's real name and by adding a list of column names after the new table name. Both table naming and column naming are optional, but table naming is required for column naming. The expression A [AS] X (<comma separated column name list>) means table A is used in the query expression as table X and its columns are named as in the given list. The original name A, or its original column names, are not visible in the scope of the query. The BNF is given below. The <correlation name> can be the same or different from the name of the table. The <derived column list> is a comma separated list of column names. The degree of this list must be equal to the degree of the table. The column names in the list must be distinct. They can be the same or different from the names of the table's columns.

<table or query name> [ [ AS ] <correlation name> [ <left paren> <derived column list> <right paren> ] ]

In the examples below, the columns of the original tables are named (a, b, c, d, e, f). The two queries are equivalent. In the second query, the table and its columns are renamed and the new names are used in the WHERE clauses:

 CREATE TABLE atable (a INT, b INT, c INT, d INT, e INT, f INT);
 SELECT d, e, f FROM atable WHERE a + b = c
 SELECT x, y, z FROM atable AS t (u, v, w, x, y, z)  WHERE u + v = w

Naming in Select List

Naming in the SELECT list logically takes place after naming in the joined table. The new names for columns are not visible in the immediate query expression or query expression. They become visible in the ORDER BY clause and in the result table that is returned to the user. Or if the query expression is used as a derived table in an enclosing query expression.

In the example below, the query is on the same table but with column renaming in the Select list. The new names are used in the ORDER BY clause:

 SELECT x + y AS xysum, y + z AS yzsum FROM atable AS t (u, v, w, x, y, z)  WHERE u + v = w ORDER BY xysum, yzsum

If the names xysum or yzsum are not used, the computed columns cannot be referenced in the ORDER BY list.

Name Resolution

In a joined table, if a column name appears in tables on both sides then any reference to the name must use the table name in order to specify which table is being referred to.

Grouping Operations

Grouping Operations

Grouping results in the elimination of duplicate rows. A grouping operation is performed after the operations discussed above. A simple form of grouping is performed by the use of DISTINCT after SELECT. This eliminates all the duplicate rows (rows that have the same value in each of their columns when compared to another row). The other form of grouping is performed with the GROUP BY clause. This form is usually used together with aggregation.

GROUP BY

<group by clause> ::= GROUP BY <grouping element> [ { <comma> <grouping element> }... ]

<grouping element> ::= <column reference> [ <collate clause> ]

The <group by clause> is a comma separated list of columns of the table in the <from clause> or expressions based on the columns.

When a <group by clause> is used, only the columns used in the <group by clause> or expressions used there, can be used in the <select list>, together with any <aggregate function> on other columns. In other words, the column names or expressions listed in the GROUP BY cluase dictate what can be used in the <select list>. After the rows of the table formed by the <from clause> and the <where clause> are finalised, the grouping operation groups together the rows that have the same values in the columns of the <group by clause>. Then any <aggregate function> in the <select list> is performed on each group, and for each group, a row is formed that contains the values of the columns of the <group by clause> and the values returned from each <aggregate function>.

When the type of <column reference> is character string, the <collate clause> can be used to specify the collation used for grouping the rows. For example, a collation that is not case sensitive can be used, or a collation for a different language than the original collation of the column.

In the first example below, a simple column reference is used in GROUP BY, while in the second example, an expression is used.

 CREATE TABLE atable (a INT, b INT, c INT, d INT, e INT, f INT);
 SELECT d, e, f FROM atable WHERE a + b = c GROUP BY d, e, f
 SELECT d + e, SUM(f) FROM atable WHERE a + b = c GROUP BY d + e HAVING SUM(f) > 2 AND d + e > 4

A <having clause> filters the rows of the table that is formed after applying the <group by clause> using its search condition. The search condition must be an expression based on the expressions in the GROUP BY list or the aggregate functions used.

DISTINCT

SELECT DISTINCT

When the keyword DISTINCT is used after SELECT, it works as a shortcut replacement for a simple GROUP BY clause. The expressions in the SELECT list are used directly as the <group by clause>. The following examples of SELECT DISTINCT and SELECT with GROUP BY are equivalent.

 SELECT DISTINCT d, e + f FROM atable WHERE a + b = c
 SELECT d, e + f FROM atable WHERE a + b = c GROUP BY d, e + f

Aggregation

Aggregation is an operation that computes a single value from the values of a column over several rows. The operation is performed with an aggregate function. The simplest form of aggregation is counting, performed by the COUNT function.

Other common aggregate functions return the maximum, minimum and average value among the values in different rows. Aggregate functions were discussed earlier in this chapter.

Set Operations

Set and Multiset Operations

While join operations generally result in laterally expanded tables, SET and COLLECTION operations are performed on two tables that have the same degree and result in a table of the same degree. The SET operations are UNION, INTERSECT and EXCEPT (difference). When each of these operations is performed on two tables, the collection of rows in each table and in the result is reduced to a set of rows, by eliminating duplicates. The set operations are then performed on the two tables, resulting in the new table which itself is a set of rows. Collection operations are similar but the tables are not reduced to sets before or after the operation and the result is not necessarily a set, but a collection of rows.

The set operations on two tables A and B are: A UNION [DISTINCT] B, A INTERSECT [DISTINCT] B and A EXCEPT [DISTINCT] B. The result table is formed in the following way: The UNION operation adds all the rows from A and B into the new table, but avoids copying duplicate rows. The INTERSECT operation copies only those rows from each table that also exist in the other table, but avoids copying duplicate rows. The EXCEPT operation copies those rows from the first table which do not exist in the second table, but avoids copying duplicate rows.

The collection operations are similar to the set operations, but can return duplicate rows. They are A UNION ALL B, A INTERSECT ALL B and A EXCEPT ALL B. The UNION ALL operation adds all the rows from A and B into the new table. The INTERSECT operation copies only those rows from each table that also exist in the other table. If n copies of a rows exists in one table, and m copies in the other table, the number of copies in the result table is the smaller of n and m. The EXCEPT operation copies those rows from the first table which do not exist in the second table. If n copies of a row exist in the first table and m copies in the second table the number of copies in the result table is n-m, or if n < m, then zero.

With Clause and Recursive Queries

The optional WITH clause can be used in a query expression. The WITH clause lists one or more named ephemeral tables that can be referenced in the query expression body. The ephemeral tables are created and populated before the rest of the query expression is executed. Their contents do not change during the execution of the <query expression body>.

<with clause> ::= WITH [ RECURSIVE ] <with list>

<with list> ::= <with list element> [ { <comma> <with list element> }... ]

<with list element> ::= <query name> [ <left paren> <with column list> <right paren> ] AS <left paren> <query expression> <right paren>

<with column list> ::= <column name list>

An example of the use of the WITH clause is given above under UNNEST. The <query expression> in the WITH clause is evaluated once and its result table can be referenced in the body of the main <query expression body> using the specified <query name>.

The RECURSIVE keyword changes the way the elements of the <with list> are interpreted. The <query expression> contained in the <with list element> must be the UNION or UNION ALL of two <query expression body> elements (simple VALUES or SELECT statements). The left element of the UNION is evaluated first and its result becomes the result of the <with list element>. After this step, the current result of the <with list element> is referenced in the right element (a SELECT statement) of the UNION, the UNION is performed between the result and previous result of the <with list element>, which is enlarged by this operation. The UNION operation is performed again and again, until the result of the <with list element> stops changing. The result of the <with list element> is now complete and is later used in the execution of the <query expression body>. When RECURSIVE is used, the <with column list> must be defined.

HyperSQL limits recursion to 265 rounds. If this is exceeded, an error is raised.

A trivial example of a recursive query is given below. Note the first column GEN. For example, if each row of the table represents a member of a family of dogs, together with its parent, the first column of the result indicates the calculated generation of each dog, ranging from first to fourth generation.

 CREATE TABLE pptree (pid INT, id INT);
 INSERT INTO pptree VALUES (NULL, 1) ,(1,2), (1,3),(2,4),(4,5),(3,6),(3,7);

 WITH RECURSIVE tree (gen, par, child) AS (
   VALUES(1, CAST(null as int), 1)
   UNION
   SELECT gen + 1, pid, id FROM pptree, tree WHERE pid = child
   ) SELECT * FROM tree;

 GEN PAR    CHILD 
 --- ------ ----- 
 1   (null) 1     
 2   1      2     
 2   1      3     
 3   2      4     
 3   3      6     
 3   3      7     
 4   4      5

if recursive queries become complex they also become very difficult to develop and debug. HyperSQL provides an alternative to this with user-defined SQL functions which return tables. Functions can perform any complex, repetitive task with better control, using loops, variables and, if necessary, recursion.

Query Expression

A query expression consists of an optional WITH clause and a query expression body. The optional WITH clause lists one or more named ephemeral tables that can be referenced, just like the database tables in the query expression body.

<query expression> ::= [ <with clause> ] <query expression body>

A query expression body refers to a table formed by using UNION and other set operations. The query expression body is evaluated from left to right and the INTERSECT operator has precedence over the UNION and EXCEPT operators. A simplified BNF is given below:

<query expression body> ::= <query term> | <query expression body> UNION | EXCEPT [ ALL | DISTINCT ] [ <corresponding spec> ] <query term>

<query term> ::= <query primary> | <query term> INTERSECT [ ALL | DISTINCT ] [ <corresponding spec> ] <query term>

<query primary> ::= <simple table> | <left paren> <query expression body> [ <order by clause> ] [ <result offset clause> ] [ <fetch first clause> ] <right paren>

<simple table> ::= <query specification> | <table value constructor> | <explicit table> <explicit table> ::= TABLE <table or query name>

<corresponding spec> ::= CORRESPONDING [ BY <left paren> <column name list> <right paren> ]

A <query term> and a <query primary> can be a SELECT statement, an <explicit table>, or a <table value constructor>.

The CORRESPONDING clause is optional. If it is not specified, then the <query term> and the <query primary> must have the same number of columns. If CORRESPONDING is specified, the two sides need not have the same number of columns. If no column list is used with CORRESPONDING, then all the column names that are common in the tables on two sides are used in the order in which they appear in the first table. If a columns list is used, it allows you to select only some columns of the tables on the left and right side to create the new table. In the example below the columns named u and v from the two SELECT statements are used to create the UNION table.

 SELECT * FROM atable UNION CORRESPONDING BY (u, v) SELECT * FROM anothertable

The type of each column of the query expression is determined by combining the types of the corresponding columns from the two participating tables.

Ordering

When the rows of the result table have been formed, it is possible to specify the order in which they are returned to the user. The ORDER BY clause is used to specify the columns used for ordering, and whether ascending or descending ordering is used. It can also specify whether NULL values are returned first or last.

 SELECT x + y AS xysum, y + z AS yzsum FROM atable AS t (u, v, w, x, y, z)  WHERE u + v = w ORDER BY xysum NULLS LAST, yzsum NULLS FIRST

The ORDER BY clause specifies one or more <value expressions>. The list of rows is sorted according to the first <value expression>. When some rows are sorted equal then they are sorted according to the next <value expression> and so on.

<order by clause> ::= ORDER BY <sort specification> [ { <comma> <sort specification> }... ]

<sort specification> ::= <value expression> [ <collate clause> ] [ ASC | DESC ] [ NULLS FIRST | NULLS LAST ]

The defaults are ASC and NULLS FIRST. Two database properties SQL NULLS FIRST and SQL NULLS ORDER can be modified to change the default behaviour.

A collation is used for columns of an ORDER BY expression that are of the type CHAR or VARCHAR. If a <collate clause> is not specified then the collation of the column, or the default collation of the database is used.

The default collation for a database is ASCII, with lowercase letters sorted after all uppercase letters. The example below shows the effect of collation on an ordered list.

 -- default collation collation for the database is ASCII
 SELECT id, lastname FROM customer ORDER BY lastname
 ID LASTNAME 
 -- -------- 
 40 Clancy   
 36 King     
 35 White    
 6  king

 -- a language collation is used, it treats king and King as adjacent entries
 SELECT id, lastname FROM customer ORDER BY lastname COLLATE "English"
 ID LASTNAME 
 -- -------- 
 40 Clancy   
 6  king     
 36 King     
 35 White

In the above example, if the LASTNAME column is itself defined in the table definition with COLLATE "English", then the COLLATE clause is not necessary in the ORDER BY expression.

An ORDER BY operation can sometimes be optimised by the engine when it can use the same index for accessing the table data and ordering. Optimisation can happen both with DESC + NULLS LAST and ASC + NULLS FIRST.

sort specification list

sort specification list

<sort specification list> ::= <value expression> [ASC | DESC] [NULLS FIRST | NULLS LAST]

Specify a sort order. A sort operation is performed on the result of a <query expression> or <query specification> and sorts the result according to one or more <value expression>. The <value expression> is usually a single column of the result, but in some cases it can be a column of the <table expression> that is not used in the select list. The default is ASC and NULLS FIRST.

Slicing

A different form of limiting the rows can be performed on the result table after it has been formed according to all the other operations (selection, grouping, ordering etc.). This is specified by the FETCH ... ROWS and OFFSET clauses of a SELECT statement. In this form, the specified OFFSET rows are removed from start of the table, then up to the specified FETCH rows are kept and the rest of the rows are discarded.

<result offset clause> ::= OFFSET <offset row count> { ROW | ROWS }

<fetch first clause> ::= FETCH { FIRST | NEXT } [ <fetch first row count> ] { ROW | ROWS } ONLY [ USING INDEX ]

<limit clause> ::= LIMIT <fetch first row count> [ USING INDEX ]

A slicing operation takes the result set that has been already processed and ordered. It then discards the specified number of rows from the start of the result set and returns the specified number of rows after the discarded rows. The <offset row count> and <fetch first row count> can be constants, dynamic variables, routine parameters, or routine variables. The type of the constants must be INTEGER.

 SELECT a, b FROM atable WHERE d < 5 ORDER BY absum OFFSET 3 FETCH 2 ROWS ONLY 
 SELECT a, b FROM atable WHERE d < 5 ORDER BY absum OFFSET 3 LIMIT 2 /* alternative keyword */ 

When the FETCH keyword is used, the specified number of rows must be at least 1, otherwise an error is returned. This behaviour is consistent with the SQL Standard. When the LIMIT keyword is used, the specified number of rows can be zero, which means return all rows (no LIMIT). In MySQL and PostgreSQL syntax modes, zero limit means no rows (empty result).

If there is an index on all the columns specified in the ORDER BY clause, it is normally used for slicing. In some queries, an index on another column may take precedence because it is used to process the WHERE condition. It is possible to add USING INDEX to the end of the slicing clause to force the use of the index for ordering and slicing, instead of the index for the WHERE condition.

Data Change Statements

Delete Statement

DELETE FROM

delete statement: searched

<delete statement: searched> ::= DELETE FROM <target table> [ [ AS ] <correlation name> ] [ WHERE <search condition> ]

Delete rows of a table. The search condition is a <boolean value expression> that is evaluated for each row of the table. If the condition is true, the row is deleted. If the condition is not specified, all the rows of the table are deleted. In fact, an implicit SELECT is performed in the form of SELECT * FROM <target table> [ WHERE <search condition>] and the selected rows are deleted. When used in JDBC, the number of rows returned by the implicit SELECT is returned as the update count.

If there are FOREIGN KEY constraints on other tables that reference the subject table, and the FOREIGN KEY constraints have referential actions, then rows from those other tables that reference the deleted rows are either deleted, or updated, according to the specified referential actions.

In the second example below the rows that have the maximum value for column A are deleted;

 DELETE FROM T WHERE C > 5
 DELETE FROM T AS TT WHERE TT.A = (SELECT MAX(A) FROM T)

Truncate Statement

TRUNCATE TABLE

truncate table statement

<truncate table statement> ::= TRUNCATE TABLE <target table> [ <identity column restart option> ] [ <truncate options> ]

<identity column restart option> ::= CONTINUE IDENTITY | RESTART IDENTITY

<truncate options> ::= AND COMMIT [ NO CHECK ]

Delete all rows of a table without firing its triggers. This statement can only be used on base tables (not views). If the table is referenced in a FOREIGN KEY constraint defined on another table, the statement causes an exception. Triggers defined on the table are not executed with this statement. The default for <identity column restart option> is CONTINUE IDENTITY. This means no change to the IDENTITY sequence of the table. If RESTART IDENTITY is specified, then the sequence is reset to its start value.

TRUNCATE is faster than ordinary DELETE. The TRUNCATE statement is an SQL Standard data change statement, therefore it is performed under transaction control and can be rolled back if the connection is not in the auto-commit mode.

HyperSQL also supports the optional AND COMMIT and NO CHECK options. If AND COMMIT is used, then the transaction is committed with the execution of the TRUNCATE statement. The action cannot be rolled back. If the additional NO CHECK option is also specified, then the TRUNCATE statement is executed even if the table is referenced in a FOREIGN KEY constraint defined on another, non-empty table. This form of TRUNCATE is faster than the default form and does not use much memory.

TRUNCATE SCHEMA

truncate schema statement

<truncate schema statement> ::= TRUNCATE SCHEMA <target schema> [ <identity column restart option> ] AND COMMIT [ NO CHECK ]

Performs the equivalent of a TRUNCATE TABLE ... AND COMMIT on all the table in the schema. If the additional NO CHECK option is also specified, then the TRUNCATE statement is executed even if any of the tables in the schema is referenced in a FOREIGN KEY constraint defined on a non-empty table in a different schema.

If RESTART IDENTITY is specified, all table IDENTITY sequences and all SEQUENCE objects in the schema are reset to their start values.

Use of this statement requires schema ownership or administrative privileges.

Insert Statement

INSERT INTO

insert statement

<insert statement> ::= INSERT INTO <target table> <insert columns and source>

<insert columns and source> ::= <from subquery> | <from constructor> | <from default>

<from subquery> ::= [ <left paren> <insert column list> <right paren> ] [ <override clause> ] <query expression>

<from constructor> ::= [ <left paren> <insert column list> <right paren> ] [ <override clause> ] <contextually typed table value constructor>

<override clause> ::= OVERRIDING USER VALUE | OVERRIDING SYSTEM VALUE

<from default> ::= DEFAULT VALUES

<insert column list> ::= <column name list>

Insert new rows in a table. An INSERT statement inserts one or more rows into the table.

The special form, INSERT INTO <target table> DEFAULT VALUES can be used with tables which have a default value for each column.

With the other forms of INSERT, the optional (<insert column list>) specifies to which columns of the table the new values are assigned.

In one form, the inserted values are from a <query expression> and all the rows that are returned by the <query expression> are inserted into the table. If the <query expression> returns no rows, nothing is inserted.

In the other form, a comma separated list of values called <contextually typed table value constructor> is used to insert one or more rows into the table. This list is contextually typed, because the keywords NULL and DEFAULT can be used for the values that are assigned to each column of the table. The keyword DEFAULT means the default value of the column and can be used only if the target column has a default value or is an IDENTITY or GENERATED column of the table.

The <override clause> must be used when a value is explicitly assigned to a column that has been defined as GENERATED ALWAYS AS IDENTITY. The clause, OVERRIDE SYSTEM VALUE means the provided values are used for the insert, while OVERRIDING USER VALUE means the provided values are simply ignored and the values generated by the system are used instead.

An array can be inserted into a column of the array type by using literals, by specifying a parameter in a prepared statement or an existing array returned by query expression. The last example below inserts an array.

The rows that are inserted into the table are checked against all the constraints that have been declared on the table. The whole INSERT operation fails if any row fails to inserted due to constraint violation. Examples:

 CREATE TABLE T (A INTEGER GENERATED BY DEFAULT AS IDENTITY, B INTEGER DEFAULT 2)
 INSERT INTO T DEFAULT VALUES /* all columns of T have DEFAULT clauses */
 INSERT INTO T (SELECT * FROM Z) /* table Z has the same columns as table T */
 INSERT INTO T (A,B) VALUES ((1,2),(3,NULL), (DEFAULT,6)) /* three rows are inserted into table T */
 ALTER TABLE T ADD COLUMN D VARCHAR(10) ARRAY /* an ARRAY column is added */
 INSERT INTO T VALUES DEFAULT, 3, ARRAY['hot','cold']

If the table contains an IDENTITY column, the value for this column for the last row inserted by a session can be retrieved using a call to the IDENTITY() function. This call returns the last value inserted by the calling session. When the insert statement is executed with a JDBC Statement or PreparedStatement method, the getGeneratedKeys() method of Statement can be used to retrieve not only the IDENTITY column, but also any GENERATED computed column, or any other column. The getGeneratedKeys() returns a ResultSet with one or more columns. This contains one row per inserted row, and can therefore return all the generated columns for a multi-row insert.

There are three methods of specifying which generated keys should be returned. The first method does not specify the columns of the table. With this method, the returned ResultSet will have a column for each column of the table that is defined as GENERATED ... AS IDENTITY or GENERATED ... AS (<expression>). The two other methods require the user to specify which columns should be returned, either by column indexes, or by column names. With these methods, there is no restriction on which columns of the inserted values to be returned. This is especially useful when some columns have a default clause which is a function, or when there are BEFORE triggers on the table that may provide the inserted value for some of the columns.

Update Statement

UPDATE

update statement: searched

<update statement: searched> ::= UPDATE <target table> [ [ AS ] <correlation name> ] SET <set clause list> [ WHERE <search condition> ]

Update rows of a table. An UPDATE statement selects rows from the <target table> using an implicit SELECT statement formed in the following manner:

SELECT * FROM <target table> [ [ AS ] <correlation name> ] [ WHERE <search condition> ]

Then it applies the SET <set clause list> expression to each selected row.

If the implicit SELECT returns no rows, no update takes place. When used in JDBC, the number of rows returned by the implicit SELECT is returned as the update count.

If there are FOREIGN KEY constraints on other tables that reference the subject table, and the FOREIGN KEY constraints have referential actions, then rows from those other tables that reference the updated rows are updated, according to the specified referential actions.

The rows that are updated are checked against all the constraints that have been declared on the table. The whole UPDATE operation fails if any row violates any constraint.

set clause list

set clause list

<set clause list> ::= <set clause> [ { <comma> <set clause> }... ]

<set clause> ::= <multiple column assignment> | <set target> <equals operator> <update source>

<multiple column assignment> ::= <set target list> <equals operator> <assigned row>

<set target list> ::= <left paren> <set target> [ { <comma> <set target> }... ] <right paren>

<assigned row> ::= <contextually typed row value expression>

<set target> ::= <column name>

<update source> ::= <value expression> | <contextually typed value specification>

Specify a list of assignments. This is used in UPDATE, MERGE and SET statements to assign values to a scalar or row target.

Apart from setting a whole target to a value, a SET statement can set individual elements of an array to new values. The last example below shows this form of assignment to the array in the column named B.

In the examples given below, UPDATE statements with single and multiple assignments are shown. Note in the third example, a SELECT statement is used to provide the update values for columns A and C, while the update value for column B is given separately. The SELECT statement must return exactly one row . In this example the SELECT statement refers to the existing value for column C in its search condition.

 UPDATE T SET A = 5 WHERE ...
 UPDATE T SET (A, B) = (1, NULL) WHERE ...
 UPDATE T SET (A, C) = (SELECT X, Y FROM U WHERE Z = C), B = 10 WHERE ...
 UPDATE T SET A = 3, B[3] = 'warm'

Merge Statement

MERGE INTO

merge statement

<merge statement> ::= MERGE INTO <target table> [ [ AS ] <merge correlation name> ] USING <table reference> ON <search condition> <merge operation specification>

<merge correlation name> ::= <correlation name>

<merge operation specification> ::= <merge when clause>...

<merge when clause> ::= <merge when matched clause> | <merge when not matched clause>

<merge when matched clause> ::= WHEN MATCHED THEN <merge update specification>

<merge when not matched clause> ::= WHEN NOT MATCHED THEN <merge insert specification>

<merge update specification> ::= UPDATE SET <set clause list>

<merge insert specification> ::= INSERT [ <left paren> <insert column list> <right paren> ] [ <override clause> ] VALUES <merge insert value list>

<merge insert value list> ::= <left paren> <merge insert value element> [ { <comma> <merge insert value element> }... ] <right paren>

<merge insert value element> ::= <value expression> | <contextually typed value specification>

Update rows, or insert new rows into the <target table>. The MERGE statement uses a second table, specified by <table reference>, to determine the rows to be updated or inserted. It is possible to use the statement only to update rows or to insert rows, but usually both update and insert are specified.

The <search condition> matches each row of the <table reference> with each row of the <target table>. If the two rows match then the UPDATE clause is used to update the matching row of the target table. Those rows of <table reference> that have no matching rows are then used to insert new rows into the <target table>. Therefore, a MERGE statement can update between 0 and all the rows of the <target table> and can insert between 0 and the number of the rows in <table reference> into the <target table>. If any row in the <target table> matches more than one row in <table reference> a cardinality error is raised. On the other hand, several rows in the <target table> can match a single row in <table reference> without any error. The constraints and referential actions specified on the database tables are enforced the same way as for an update and an insert statement.

The MERGE statement can be used with only the WHEN NOT MATCHED clause as a conditional INSERT statement that inserts a row if no existing rows match a condition.

In the first example below, the table originally contains two rows for different furniture. The <table reference> is the (VALUES(1, 'conference table'), (14, 'sofa'), (5, 'coffee table')) expression, which evaluates to a table with 3 rows. When the x value for a row matches an existing row, then the existing row is updated. When the x value does not match, the row is inserted. Therefore one row of table t is updated from 'dining table' to 'conference table', and two rows are inserted into table t. The second example uses a SELECT statement as the source of the values for the MERGE.

In the third example, a new row in inserted into the table only when the primary key for the new row does not exist. This example uses parameters and should be executed as a JDBC PreparedStatement. The parameter is cast as INTEGER because the MERGE statement does not use determine the types of values in the USING clause.

 CREATE TABLE t (id INT PRIMARY KEY, description VARCHAR(100))
 INSERT INTO t VALUES (1, 'dining table'), (2, 'deck chair')

 MERGE INTO t USING (VALUES(1, 'conference table'), (14, 'sofa'), (5, 'coffee table')) 
   AS vals(x,y) ON t.id = vals.x
   WHEN MATCHED THEN UPDATE SET t.description = vals.y
   WHEN NOT MATCHED THEN INSERT VALUES vals.x, vals.y

 MERGE INTO t USING (SELECT * FROM tt WHERE acol = 2) AS vals(x,y) ON t.id = vals.x
   WHEN MATCHED THEN UPDATE SET t.description = vals.y
   WHEN NOT MATCHED THEN INSERT VALUES vals.x, vals.y

 MERGE INTO t USING (VALUES(CAST(? AS INT))) AS vals(x) ON t.id = vals.x
   WHEN NOT MATCHED THEN INSERT VALUES vals.x, ?

Diagnostics and State

HyperSQL supports some SQL statements, expressions, functions and Java methods that report on the most recently executed statement.

The IDENTITY() function returns the last inserted identity value for the current session.

The GET DIAGNOSTICS statement is supported to a limited extent. The built-in function DIAGNOSTICS() is an alternative. These are normally used in SQL/PSM routines to check the result of the last data update operation.

GET DIAGNOSTICS

get diagnostics statement

<get diagnostics statement> ::= GET DIAGNOSTICS <simple target value specification> = ROW_COUNT

The <simple target value specification> is a session variable, or a routine variable or OUT parameter.

The keyword ROW_COUNT specifies the row count returned by the last executed statement. For INSERT, UPDATE, DELETE and MERGE statements, this is the number of rows affected by the statement. This is the same value as returned by JDBC executeUpdate() methods. For all other statements, zero is returned.

The value of ROW_COUNT is stored in the specified target.

In future versions, more options will be supported for diagnostics values.

Chapter 8. SQL-Invoked Routines

Fred Toussi

The HSQL Development Group

$Revision: 5271 $

Copyright 2010-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

SQL-invoked routines are functions and procedures called from SQL. HyperSQL 2.0 supports routines conforming to two parts of the SQL Standard. Routines written in the SQL language are supported in conformance to SQL/PSM (Persistent Stored Modules) specification. Routines written in Java are supported in broad conformance to SQL/JRT specification. In addition, HyperSQL's previous non-standard support for calling Java routines without prior method definition is retained and enhanced in the latest version by extending the SQL/JRT specification.

HyperSQL also supports user defined aggregate functions written in the SQL language or Java. This feature is an extension to the SQL Standard.

SQL-invoked routines are schema objects. Naming and referencing follows conventions common to all schema objects. The same routine name can be defined in two different schemas and used with schema-qualified references.

A routine is either a procedure or a function.

A function:

  • is defined with CREATE FUNCTION

  • always returns a single value or a single table

  • does not modify the data in the database

  • is used as part of an SQL statement

  • can have parameters

  • can be polymorphic

A procedure:

  • is defined with CREATE PROCEDURE

  • can return zero to multiple values or result sets

  • can modify the data in the database

  • is called separately, using the CALL statement

  • can have parameters

  • can be polymorphic

Definition of routine signature and characteristics, name resolution and invocation are all implemented uniformly for routines written in SQL or Java.

Routine Definition

SQL-Invoked Routines, whether PSM or JRT, are defined using a SQL statement with the same syntax. The part that is different is the <routine body> which consists of SQL statements in PSM routines or a reference to a Java method in JRT routines.

Details of Routine definition are discussed in this section. You may start by reading the next two sections which provide several examples before reading this section for the details.

Routine definition has several mandatory or optional clauses. The complete BNF supported by HyperSQL and the remaining clauses are documented in this section.

CREATE FUNCTION

CREATE PROCEDURE

routine definition

Routine definition is similar for procedures and functions. A function definition has the mandatory <returns clause> which is discussed later. The description given so far covers the essential elements of the specification with the BNF given below.

<schema procedure> ::= CREATE PROCEDURE <schema qualified routine name> <SQL parameter declaration list> <routine characteristics> <routine body>

<schema function> ::= CREATE FUNCTION <schema qualified routine name> <SQL parameter declaration list> <returns clause> <routine characteristics> <routine body>

Parameter declaration list has been described above. For SQL/JRT routines, the <SQL parameter name> is optional while for SQL/PSM routines, it is required. If the <parameter mode> of a parameter is OUT or INOUT, it must be specified. The BNF is given below:

<SQL parameter declaration list> ::= <left paren> [ <SQL parameter declaration> [ { <comma> <SQL parameter declaration> }... ] ] <right paren>

<SQL parameter declaration> ::= [ <parameter mode> ] [ <SQL parameter name> ] <parameter type>

<parameter mode> ::= IN | OUT | INOUT

<parameter type> ::= <data type>

Return Value and Table Functions

RETURNS

returns clause

The <returns clause> specifies the type of the return value of a function (not a procedure). For all SQL/PSM functions and ordinary SQL/JRT functions, this is simply a type definition which can be a built-in type, a DOMAIN type or a DISTINCT type, or alternatively, a TABLE definition. For example, RETURNS INTEGER.

For a SQL/JRT function, it is possible to define a <returns table type> for a Java method that returns a java.sql.ResultSet object. Such SQL/JRT functions are called table functions. Table functions are used differently from normal functions. A table function can be used in an SQL query expression exactly where a normal table or view is allowed. At the time of invocation, the Java method is called and the returned ResultSet is transformed into an SQL table. The column types of the declared TABLE must match those of the ResultSet, otherwise an exception is raised at the time of invocation.

If a <returns table type> is defined for an SQL/PSM function, the following expression is used inside the function to return a table: RETURN TABLE ( <query expression> ); In the example blow, a table with two columns is returned.

 RETURN TABLE ( SELECT a, b FROM atable WHERE e = 10 );

Functions that return a table are designed to be used in SELECT statements using the TABLE keyword to form a joined table.

When a JDBC CallableStatement is used to CALL the function, the table returned from the function call is returned and can be accessed with the getResultSet() method of the CallableStatement.

<returns clause> ::= RETURNS <returns type>

<returns type> ::= <returns data type> | <returns table type>

<returns table type> ::= TABLE <table function column list>

<table function column list> ::= <left paren> <table function column list element> [ { <comma> <table function column list element> } ... ] <right paren>

<table function column list element> ::= <column name> <data type>

<returns data type> ::= <data type>

routine body

routine body

Routine body is either one or more SQL statements or a Java reference. The user that defines the routine by issuing the CREATE FUNCTION or CREATE SCHEMA command must have the relevant access rights to all tables, sequences, routines, etc. that are accessed by the routine. If another user is given EXECUTE privilege on the routine, then there are two possibilities, depending on the <rights clause>. This clause refers to the access rights that are checked when a routine is invoked. The default is SQL SECURITY DEFINER, which means access rights of the definer are used; therefore no extra checks are performed when the other user invokes the routine. The alternative SQL SECURITY INVOKER means access rights on all the database objects referenced by the routine are checked for the invoker. This alternative is not supported by HyperSQL.

<routine body> ::= <SQL routine spec> | <external body reference>

<SQL routine spec> ::= [ <rights clause> ] <SQL routine body>

<rights clause> ::= SQL SECURITY INVOKER | SQL SECURITY DEFINER

SQL routine body

SQL routine body

The routine body of a an SQL routine consists of an statement.

<SQL routine body> ::= <SQL procedure statement>

EXTERNAL NAME

external body reference

External name specifies the qualified name of the Java method associated with this routine. Early releases of HyperSQL 2.0 only supports Java methods within the classpath. The <external Java reference string> is a quoted string which starts with CLASSPATH: and is followed by the Java package, class and method names separated with dots. HyperSQL does not currently support the optional <Java parameter declaration list>.

<external body reference> ::= EXTERNAL NAME <external Java reference string>

<external Java reference string> ::= <jar and class name> <period> <Java method name> [ <Java parameter declaration list> ]

Routine Characteristics

The <routine characteristics> clause covers several sub-clauses

<routine characteristics> ::= [ <routine characteristic>... ]

<routine characteristic> ::= <language clause> | <parameter style clause> | SPECIFIC <specific name> | <deterministic characteristic> | <SQL-data access indication> | <null-call clause> | <returned result sets characteristic> | <savepoint level indication>

LANGUAGE

language clause

The <language clause> refers to the language in which the routine body is written. It is either SQL or Java. The default is SQL, so JAVA must be specified for SQL/JRT routines.

<language clause> ::= LANGUAGE <language name>

<language name> ::= SQL | JAVA

The parameter style is not allowed for SQL routines. It is optional for Java routines and, in HyperSQL, the only value allowed is JAVA.

<parameter style> ::= JAVA

SPECIFIC NAME

specific name

The SPECIFIC <specific name> clause is optional but the engine will creates an automatic name if it is not present. When there are several versions of the same routine, the <specific name> is used in schema manipulation statements to drop or alter a specific version. The <specific name> is a user-defined name. It applies to both functions and procedures. In the examples below, a specific name is specified for each function.

 CREATE FUNCTION an_hour_before_or_now(t TIMESTAMP)
   RETURNS TIMESTAMP
   NO SQL
   LANGUAGE JAVA PARAMETER STYLE JAVA
   SPECIFIC an_hour_before_or_now_with_timestamp
   EXTERNAL NAME 'CLASSPATH:org.npo.lib.nowLessAnHour'

 CREATE FUNCTION an_hour_before_max (e_type INT)
   RETURNS TIMESTAMP SPECIFIC an_hour_before_max_with_int
   RETURN (SELECT MAX(event_time) FROM atable WHERE event_type = e_type) - 1 HOUR

DETERMINISTIC

deterministic characteristic

The <deterministic characteristic> clause indicates that a routine is deterministic or not. Deterministic means the routine does not reference random values, external variables, or time of invocation. The default is NOT DETERMINISTIC. It is essential to declare this characteristics correctly for an SQL/JRT routine, as the engine does not know the contents of the Java code, which could include calls to methods returning random or time sensitive values.

<deterministic characteristic> ::= DETERMINISTIC | NOT DETERMINISTIC

SQL DATA access

SQL DATA access characteristic

The <SQL-data access indication>  clause indicates the extent to which a routine interacts with the database or the data stored in the database tables in different schemas (SQL DATA).

NO SQL means no SQL command is issued in the routine body and can be used only for SQL/JRT functions.

CONTAINS SQL means some SQL commands are used, but they do not read or modify the SQL data. READS SQL DATA and MODIFIES SQL DATA are self explanatory.

A CREATE PROCEDURE definition can use MODIFIES SQL DATA. This is not allowed in CREATE FUNCTION. Note that a PROCEDURE or a FUNCTION may have internal tables or return a table which are populated by the routine's statements. These tables are not considered SQL DATA, therefore there is no need to specify MODIFIES SQL DATA for such routines.

<SQL-data access indication> ::= NO SQL | CONTAINS SQL | READS SQL DATA | MODIFIES SQL DATA

NULL INPUT

null call clause

Null Arguments

The <null-call clause> is used only for functions. If a function returns NULL when any of the calling arguments is null, then by specifying RETURNS NULL ON NULL INPUT, calls to the function are known to be redundant and do not take place when an argument is null. This simplifies the coding of the SQL/JRT Java methods and improves performance at the same time.

<null-call clause> ::= RETURNS NULL ON NULL INPUT | CALLED ON NULL INPUT

SAVEPOINT LEVEL

transaction impact

The <savepoint level indication> is used only for procedures and refers to the visibility of existing savepoints within the body of the procedure. If NEW SAVEPOINT LEVEL is specified, savepoints that have been declared prior to calling the procedure become invisible within the body of the procedure. HyperSQL’s implementation accepts only NEW SAVEPOINT LEVEL, which must be specified.

<savepoint level indication> ::= NEW SAVEPOINT LEVEL | OLD SAVEPOINT LEVEL

DYNAMIC RESULT SETS

returned result sets characteristic

The <returned result sets characteristic> is used with SQL/PSM and SQL/JRT procedures (not with functions). The maximum number of result sets that a procedure may return can be specified with the clause below. The default is zero. If you want your procedure to return result sets, you must specify the maximum number of result sets that your procedure may return. Details are discussed in the next sections.

<returned result sets characteristic> ::= DYNAMIC RESULT SETS <maximum returned result sets>

SQL Language Routines (PSM)

The PSM (Persistent Stored Module) specification extends the SQL language with structures and control statements such as conditional and loop statements. Both SQL Function and SQL procedure bodies use the same syntax, with minor exceptions.

The routine body is a SQL statement. In its simplest form, the body is a single SQL statement. A simple example of a function is given below:

 CREATE FUNCTION an_hour_before (t TIMESTAMP)
   RETURNS TIMESTAMP
   RETURN t - 1 HOUR

An example of the use of the function in an SQL statement is given below:

 SELECT an_hour_before(event_timestamp) AS notification_timestamp, event_name FROM events;

A simple example of a procedure is given below:

 CREATE PROCEDURE new_customer(firstname VARCHAR(50), lastname VARCHAR(50))
   MODIFIES SQL DATA
   INSERT INTO CUSTOMERS VALUES (DEFAULT, firstname, lastname, CURRENT_TIMESTAMP)

The procedure inserts a row into an existing table with the definition given below:

 CREATE TABLE customers(id INTEGER GENERATED BY DEFAULT AS IDENTITY, firstname VARCHAR(50), lastname VARCHAR(50), added TIMESTAMP);

An example of the use of the procedure is given below:

 CALL new_customer('JOHN', 'SMITH');

The routine body is often a compound statement. A compound statement can contain one or more SQL statements, which can include control statements, as well as nested compound statements.

Please note carefully the use of <semicolon>, which is required at the end of some statements but not accepted at the end of others.

Advantages and Disadvantages

SQL Language Routines (PSM) have certain advantages over Java Language Routines (SQL/JRT) and a couple of disadvantages.

  • SQL language routines (PSM) do not rely on custom Java classes to be present on the classpath. The databases that use them are therefore more portable.

  • For a routine that accesses SQL DATA, all the SQL statements in an SQL routine are known and monitored by the engine. The engine will not allow a table, routine or sequence that is referenced in an SQL routine to be dropped, or its structure modified in a way that will break the routine execution. The engine does not keep this information about a Java routine.

  • Because the statements in an SQL routine are known to the engine, the execution of an SQL routine locks all the database objects it needs to access before the actual execution. With Java routines, locks are obtained during execution and this may cause additional delays in multi threaded access to the database.

  • For routines that do not access SQL DATA, Java routines (SQL/JRT) may be faster if they perform extensive calculations.

  • Only Java routines can access external programs and resources directly.

Routine Statements

The following SQL Statements can be used only in routines. These statements are covered in this section.

<handler declaration>

<table variable declaration>

<variable declaration>

<declare cursor>

<assignment statement>

<compound statement>

<case statement>

<if statement>

<while statement>

<repeat statement>

<for statement>

<loop statement>

<iterate statement

<leave statement>

<signal statement>

<resignal statement>

<return statement>

<select statement: single row>

<open statement>

The following SQL Statements can be used in procedures but not in generally in functions (they can be used in functions only to change the data in a local table variable) . These statements are covered in other chapters of this Guide.

<call statement>

<delete statement>

<insert statement>

<update statement>

<merge statement>

As shown in the examples below, the formal parameters and the variables of the routine can be used in statements, similar to the way a column reference is used.

Compound Statement

A compound statement is enclosed in a BEGIN / END block with optional labels. It can contain one or more <SQL variable declaration>, <declare cursor> or <handler declaration> before at least one SQL statement. The BNF is given below:

<compound statement> ::= [ <beginning label> <colon> ] BEGIN [[NOT] ATOMIC]

[{<SQL variable declaration> <semicolon>} ...]

[{<declare cursor> <semicolon>} ...]

[{<handler declaration> <semicolon>}...]

{<SQL procedure statement> <semicolon>} ...

END [ <ending label> ]

An example of a simple compound statement body is given below. It performs the common task of inserting related data into two table. The IDENTITY value that is automatically inserted in the first table is retrieved using the IDENTITY() function and inserted into the second table. Other examples show more complex compound statements.

 CREATE PROCEDURE new_customer(firstname VARCHAR(50), lastname  VARCHAR(50), address VARCHAR(100))
   MODIFIES SQL DATA
     BEGIN ATOMIC
     INSERT INTO customers VALUES (DEFAULT, firstname, lastname, CURRENT_TIMESTAMP);
     INSERT INTO addresses VALUES (DEFAULT, IDENTITY(), address);
   END

Table Variables

A <table variable declaration> defines the name and columns of a local table, that can be used in the routine body. The table cannot have constraints. Table variable declarations are made before scalar variable declarations.

 BEGIN ATOMIC
   DECLARE TABLE temp_table (col_a INT, col_b VARCHAR(20);
   DECLARE temp_id INTEGER;
   -- more statements
 END

Variables

A <variable declaration> defines the name and data type of the variable and, optionally, its default value. In the next example, a variable is used to hold the IDENTITY value. In addition, the formal parameters of the procedure are identified as input parameters with the use of the optional IN keyword. This procedure does exactly the same job as the procedure in the previous example.

 CREATE PROCEDURE new_customer(IN firstname VARCHAR(50), IN lastname VARCHAR(50), IN address VARCHAR(100))
     MODIFIES SQL DATA
   BEGIN ATOMIC
     DECLARE temp_id INTEGER;
     INSERT INTO CUSTOMERS VALUES (DEFAULT, firstname, lastname, CURRENT_TIMESTAMP);
     SET temp_id = IDENTITY();
     INSERT INTO ADDRESSES VALUES (DEFAULT, temp_id, address);
   END

The BNF for variable declaration is given below:

DECLARE variable

SQL variable declaration

<SQL variable declaration> ::= DECLARE <variable name list> <data type> [DEFAULT <default value>]

<variable name list> ::= <variable name> [ { <comma> <variable name> }... ]

Examples of variable declaration are given below. Note that in a DECLARE statement with multiple comma-separated variable names, the type and the default value applies to all the variables in the list:

 BEGIN ATOMIC
   DECLARE temp_zero DATE;
   DECLARE temp_one, temp_two INTEGER DEFAULT 2;
   DECLARE temp_three VARCHAR(20) DEFAULT 'no name';
   -- more statements ...
   SET temp_zero = DATE '2010-03-18';
   SET temp_two = 5;
   -- more statements ...
 END

Cursors

A <declare cursor> statement is used to declare a SELECT statement. The current usage of this statement in early versions of HyperSQL 2.0 is exclusively to return a result set from a procedure. The result set is returned to the JDBC CallableStatement object that calls the procedure. The getResultSet() method of CallableStatement is then used to retrieve the JDBC ResultSet.

In the <routine definition>, the DYNAMIC RESULT SETS clause must be used to specify a value above zero. The DECLARE CURSOR statement is used after any variable declaration in compound statement block. The SELECT statement should be followed with FOR READ ONLY to avoid possible error messages. The <open statement> is then executed for the cursor at the point where the result set should be populated.

After the procedure is executed with a JDBC CallableStatement execute() method, all the result sets that were opened are returned to the JDBC CallableStatement.

Calling getResultSet() will return the first ResultSet. When there are multiple result sets, the getMoreResults() method of the Callable statement is called to move to the next ResultSet, before getResultSet() is called to return the next ResultSet. See the Data Access and Change chapter on the syntax for declaring the cursor.

 BEGIN ATOMIC
   DECLARE temp_zero DATE;
   DECLARE result CURSOR WITH RETURN FOR SELECT * FROM INFORMATION_SCHEMA.TABLES FOR READ ONLY;
   -- more statements ...
   OPEN result;
 END

Handlers

A <handler declaration> defines the course of action when an exception or warning is raised during the execution of the compound statement. A compound statement may have one or more handler declarations. These handlers become active when code execution enters the compound statement block and remain active in any sub-block and statement within the block. The handlers become inactive when code execution leaves the block.

In the previous example of the new_customer procedure, if an exception is thrown during the execution of either SQL statement, the execution of the compound statement is terminated and the exception is propagated and thrown by the CALL statement for the procedure. All changes made by the procedure are rolled back.

A handler declaration can resolve the thrown exception within the compound statement without propagating it, and allow the execution of the compound statement to continue.

In the example below, the UNDO handler declaration catches any exception that is thrown during the execution of the compound statement inside the BEGIN ... END block. As it is an UNDO handler, all the changes to data performed within the compound statement ( BEGIN ... END block) are rolled back. The procedure then returns without throwing an exception.

 CREATE PROCEDURE new_customer(IN firstname VARCHAR(50), IN lastname VARCHAR(50), IN address VARCHAR(100))
     MODIFIES SQL DATA
   label_one: BEGIN ATOMIC
     DECLARE temp_id INTEGER;
     DECLARE UNDO HANDLER FOR SQLEXCEPTION;
     INSERT INTO CUSTOMERS VALUES (DEFAULT, firstname, lastname, CURRENT_TIMESTAMP);
     SET temp_id = IDENTITY();
     INSERT INTO ADDRESSES VALUES (DEFAULT, temp_id, address);
   END

Other types of hander are CONTINUE and EXIT handlers. A CONTINUE handler ignores any exception and proceeds to the next statement in the block. An EXIT handler terminates execution without undoing the data changes performed by the previous (successful) statements.

The conditions can be general conditions, or specific conditions.

Among general conditions that can be specified, SQLEXCEPTION covers all exceptions, SQLWARNING covers all warnings, while NOT FOUND covers the not-found condition, which is raised when a DELETE, UPDATE, INSERT or MERGE statement completes without actually affecting any row.

Alternatively, one or more specific conditions can be specified (separated with commas) which apply to specific exceptions or warnings or classes or exceptions or warnings. A specific condition is specified with SQLSTATE <value>, for example SQLSTATE 'W_01003' specifies the warning raised after a SQL statement is executed which contains an aggregate function which encounters a null value during execution. An example is given below which activates the handler when either of the two warnings is raised:

 DECLARE UNDO HANDLER FOR SQLSTATE 'W_01003', 'W_01004';

The BNF for <handler declaration> is given below:

DECLARE HANDLER

declare handler statement

<handler declaration> ::= DECLARE {UNDO | CONTINUE | EXIT} HANDLER FOR {SQLEXCEPTION | SQLWARNING | NOT FOUND} | { SQLSTATE <state value> [, ...]} [<SQL procedure statement>];

A handler declaration may specify an <SQL procedure statement> to be performed when the handler is activated. In the example below the handler performs the UNDO as in the previous example then inserts the (invalid) data into a separate table.

 CREATE PROCEDURE new_customer(IN firstname VARCHAR(50), IN lastname VARCHAR(50), IN address VARCHAR(100))
     MODIFIES SQL DATA
   label_one: BEGIN ATOMIC
     DECLARE temp_id INTEGER;
     DECLARE UNDO HANDLER FOR SQLEXCEPTION
     INSERT INTO invalid_customers VALUES(firstanme, lastname, address);
     -- last statement is part of the handler
     INSERT INTO CUSTOMERS VALUES (DEFAULT, firstname, lastname, CURRENT_TIMESTAMP);
     SET temp_id = IDENTITY();
     INSERT INTO ADDRESSES VALUES (DEFAULT, temp_id, address);
   END

The <SQL procedure statement> in the handler declaration is required by the SQL Standard but is optional in HyperSQL. If the execution of the <SQL procedure statement> specified in the handler declaration throws an exception itself, then it is handled by the handlers that are currently active at an enclosing (outer) BEGIN ... END block. The <SQL procedure statement> can itself be a compound statement with its own handlers.

When a handler handles an exception condition such as the general SQLEXCEPTION or some specific SQLSTATE, any changes made by the statement that caused the exception will be rolled back. For example, execution of a single update statement that modifies several rows will not change any row if an exception occurs during the update of one of the rows. The handler action affects the changes made by statements that were executed successfully before the exception occured.

Actions performed by different types of handler are listed below:

  • An UNDO handler rolls back all the data changes within the BEGIN ... END block which contains the handler declaration. The execution of the BEGIN ... END block is considered complete. If an <SQL procedure statement> is specified, it is executed after the roll back.

  • A CONTINUE handler does not roll back the data changes. It continues execution as if the last statement was successful. If an <SQL procedure statement> is specified, it is executed before continuing execution.

  • An EXIT handler does not roll back the data changes. It aborts the execution of the BEGIN ... END block which contains the handler declaration. The execution of the BEGIN ... END block is considered complete, but unlike the UNDO handler the actions are not rolled back. If an <SQL procedure statement> is specified, it is executed before aborting.

Assignment Statement

The SET statement is used for assignment. It can be used flexibly with rows or single values. The BNF is given below:

<assignment statement> ::= <singleton variable assignment> | <multiple variable assignment>

<singleton variable assignment> ::= SET <assignment target> <equals operator> <assignment source>

<multiple variable assignment> ::= SET (<variable or parameter>, ...) = <row value expression>

In the example below, the result of the SELECT is assigned to two OUT or INOUT arguments. The SELECT must return one row. If it returns more than one, an exception is raised. If it returns no row, no change is made to ARG1 and ARG2.

 SET (arg1, arg2) = (SELECT col1, col2 FROM atable WHERE id = 10);

In the example below, the result of a function call is assigned to VAR1.

 SET var1 = SQRT(var2);

Select Statement : Single Row

A special form of SELECT can also be used for assigning values from a query to one or more arguments or variables. This works similar to a SET statement that has a SELECT statement as the source.

SELECT : SINGLE ROW

select statement: single row

<select statement: single row> ::= SELECT [ <set quantifier> ] <select list> INTO <select target list> <table expression>

<select target list> ::= <target specification> [ { <comma> <target specification> }... ]

Retrieve values from a specified row of a table and assign the fields to the specified targets. The example below has an identical effect to the example of SET statement given above.

SELECT col1, col2 INTO arg1, arg2 FROM atable WHERE id = 10;

Formal Parameters

Each parameter of a procedure can be defined as IN, OUT or INOUT. An IN parameter is an input to the procedure and is passed by value. The value cannot be modified inside the procedure body. An OUT parameter is a reference for output. An INOUT parameter is a reference for both input and output. An OUT or INOUT parameter argument is passed by reference, therefore only a dynamic parameter argument or a variable within an enclosing procedure can be passed for it. The assignment statement is used to assign a value to an OUT or INOUT parameter.

In the example below, the procedure is declared with an OUT parameter. It assigns the auto-generated IDENTITY value from the INSERT statement to the OUT argument.

 CREATE PROCEDURE new_customer(OUT newid INT, IN firstname VARCHAR(50), IN lastname VARCHAR(50), IN address VARCHAR(100))
   MODIFIES SQL DATA
   BEGIN ATOMIC
     DECLARE temp_id INTEGER;
     INSERT INTO CUSTOMERS VALUES (DEFAULT, firstname, lastname, CURRENT_TIMESTAMP);
     SET temp_id = IDENTITY();
     INSERT INTO ADDRESSES VALUES (DEFAULT, temp_id, address);
     SET newid = temp_id;
   END

In the SQL session, or in the body of another stored procedure, a variable must be assigned to the OUT parameter. After the procedure call, this variable will hold the new identity value that was generated inside the procedure. If the procedure is called directly, using the JDBC CallableStatement interface, then the value of the first, OUT argument can be retrieved with a call to getInt(1)after calling the execute() method.

In the example below, a session variable, the_new_id is declared. After the call to new_customer, the value for the identity is stored in the_new_id variable. This is returned via the next CALL statement. Alternatively, the_new_id can be used as an argument to another CALL statement.

 DECLARE the_new_id INT DEFAULT NULL;
 CALL new_customer(the_new_id, 'John', 'Smith', '10 Parliament Square'); 
 CALL the_new_id;

Iterated Statements

Various iterated statements can be used in routines. In these statements, the <SQL statement list> consists of one or more SQL statements. The <search condition> can be any valid SQL expression of BOOLEAN type.

LOOP

loop statement

<loop statement> ::= [ <beginning label> <colon> ] LOOP <SQL statement list> END LOOP [ <ending label> ]

WHILE

while statement

<while statement> ::= [ <beginning label> <colon> ] WHILE <search condition> DO <SQL statement list> END WHILE [ <ending label> ]

REPEAT

repeat statement

<repeat statement> ::= [ <beginning label> <colon> ]

REPEAT <SQL statement list> UNTIL <search condition> END REPEAT [ <ending label>

In the example below, a multiple rows are inserted into a table in a WHILE loop:

 loop_label: WHILE my_var > 0 DO
   INSERT INTO CUSTOMERS VALUES (DEFAULT, my_var);
   SET my_var = my_var - 1;
   IF my_var = 10 THEN SET my_var = 8; END IF;
   IF my_var = 22 THEN LEAVE loop_label; END IF;
 END WHILE loop_label;

Iterated FOR Statement

The <for statement> is similar to other iterated statement, but it is always used with a cursor declaration to iterate over the rows of the result set of the cursor and perform operations using the values of each row.

FOR

for statement

<for statement> ::= [ <beginning label> <colon> ] FOR <query expression> DO <SQL statement list> END FOR [ <ending label> ]

The <query expression> is a SELECT statement. When the FOR statement is executed, the query expression is executed first and the result set is formed. Then for each row of the result set, the <SQL statement list> is executed. What is special about the FOR statement is that all the columns of the current row can be accessed by name in the statements in the <SQL statement list>. The columns are read only and cannot be updated. For example, if the column names for the select statement are ID, FIRSTNAME, LASTNAME, then these can be accessed as a variable name. The column names must be unique and not equivalent to any parameter or variable name in scope.

The FOR statement is useful for computing values over multiple rows of the result set, or for calling a procedure for some row of the result set.

In the example below, the procedure uses a FOR statement to iterate over the rows for a customer with lastname equal to name_p. No action is performed for the first row, but for all the subsequent rows, the row is deleted from the table.

Note the following: The result set for the SELECT statement is built only once, before processing the statements inside the FOR block begins. For all the rows of the SELECT statement apart from the first row, the row is deleted from the customer table. The WHERE condition uses the automatic variable id, which holds the customer.id value for the current row of the result set, to delete the row. The procedure updates the val_p argument and when it returns, the val_p represents the total count of rows with the given lastname before the duplicates were deleted.

 CREATE PROCEDURE test_proc(INOUT val_p INT, IN lastname_p VARCHAR(20)) 
 MODIFIES SQL DATA
 BEGIN ATOMIC
   SET val_p = 0;
   for_label: FOR SELECT * FROM customer WHERE lastname = lastname_p DO
     IF  val_p > 0 THEN
       DELETE FROM customer WHERE customer.id = id;
     END IF;
     SET val_p = val_p + 1;
   END FOR for_label;
 END

Conditional Statements

There are two types of CASE ... WHEN statement and the IF ... THEN statement.

CASE WHEN

case when statement

The simple case statement uses a <case operand> as the predicand of one or more predicates. For the right part of each predicate, it specifies one or more SQL statements to execute if the predicate evaluates TRUE. If the ELSE clause is not specified, at least one of the search conditions must be true, otherwise an exception is raised.

<simple case statement> ::= CASE <case operand> <simple case statement when clause>... [ <case statement else clause> ] END CASE

<simple case statement when clause> ::= WHEN <when operand list> THEN <SQL statement list>

<case statement else clause> ::= ELSE <SQL statement list>

A skeletal example is given below. The variable var_one is first tested for equality with 22 or 23 and if the test evaluates to TRUE, then the INSERT statement is performed and the statement ends. If the test does not evaluate to TRUE, the next condition test, which is an IN predicate, is performed with var_one and so on. The statement after the ELSE clause is performed if none the previous tests returns TRUE.

CASE var_one
  WHEN 22, 23 THEN INSERT INTO t_one ...;
  WHEN IN (2, 4, 5) THEN DELETE FROM t_one WHERE ...;
  ELSE UPDATE t_one ...;
  END CASE

The searched case statement uses one or more search conditions, and for each search condition, it specifies one or more SQL statements to execute if the search condition evaluates TRUE. An exception is raised if there is no ELSE clause and none of the search conditions evaluates TRUE.

<searched case statement> ::= CASE <searched case statement when clause>... [ <case statement else clause> ] END CASE

<searched case statement when clause> ::= WHEN <search condition> THEN <SQL statement list>

The example below is partly a rewrite of the previous example, but a new condition is added:

 CASE WHEN var_one = 22 OR var_one = 23 THEN INSERT INTO t_one ...;
   WHEN var_one IN (2, 4, 5) THEN DELETE FROM t_one WHERE ...;
   WHEN var_two IS NULL THEN UPDATE t_one ...;
   ELSE UPDATE t_one ...;
   END CASE

IF

if statement

The if statement is very similar to the searched case statement. The difference is that no exception is raised if there is no ELSE clause and no search condition evaluates TRUE.

<if statement> ::= IF <search condition> <if statement then clause> [ <if statement elseif clause>... ] [ <if statement else clause> ] END IF

<if statement then clause> ::= THEN <SQL statement list>

<if statement elseif clause> ::= ELSEIF <search condition> THEN <SQL statement list>

<if statement else clause> ::= ELSE <SQL statement list>

Return Statement

The RETURN statement is required and used only in functions. The body of a function is either a RETURN statement, or a compound statement that contains a RETURN statement.

The return value of a FUNCTION can be assigned to a variable, or used inside an SQL statement.

An SQL/PSM function or an SQL/JRT function can return a single result when the function is defined as RETURNS TABLE ( .. )

To return a table from a SELECT statement, you should use a return statement such as RETURN TABLE( SELECT ...) in an SQL/PSM function. For an SQL/JRT function, the Java method should return a JDBCResultSet instance.

To call a function from JDBC, use a java.sql.CallableStatement instance. The getResultSet() call can be used to access the ResultSet returned from a function that returns a result set. If the function returns a scalar value, the returned result has a single column and a single row which contains the scalar returned value.

RETURN

return statement

<return statement> ::= RETURN <return value>

<return value> ::= <value expression> | NULL

Return a value from an SQL function. If the function is defined as RETURNS TABLE, then the value is a TABLE expression such as RETURN TABLE(SELECT ...) otherwise, the value expression can be any scalar expression. In the examples below, the same function is written with or without a BEGIN END block. In both versions, the RETURN value is a scalar expression.

 CREATE FUNCTION an_hour_before_max (e_type INT)
   RETURNS TIMESTAMP
   RETURN (SELECT MAX(event_time) FROM atable WHERE event_type = e_type) - 1 HOUR

 CREATE FUNCTION an_hour_before_max (e_type INT)
   RETURNS TIMESTAMP
   BEGIN ATOMIC
     DECLARE max_event TIMESTAMP;
     SET max_event = SELECT MAX(event_time) FROM atable WHERE event_type = e_type;
     RETURN max_event - 1 HOUR;
   END

Control Statements

In addition to the RETURN statement, the following statements can be used in specific contexts.

ITERATE STATEMENT

The ITERATE statement can be used to cause the next iteration of a labelled iterated statement (a WHILE, REPEAT or LOOP statement). It is similar to the "continue" statement in C and Java.

<iterate statement> ::= ITERATE <statement label>

LEAVE STATEMENT

The LEAVE statement can be used to leave a labelled block. When used in an iterated statement, it is similar to the "break" statement is C and Java. But it can be used in compound statements as well.

<leave statement> ::= LEAVE <statement label>

Raising Exceptions

Signal and Resignal Statements allow the routine to throw an exception. If used with the IF or CASE conditions, the exception is thrown conditionally.

SIGNAL

signal statement

The SIGNAL statement is used to throw an exception (or force an exception). When invoked, any exception handler for the given exception is in turn invoked. If there is no handler, the exception is propagated to the enclosing context. In its simplest form, when there is no exception handler for the given exception, routine execution is halted, any change of data is rolled back and the routine throws the exception. By default, the message for the exception is taken from the predefined exception message for the specified SQLSTATE. A custom message can be specified with the optional SET clause.

<signal statement> ::= SIGNAL SQLSTATE <state value> [ SET MESSAGE_TEXT = <character string literal> ]

RESIGNAL

resignal statement

The RESIGNAL statement is used to throw an exception from an exception handler's <SQL procedure statement>, in effect propagating the exception to the enclosing context without further action by the currently active handlers. By default, the message for the exception is taken from the predefined exception message for the specified SQLSTATE. A custom message can be specified with the optional SET clause.

<resignal statement> ::= RESIGNAL SQLSTATE <state value> [ SET MESSAGE_TEXT = <character string literal> ]

Routine Polymorphism

More than one version of a routine can be created.

For procedures, the different versions must have different parameter counts. When the procedure is called, the parameter count determines which version is called.

For functions, the different versions can have the same or different parameter counts. When the parameter count of two versions of a function is the same, the type of parameters must be different. When the function is called, the best matching version of the function is used, according to both the parameter count and parameter types. The return type of different versions of a function can be the same or different.

Two versions of an overloaded function are given below. One version accepts TIMESTAMP while the other accepts TIME arguments.

 CREATE FUNCTION an_hour_before_or_now(t TIMESTAMP)
   RETURNS TIMESTAMP
   IF t > CURRENT_TIMESTAMP THEN
     RETURN CURRENT_TIMESTAMP;
   ELSE
     RETURN t - 1 HOUR;
   END IF

 CREATE FUNCTION an_hour_before_or_now(t TIME)
   RETURNS TIME
   CASE t
     WHEN > CURRENT_TIME THEN
       RETURN CURRENT_TIME;
     WHEN >= TIME'01:00:00' THEN
       RETURN t - 1 HOUR;
     ELSE
       RETURN CURRENT_TIME;
   END CASE

It is perfectly possible to have different versions of the routine as SQL/JRT or SQL/PSM routines.

Returning Data From Procedures

The OUT or INOUT parameters of a PROCEDURE are used to assign simple values to dynamic parameters or to variables in the calling context.

According to the Standard, an SQL/PSM or SQL/JRT procedure may also return result sets to the calling context. These result sets are dynamic in the sense that a procedure may return a different number of result sets or none at all in different invocations. The SQL Standard uses a mechanism called CURSORS for accessing and modifying rows of a result set one by one. This mechanism is necessary when the database is accessed from an external application program. The JDBC ResultSet interface allows this method of access from Java programs and is supported by HyperSQL.

HyperSQL support this method of returning single or multiple result sets from SQL/PSM procedures only via the JDBC CallableStatement interface. Cursors are declared and opened within the body of the procedure. No further operation is performed on the cursors within the procedure. When the execution of the procedure is complete, the cursors become available as Java ResultSet objects via the CallableStatement instance that called the SQL/PSM procedure.

The JDBC CallableStatement class is used with the SQL statement CALL <routine name> ( <argument 1>, ... ) to call procedures (also to call functions). After the call to execute(), the getXXX() methods can be used to retrieve INOUT or OUT arguments after the call. The getMoreResults() method and the getResultSet() method can be used to access the ResultSet(s) returned by a procedure that returns one or more results. If the procedure returns more than one result set, the getMoreResults() call moves to the next result.

In the example below, the procedure inserts a row into the customer table. It then performs the SELECT statement to return the latest inserted row as a result set. Therefore the definition includes the DYNAMIC RESULT SETS 1 clause. You must specify correctly the maximum number of result sets that the procedure may return.

 CREATE PROCEDURE new_customer(firstname VARCHAR(50), lastname VARCHAR(50))
   MODIFIES SQL DATA DYNAMIC RESULT SETS 1
   BEGIN ATOMIC
     DECLARE result CURSOR FOR SELECT * FROM CUSTOMERS WHERE ID = IDENTITY();
     INSERT INTO CUSTOMERS VALUES (DEFAULT, firstname, lastname, CURRENT_TIMESTAMP);
     OPEN result;    
   END

The above procedure is called in Java using a CallableStatement

 Connection conn = ...;
 CallableStatement call = conn.prepareCall("call new_customer(?, ?)");
 call.setString(1, "Paul");
 call.setString(2, "Smith");
 call.execute();
 if (call.getMoreResults())
     ResultSet result = call.getResultSet();

In the example below a procedure has one IN argument and two OUT arguments. The JDBC CallableStatement is used to retrieve the values returned in the OUT arguments.

 CREATE PROCEDURE get_customer(IN id INT, OUT firstname VARCHAR(50), OUT lastname VARCHAR(50)) 
   READS SQL DATA
   BEGIN ATOMIC
     -- this statement uses the id to get firstname and lastname
     SELECT first_name, last_name INTO firstname, lastname FROM customers WHERE cust_id = id;
   END

 Connection conn = ...;
 CallableStatement call = conn.prepareCall("call get_customer(?, ?, ?)");
 call.setInt(1, 121); // only the IN (or INOUT) arguments should be set before the call
 call.execute();
 String firstname = call.getString(2); // the OUT (or INOUT) arguments are retrieved after the call
 String lastname = call.getString(3);

SQL/JRT procedures are discussed in the Java Language Procedures section below. Those routines are called exactly the same way as SQL/PSM procedures, using the JDBC CallableStatement interface.

It is also possible to use a JDBC Statement or PreparedStatement object to call a procedure if the procedure arguments are constant. If the procedure returns one or more result sets, the Statement#getMoreResults() method should be called before retrieving the ResultSet.

Java functions are called from JDBC similar to procedures. With functions, the getMoreResuls() method should not be called at all.

Recursive Routines

Routines can be recursive. Recursive functions are often functions that return arrays or tables. To create a recursive routine, the routine definition must be created first with a dummy body. Then the ALTER ROUTINE statement is used to define the routine body.

In the example below, the table contains a tree of rows each with a parent. The routine returns an array containing the id list of all the direct and indirect children of the given parent. The routine appends the array variable id_list with the id of each direct child and for each child appends the array with the id array of its children by calling the routine recursively.

The routine can be used in a SELECT statement as the example shows.

 CREATE TABLE ptree (pid INT, id INT);
 INSERT INTO ptree VALUES (NULL, 1) ,(1,2), (1,3),(2,4),(4,5),(3,6),(3,7);

 -- the function is created and always throws an exception when used
 CREATE FUNCTION child_arr(p_pid INT) RETURNS INT ARRAY
   SPECIFIC child_arr_one
   READS SQL DATA
   SIGNAL SQLSTATE '45000'

 -- the actual body of the function is defined, replacing the statement that throws the exception
 ALTER SPECIFIC ROUTINE child_arr_one
   BEGIN ATOMIC
     DECLARE id_list INT ARRAY DEFAULT ARRAY[];
     for_loop:
     FOR SELECT id FROM ptree WHERE pid = p_pid DO
       SET id_list[CARDINALITY(id_list) + 1] = id;
       SET id_list = id_list || child_arr(id);
     END FOR for_loop;
     RETURN id_list;
   END

 -- the function can now be used in SQL statements
 SELECT * FROM TABLE(child_arr(2))

In the next example, a table with two columns is returned instead of an array. In this example, a local table variable is declared and filled with the children and the children's children.

 CREATE FUNCTION child_table(p_pid INT) RETURNS TABLE(r_pid INT, r_id INT)
   SPECIFIC child_table_one
   READS SQL DATA
   SIGNAL SQLSTATE '45000'

 ALTER SPECIFIC ROUTINE child_table_one
   BEGIN ATOMIC
     DECLARE TABLE child_tree (pid INT, id INT);
     for_loop:
     FOR SELECT pid, id FROM ptree WHERE pid = p_pid DO
       INSERT INTO child_tree VALUES pid, id;
       INSERT INTO child_tree SELECT r_pid, r_id FROM TABLE(child_table(id));
     END FOR for_loop;
     RETURN TABLE(SELECT * FROM child_tree);
   END

 SELECT * FROM TABLE(child_table(1))

Infinite recursion is not possible as the routine is terminated when a given depth is reached.

Java Language Routines (SQL/JRT)

The general features of SQL-Invoked Routines are shared between PSM and JRT routines. These features are covered in the previous section. This section deals with specific aspects of JRT routines.

The body of a Java language routine is a static method of a Java class, specified with a fully qualified method name in the routine definition. A simple CREATE FUNCTION example is given below, which defines the function to call the java.lang.Math.sinh(double d) Java method. The function can be called in SQL statements just like any built-in function.

 CREATE FUNCTION sinh(v DOUBLE) RETURNS DOUBLE
   LANGUAGE JAVA DETERMINISTIC NO SQL
   EXTERNAL NAME 'CLASSPATH:java.lang.Math.sinh'

 SELECT sinh(doublecolumn) FROM mytable

In the example below, the static method named toZeroPaddedString is specified to be called when the function is invoked.

 CREATE FUNCTION zero_pad(x BIGINT, digits INT, maxsize INT)
   RETURNS CHAR VARYING(100)
   LANGUAGE JAVA DETERMINISTIC NO SQL
   EXTERNAL NAME 'CLASSPATH:org.hsqldb.lib.StringUtil.toZeroPaddedString'

The signature of the Java method (used in the Java code but not in SQL code to create the function) is given below:

 public static String toZeroPaddedString(long value, int precision, int maxSize)

The parameter and return types of the SQL routine definition must match those of the Java method according to the table below:

SMALLINT  

short or Short

INT

int or Integer

BIGINT

long or Long

NUMERIC  or DECIMAL

BigDecimal

FLOAT  or DOUBLE

double or Double

CHAR or VARCHAR

String

DATE

java.sql.Date

TIME

java.sql.Time

TIMESTAMP

java.sql.Timestamp

BINARY

Byte[]

BOOLEAN

boolean or Boolean

ARRAY of any typejava.sql.Array

TABLE

java.sql.ResultSet

If the specified Java method is not found or its parameters and return types do not match the definition, an exception is raised. If more than one version of the Java method exist, then the one with matching parameter and return types is found and registered. If two “equivalent” methods exist, the first one is registered. (This situation arises only when a parameter is a primitive in one version and an Object in another version, e.g. long and java.lang.Long.).

When the Java method of an SQL/JRT routine returns a value, it should be within the size and precision limits defined in the return type of the SQL-invoked routine, otherwise an exception is raised. The scale difference are ignored and corrected. For example, in the above example, the RETURNS CHAR VARYING(100) clause limits the length of the strings returned from the Java method to 100. But if the number of digits after the decimal point (scale) of a returned BigDecimal value is larger than the scale specified in the RETURNS clause, the decimal fraction is silently truncated and no exception of warning is raised.

When the function is specified as RETURNS TABLE(...) the static Java method should return a JDBCResultSet instance. For an example of how to construct a JDBCResultSet for this purpose, see the source code for the org.hsqldb.jdbc.JDBCArray class.

Polymorphism

If two versions of the same SQL invoked routine with different parameter types are required, they can be defined to point to the same method name or different method names, or even methods in different classes. In the example below, the first two definitions refer to the same method name in the same class. In the Java class, the two static methods are defined with corresponding method signatures.

In the third example, the Java function returns a result set and the SQL declaration includes RETURNS TABLE.

 CREATE FUNCTION an_hour_before_or_now(t TIME)
   RETURNS TIME
   NO SQL
   LANGUAGE JAVA PARAMETER STYLE JAVA
   EXTERNAL NAME 'CLASSPATH:org.npo.lib.nowLessAnHour'

 CREATE FUNCTION an_hour_before_or_now(t TIMESTAMP)
   RETURNS TIMESTAMP
   NO SQL
   LANGUAGE JAVA PARAMETER STYLE JAVA
   EXTERNAL NAME 'CLASSPATH:org.npo.lib.nowLessAnHour'

 CREATE FUNCTION testquery(i INTEGER) 
   RETURNS TABLE(n VARCHAR(20), i INT) 
   READS SQL DATA
   LANGUAGE JAVA
   EXTERNAL NAME 'CLASSPATH:org.hsqldb.test.TestJavaFunctions.getQueryResult'

In the Java class the definitions are as follows. Note the definition of the getQueryResult method begins with a java.sql.Connection parameter. This parameter is ignored when choosing the Java method. The parameter is used to pass the current JDBC connection to the Java method.

 public static java.sql.Time nowLessAnHour(java.sql.Time value) {
     ...
 }

 public static java.sql.Timestamp nowLessAnHour(java.sql.Timestamp value)
     ...
 }

 public static ResultSet getQueryResult(Connection connection, int i) throws SQLException {
     Statement st = connection.createStatement();
     return st.executeQuery("SELECT * FROM T WHERE I < " + i);
 }

Java Language Procedures

Java procedures are defined similarly to functions. The differences are:

  • The return type of the Java static method must be void.

  • If a parameter is defined as OUT or INOUT, the corresponding Java static method parameter must be defined as an array of the JDBC non-primitive type.

  • When the Java static method is invoked, the OUT and INOUT arguments are passed to the Java method as a single-element array.

  • The static method can modify the OUT or INOUT argument by assigning a value to the sole element of the argument array.

  • A procedure can return one or more result sets. These are instantiated as JDBC ResultSet objects by the Java static and returned in array arguments of the method. The signature of the Java method for a procedure that has N declared parameters and returns M result sets has the following pattern. The N parameters corresponding to the signature of the declared SQL procedure are defined first, followed by M parameters as ResultSet arrays.

    When the SQL procedure is executed, the Java method is called with single element array arguments passed for OUT and INOUT SQL parameters, and single element arrays of ResultSet for the returned ResultSet objects. The Java method may call the execute() or executeQuery() methods of JDBC Statement or PreparedStatement objects that are declared within the method and assign the ResultSet objects to the first element of each ResultSet[] argument. For the returned ResultSet objects, the Java method should not call the methods of java.sql.ResultSet before returning.

    void methodName(<arg1>, ... <argN>, ResultSet[] r1, ..., ResultSet[] rM)

  • If the procedure contains SQL statements, only statements for data access and manipulation are allowed. The Java method should not perform commit or rollback. The SQL statements should not change the session settings and should not include statements that create or alter tables or other database objects. These rules are generally enforced by the engine, but additional enforcement may be added in future versions

An example of a procedure definition, together with its Java signature, is given below. This procedure is the SQL/JRT version of the example discussed above for SQL/PSM.

 CREATE PROCEDURE get_customer(IN id INT, OUT firstname VARCHAR(50), OUT lastname VARCHAR(50)) 
   READS SQL DATA
   LANGUAGE JAVA
   EXTERNAL NAME 'CLASSPATH:org.hsqldb.test.Test01.getCustomerProcedure'

   public static void getCustomerProcedure(int id, String[] firstn, String[] lastn)
       throws java.sql.SQLException {
       firstn[0] = somevalue;  // parameter out value is assigned
       lastn[0] = somevalue;   // parameter out value is assigned
   }

In the next example a procedure is defined to return a result set. The signature of the Java method is also given. The Java method assigns a ResultSet object to the zero element of the result parameter.

 CREATE PROCEDURE new_customer(firstname VARCHAR(50), lastname VARCHAR(50))
   MODIFIES SQL DATA 
   LANGUAGE JAVA
   DYNAMIC RESULT SETS 1
   EXTERNAL NAME 'CLASSPATH:org.hsqldb.test.Test01.newCustomerProcedure'

   public static void newCustomerProcedure(String firstn, String lastn,
                       ResultSet[] result) throws java.sql.SQLException {
       result[0] = someresultset;  // dynamic result set is assigned
   }

Java language procedures SQL/JRT are used in an identical manner to SQL/PSM routines. See the section under SQL/PSM routines, Returning Data From Procedures, on how to use the JDBC CallableStatement interface to call the procedure and to get the OUT and INOUT arguments and to use the ResultSet objects returned by the procedure.

Java Static Methods

The static methods that are used for procedures and functions must be declared in a public class. The methods must be declared as public static. For functions, the method return type must be one of the JDBC supported types. The IN parameters of the method must be declared as one of the supported types. The OUT and INOUT parameters must be declared as Java arrays of supported types. If the SQL definition of a function includes RETURNS NULL ON NULL INPUT, then the IN parameters of the Java static function can be int or long primitives, otherwise, they must be Integer or Long. The declared Java arrays for OUT and INOUT parameters for SQL INTEGER or BIGINT must be Integer[] or Long[] respectively.

If the SQL definition of the routine includes NO SQL, then no JDBC method call is allowed to execute in the method body. Otherwise, a JDBC Connection can be used within the Java method to access the database. If the definition includes CONTAINS SQL, then no table data can be read. If the definition includes READS SQL DATA, then no table data can be modified. If the definition includes MODIFIES SQL DATA, then data can be modified. In all modes, it is not allowed to execute DDL statements that change the schema definition.

It is possible to use DECLARE LOCAL TEMPORARY TABLE in a Java method, as this is in the session scope.

There are two ways to use the JDBC Connection object.

  1. Define the Java method with a Connection parameter as the first parameter. This parameter is "hidden" and only visible to the engine. The rest of the parameters, if any, are used to choose the method according to the required types of parameters.

  2. Use the SQL/JRT Standard "jdbc:default:connection" method. With this approach, the Java method does not include a Connection parameter. In the method body, the connection is established with a method call to DriverManager, as in the example below:

    Connection con = DriverManager.getConnection("jdbc:default:connection");

Both methods return a connection that is based on the current session. This connection has some extra properties, for example, the Close() method does not actually close it.

An example of an SQL PROCEDURE with its Java method definition is given below. The CREATE PROCEDURE statement is the same with or without the Connection parameter:

 CREATE PROCEDURE proc1(IN P1 INT, IN P2 INT, OUT P3 INT)
 SPECIFIC P2 LANGUAGE JAVA DETERMINISTIC MODIFIES SQL DATA EXTERNAL NAME 'CLASSPATH:org.hsqldb.test.TestStoredProcedure.procTest2'");

In the first example, the "jdbc:default:connection" method is used. In the second example, a connection parameter is used

 public static void procTest2(int p1, int p2,
                    Integer[] p3) throws java.sql.SQLException {

     Connection conn =
         DriverManager.getConnection("jdbc:default:connection");
     java.sql.Statement stmt = conn.createStatement();

     stmt.execute("INSERT INTO MYTABLE VALUES(" + p1 + ",'test1')");
     stmt.execute("INSERT INTO MYTABLE VALUES(" + p2 + ",'test2')");

     java.sql.ResultSet rs = stmt.executeQuery("select * from MYTABLE");
     java.sql.ResultSetMetaData meta = rs.getMetaData();

     int cols  = meta.getColumnCount();
     p3[0] = Integer.valueOf(cols);

     rs.close();
     stmt.close();
 }

//  alternative declaration with Connection parameter
//  public static void procTest2(Connection conn, int p1, int p2,
//                    Integer[] p3) throws java.sql.SQLException {

When the stored procedure is called by the user's program, the value of the OUT parameter can be read after the call.

 // a CallableStatement is used to prepare the call
 // the OUT parameter contains the return value
 CallableStatement c = conn.prepareCall("call proc1(1,2,?)");
 c.execute();
 int value = c.getInt(1);

Legacy Support

The legacy HyperSQL statement, CREATE ALIAS <name> FOR <fully qualified Java method name> is no longer supported directly. It is supported when importing databases and translates to a special CREATE FUNCTION <name> statement that creates the function in the PUBLIC schema.

The direct use of a Java method as a function is still supported but deprecated. It is internally translated to a special CREATE FUNCTION statement where the name of the function is the double quoted, fully qualified name of the Java method used.

Securing Access to Classes

By default, the static methods of any class that is on the classpath are available to be used. This can compromise security in some systems. The optional Java system property hsqldb.method_class_names allows preventing access to classes other than java.lang.Math or specifying a semicolon-separated list of allowed classes. A property value that ends with .* is treated as a wild card and allows access to all class or method names formed by substitution of the * (asterisk).

In the example below, the property has been included as an argument to the Java command.

 java -Dhsqldb.method_class_names="org.me.MyClass;org.you.YourClass;org.you.lib.*" [the rest of the command line]

The above example allows access to the methods in the two classes: org.me.MyClass and org.you.YourClass together with all the classes in the org.you.lib package. Note that if the property is not defined, no access control is performed at this level.

Once the routine has been defined, the normal database access control still applies. The routine can be executed only by the users who have been granted EXECUTE privileges on it. The user who executes a Java routine must also have the relevant access privileges on the tables that are used inside the Java method.

Warning

The definition of SQL/JRT routines referencing the user's Java static methods is stored in the .script file of the database.

If the database is opened in a Java environment that does not have access to the referenced Java static methods on its classpath, the SQL/JRT routines are not created when the database is opened. When the database is closed, the routine definitions are lost.

There is a workaround to prevent opening the database when the static method are not on the classpath. You can create an SQL/PSM procedure which calls all the SQL/JRT functions and procedures in your database. The calls should have the necessary dummy arguments. This procedure will fail to be created when the referenced methods are not accessible and will return "Error in script file". There is no need ever to execute the procedure. However, to avoid accidental use, you can ensure that it does not execute the SQL/JRT routines by adding a line such as IF TRUE THEN RETURN; before any references to the SQL/JRT routines.

User Defined Aggregate Functions

HyperSQL adds an extension to the SQL Standard to allow user-defined aggregate functions. A user-defined aggregate function has a single parameter when it is used in SQL statements. Unlike the predefined aggregate functions, the keyword DISTINCT cannot be used when a user defined aggregate function is invoked. Like all user-defined functions, an aggregate function belongs to a schema and can be polymorphic (using multiple function definitions with the same name but different parameter types).

A user defined aggregate function can be used in SQL statements where a predefined aggregate function is allowed.

Definition of Aggregate Functions

An aggregate function is always defined with 4 parameters. The first parameter is the parameter that is used when the function is invoked in SQL statements, the rest of the parameter are invisible to the invoking SQL statement. The type of the first parameter is user defined. The type of the second parameter must be BOOLEAN. The third and fourth parameters have user defined types and must be defined as INOUT parameters. The defined return type of the function determines the type of the value returned when the function is invoked.

CREATE AGGREGATE FUNCTION

user defined aggregate function definition

Aggregate function definition is similar to normal function definition and has the mandatory <returns clause>. The BNF is given below.

<user defined aggregate function> ::= CREATE AGGREGATE FUNCTION <schema qualified routine name> <SQL aggregate parameter declaration list> <returns clause> <routine characteristics> <routine body>

The parameter declaration list BNF is given below. The type of the first parameter is used when the function is invoked as part of an SQL statement. When multiple versions of a function are required, each version will have the first parameter of a different type.

<SQL aggregate declaration list> ::= <left paren> [IN] [ <SQL parameter name> ] <parameter type> <comma> [IN] [ <SQL parameter name> ] BOOLEAN <comma> INOUT [ <SQL parameter name> ] <parameter type> <comma> INOUT [ <SQL parameter name> ] <parameter type> <right paren>

The return type is user defined. This is the type of the resulting value when the function is called. Usually an aggregate function is defined with CONTAINS SQL, as it normally does not read the data in database tables, but it is possible to define the function with READS SQL DATA and access the database tables.

When a SQL statement that uses the aggregate function is executed, HyperSQL invokes the aggregate function, with all the arguments set, once per each row in order to compute the values. Finally, it invokes the function once more to return the final result.

In the computation phase, the first argument is the value of the user argument as specified in the SQL statement, computed for the current row. The second argument is the boolean FALSE. The third and fourth argument values can have any type and are initially null, but they can be updated in the body of the function during each invocation. The third and fourth arguments act as registers and hold their values between invocations. The return value of the function is ignored during the computation phase (when the second parameter is FALSE).

After the computation phase, the function is invoked once more to get the final result. In this invocation, the first argument is NULL and the second argument is boolean TRUE. The third and fourth arguments hold the values they held at the end of the last invocation. The value returned by the function in this invocation is used as the result of the aggregate function computation in the invoking SQL statement. In SQL queries with GROUP BY, the call sequence is repeated separately for each separate group.

SQL PSM Aggregate Functions

The example below features a user defined version of the Standard AVG(<value expression>) aggregate function for INTEGER input and output types. This function behaves differently from the Standard AVG function as it returns 0 when all the input values are null.

 CREATE AGGREGATE FUNCTION udavg(IN x INTEGER, IN flag BOOLEAN, INOUT addup BIGINT, INOUT counter INT)
   RETURNS INTEGER
   CONTAINS SQL
   BEGIN ATOMIC
     IF flag THEN
       RETURN addup / counter;
     ELSE
       SET counter = COALESCE(counter, 0) + 1;
       SET addup = COALESCE(addup, 0) || COALESCE(x, 0);
       RETURN NULL;
     END IF;
   END

The user defined aggregate function is used in a select statement in the example below. Only the first parameter is visible and utilised in the select statement.

 SELECT udavg(id) FROM customers GROUP BY lastname;

In the example below, the function returns an array that contains all the values passed for the aggregated column. For use with longer arrays, you can optimise the function by defining a larger array in the first iteration, and using the TRIM_ARRAY function on the RETURN to cut the array to size. This function is similar to the built-in ARRAY_AGG function

 CREATE AGGREGATE FUNCTION array_aggregate(IN val VARCHAR(100), IN flag boolean, INOUT buffer VARCHAR(100) ARRAY, INOUT counter INT)
   RETURNS VARCHAR(100) ARRAY
   CONTAINS SQL
   BEGIN ATOMIC
     IF flag THEN
       RETURN buffer;
     ELSE
       IF val IS NULL THEN RETURN NULL; END IF;
       IF counter IS NULL THEN SET counter = 0; END IF;
       SET counter = counter + 1;
       IF counter = 1 THEN SET buffer = ARRAY[val];
       ELSE SET buffer[counter] = val; END IF;
       RETURN NULL;
     END IF;
   END

The tables and data for the select statement below are created with the DatabaseManager or DatabaseManagerSwing GUI apps. (You can find the SQL in the TestSelf.txt file in the zip). Part of the output is shown. Each row of the output includes an array containing the values for the invoices for each customer.

 SELECT ID, FIRSTNAME, LASTNAME, ARRAY_AGGREGATE(CAST(INVOICE.TOTAL AS VARCHAR(100))) 
   FROM customer JOIN INVOICE ON ID =CUSTOMERID
   GROUP BY ID, FIRSTNAME, LASTNAME

 11 Susanne   Karsen    ARRAY['3988.20']                               
 12 John      Peterson  ARRAY['2903.10','4382.10','4139.70','3316.50'] 
 13 Michael   Clancy    ARRAY['6525.30']                               
 14 James     King      ARRAY['3665.40','905.10','498.00']             
 18 Sylvia    Clancy    ARRAY['634.20','4883.10']                      
 20 Bob       Clancy    ARRAY['3414.60','744.60']

In the example below, the function returns a string that contains the comma-separated list of all the values passed for the aggregated column. This function is similar to the built in GROUP_CONCAT function.

 CREATE AGGREGATE FUNCTION group_concatenate
     (IN val VARCHAR(100), IN flag BOOLEAN, INOUT buffer VARCHAR(1000), INOUT counter INT)
     RETURNS VARCHAR(1000)
     CONTAINS SQL
   BEGIN ATOMIC
     IF FLAG THEN
       RETURN BUFFER;
     ELSE
       IF val IS NULL THEN RETURN NULL; END IF;
       IF buffer IS NULL THEN SET BUFFER = ''; END IF;
       IF counter IS NULL THEN SET COUNTER = 0; END IF;
       IF counter > 0 THEN SET buffer = buffer || ','; END IF;
       SET buffer = buffer + val;
       SET counter = counter + 1;
       RETURN NULL;
     END IF;
   END

The same tables and data as for the previous example is used. Part of the output is shown. Each row of the output is a comma-separated list of names.

 SELECT group_concatenate(firstname || ' ' || lastname) FROM customer GROUP BY lastname
  
 Laura Steel,John Steel,John Steel,Robert Steel                                   
 Robert King,Robert King,James King,George King,Julia King,George King            
 Robert Sommer,Janet Sommer                                                       
 Michael Smith,Anne Smith,Andrew Smith                                            
 Bill Fuller,Anne Fuller                                                          
 Laura White,Sylvia White                                                         
 Susanne Clancy,Michael Clancy,Sylvia Clancy,Bob Clancy,Susanne Clancy,John Clancy

Java Aggregate Functions

A Java aggregate function is defined similarly to PSM functions, apart from the routine body, which is defined as EXTERNAL NAME ... The Java function signature must follow the rules for both nullable and INOUT parameters, therefore:

No argument is defined as a primitive or primitive array type. This allows nulls to be passed to the function. The second and third arguments must be defined as arrays of the JDBC non-primitive types listed in the table in the previous section.

In the example below, a user-defined aggregate function for geometric mean is defined.

 CREATE AGGREGATE FUNCTION geometric_mean(IN val DOUBLE, IN flag BOOLEAN, INOUT register DOUBLE, INOUT counter INT)
     RETURNS DOUBLE
     NO SQL
     LANGUAGE JAVA
     EXTERNAL NAME 'CLASSPATH:org.hsqldb.test.Test01.geometricMean'

The Java function definition is given below:

 public static Double geometricMean(Double in, Boolean flag,
         Double[] register, Integer[] counter) {
     if (flag) {
         if (register[0] == null) { return null; }
         double a = register[0].doubleValue();
         double b = 1 / (double) counter[0];
         return Double.valueOf(java.lang.Math.pow(a, b));
     }
     if (in == null) { return null; }
     if (in.doubleValue() == 0) { return null; }
     if (register[0] == null) {
         register[0] = in;
         counter[0]  = Integer.valueOf(1);
     } else {
         register[0] = Double.valueOf(register[0].doubleValue() * in.doubleValue());
         counter[0] = Integer.valueOf(counter[0].intValue() + 1);
     }
     return null;
 }

In a select statement, the function is used exactly like the built-in aggregate functions:

 SELECT geometric_mean(age) FROM  FROM customer

Chapter 9. Triggers

Fred Toussi

The HSQL Development Group

$Revision: 3042 $

Copyright 2010-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Overview

Trigger functionality first appeared in SQL:1999. Triggers embody the live database concept, where changes in SQL data can be monitored and acted upon. This means each time a DELETE, UPDATE or INSERT is performed, additional actions are taken by the declared triggers. SQL Standard triggers are imperative while the relational aspects of SQL are declarative. Triggers allow performing an arbitrary transformation of data that is being updated or inserted, or to prevent insert, updated or deletes, or to perform additional operations.

Some bad examples of SQL triggers in effect enforce an “integrity constraint” which would better be expressed as a CHECK constraint. A trigger that causes an exception if the value inserted in a column is negative is such an example. A check constraint that declares CHECK VALUE >= 0 (declarative) is a better way of expressing an integrity constraint than a trigger that throws an exception if the same condition is false.

Usage constraints cannot always be expressed by SQL’s integrity constraint statements. Triggers can enforce these constraints. For example, it is not possible to use a check constraint to prevent inserts or deletes on weekends. A trigger can be used to enforce the time when each operation is allowed.

A trigger is declared to activate when an UPDATE, INSERT or DELETE action is performed on a table. These actions may be direct or indirect. Indirect actions may arise from CASCADE actions of FOREIGN KEY constraints, or from data change statements performed on a VIEW that is based on the table that in.

It is possible to declare multiple triggers on a single table. The triggers activate one by one according to the order in which they were defined. HyperSQL supports an extension to the CREATE TRIGGER statement, which allows the user to specify the execution order of the new trigger.

A row level trigger allows access to the deleted or inserted rows. For UPDATE actions there is both an old and new version of each row. A trigger can be specified to activate before or after the action has been performed.

BEFORE Triggers

A trigger that is declared as BEFORE DELETE cannot modify the deleted row. In other words, it cannot decide to delete a different row by changing the column values of the row. A trigger that is declared as BEFORE INSERT and BEFORE UPDATE can modify the values that are inserted into the database. For example, a badly formatted string can be cleaned up by a trigger before INSERT or UPDATE.

BEFORE triggers cannot modify the other tables of the database. All BEFORE triggers can veto the action by throwing an exception.

Because BEFORE triggers can modify the inserted or updated rows, all constraint checks are performed after the execution of the BEFORE triggers. The checks include NOT NULL constraints, length of strings, CHECK constraints, and FOREIGN key constraints.

AFTER Triggers

AFTER triggers can also perform additional data changes, for example inserting an additional row into a different table for data audits. These triggers cannot modify the rows that have been modified by the INSERT or UPDATE action.

INSTEAD OF Triggers

A trigger that is declared on a VIEW, is an INSTEAD OF trigger. This term means when an INSERT, UPDATE or DELETE statement is executed with the view as the target, the trigger action is all that is performed, and no further data change takes place on the view. The trigger action can include all the statements that are necessary to change the data in the tables that underlie the view, or even other tables, such as audit tables. With the use of INSTEAD OF triggers a read-only view can effectively become updatable or insertable-into.

Trigger Properties

A trigger is declared on a specific table or view. Various trigger properties determine when the trigger is executed and how.

Trigger Event

The trigger event specifies the type of SQL statement that causes the trigger to execute. Each trigger is specified to execute when an INSERT, DELETE or UPDATE takes place.

The event can be filtered by two separate means. For all triggers, the WHEN clause can specify a condition against the rows that are the subject of the trigger, together with the data in the database. For example, a trigger can activate when the size of a table becomes larger than a certain amount. Or it can activate when the values in the rows being modified satisfy certain conditions.

An UPDATE trigger can be declared to execute only when certain columns are the subject of an update statement. For example, a trigger declared as AFTER UPDATE OF (datecolumn) will activate only when the UPDATE statement that is executed includes the column, datecolumn, as one of the columns specified in its SET statements.

Granularity

A statement level trigger is performed once for the executed SQL statement and is declared as FOR EACH STATEMENT.

A row level trigger is performed once for each row that is modified during the execution of an SQL statement and is declared as FOR EACH ROW. Note that an SQL statement can INSERT, UPDATE or DELETE zero or more rows.

If a statement does not apply to any row, then the trigger is not executed.

If FOR EACH ROW or FOR EACH STATEMENT is not specified, then the default is FOR EACH STATEMENT.

The granularity dictates whether the REFERENCING clause can specify OLD ROW, NEW ROW, or OLD TABLE, NEW TABLE.

A trigger declared as FOR EACH STATEMENT can only be an AFTER trigger.

Trigger Action Time

A trigger is executed BEFORE, AFTER or INSTEAD OF the trigger event.

INSTEAD OF triggers are allowed only when the trigger is declared on a VIEW. With this type of trigger, the event (SQL statement) itself is not executed, only the trigger.

BEFORE or AFTER triggers are executed just before or just after the execution of the event. For example, just before a row is inserted into a table, the BEFORE trigger is activated, and just after the row is inserted, the AFTER trigger is executed.

BEFORE triggers can modify the row that is being inserted or updated. AFTER triggers cannot modify rows. They are usually used to perform additional operations, such as inserting rows into other tables.

A trigger declared as FOR EACH STATEMENT can only be an AFTER trigger.

References to Rows

If the old rows or new rows are referenced in the SQL statements in the trigger action, they must have names. The REFERENCING clause is used to give names to the old and new rows. The clause, REFERENCING OLD | NEW TABLE is used for statement level triggers. The clause, REFERENCING OLD | NEW ROW is used for row level triggers. If the old rows or new rows are referenced in the SQL statements in the trigger action, they must have names. In the SQL statements, the columns of the old or new rows are qualified with the specified names.

Trigger Condition

The WHEN clause can specify a condition for the columns of the row that is being changed. Using this clause you can simply avoid unnecessary trigger activation for rows that do not need it.

For UPDATE trigger, you can specify a list of columns of the table. If a list of columns is specified, then if the UPDATE statement does not change the columns with SET clauses, then the trigger is not activated at all.

Trigger Action in SQL

The trigger action specifies what the trigger does when it is activated. This is usually written as one or more SQL statements.

When a row level trigger is activated, there is an OLD ROW, or a NEW ROW, or both. An INSERT statement supplies a NEW ROW row to be inserted into a table. A DELETE statement supplies an OLD ROW be deleted. An UPDATE statement supplies both OLD ROW and NEW ROW that represent the updated rows before and after the update. The REFERENCING clause gives names to these rows, so that the rows can be referenced in the trigger action.

In the example below, a name is given to the NEW ROW and it is used both in the WHEN clause and in the trigger action SQL to insert a row into a triglog table after each row insert into the testtrig table.

 CREATE TRIGGER trig AFTER INSERT ON testtrig 
   REFERENCING NEW ROW AS newrow
   FOR EACH ROW WHEN (newrow.id > 1)
   INSERT INTO TRIGLOG VALUES (newrow.id, newrow.data, 'inserted')

In the example blow, the trigger code modifies the updated data if a condition is true. This type of trigger is useful when the application does not perform the necessary checks and modifications to data. The statement block that starts with BEGIN ATOMIC is similar to an SQL/PSM block and can contain all the SQL statements that are allowed in an SQL/PSM block.

 CREATE TRIGGER t BEFORE UPDATE ON customer
   REFERENCING NEW AS newrow FOR EACH ROW
   BEGIN ATOMIC
     IF LENGTH(newrow.firstname) > 10 THEN
       SET newrow.firstname = LOWER(newrow.firstname);
     END IF;
   END

Trigger Action in Java

A trigger action can be written as a Java class that implements the org.hsqldb.Trigger interface. This interface has a single method which is called when the trigger is activated, either before or after the event. When the method is called by the engine, it supplies the type of trigger as an int value defined by the interface(as type argument), the name of the trigger (as trigName argument), the name of the table (as tabName argument), the OLD ROW (as oldRow argument) and the NEW ROW (as newRow argument). The oldRow argument is null for row level INSERT triggers. The newRow argument is null for row level DELETE triggers. For table level triggers, both arguments are null (that is, there is no access to the data). The triggerType argument is one of the constants in the org.hsqldb.Trigger interface which indicate the type of trigger, for example, INSERT_BEFORE_ROW or UPDATE_AFTER_ROW.

The Java class for the trigger can be reused for several triggers on different tables. The method code can distinguish between the different tables and triggers using the supplied arguments and take appropriate action.

 fire (int triggerType, String name, String table, Object row1[], Object row2[])

The Java method for a synchronous trigger (see below) can modify the values in newRow2 in a BEFORE trigger. Such modifications are reflected in the row that is being inserted or updated. Any other modifications are ignored by the engine.

A Java trigger that uses an instance of org.hsqldb.Trigger has two forms, synchronous, or asynchronous (immediate or queued). By default, or when QUEUE 0 is specified, the action is performed immediately by calling the Java method. This is similar to SQL trigger actions.

When QUEUE n is specified with n larger than 0, the engine uses a separate thread to execute the Java method, using a queue with the size n. For certain applications, such as real-time systems this allows asynchronous notifications to be sent by the trigger event, without introducing delays in the engine. With asynchronous triggers, an extra parameter, NOWAIT can be used in trigger definition. This overcomes the queue full condition. In this mode, old calls that are still in the queue are discarded one by one and replaced with new calls.

Java row level triggers that are declared with BEFORE trigger action time can modify the row data. Triggers with AFTER trigger action time can modify the database, e.g. insert new rows. If the trigger needs to access the database, the same method as in Java Language Routines SQL/JRT can be used. The Java code should connect to the URL "jdbc:default:connection" and use this connection to access the database.

For sample trigger classes and test code see, org.hsqldb.sample.TriggerSample, org.hsqldb.test.TestTriggers, org.hsqldb.test.TriggerClass and the associated text script TestTriggers.txt in the /testrun/hsqldb/ directory. In the example below, the trigger is activated only if the update statement includes SET clauses that modify any of the specified columns (c1, c2, c3). Furthermore, the trigger is not activated if the c2 column in the updated row is null.

 CREATE TRIGGER TRIGBUR BEFORE UPDATE OF c1, c2, c3 ON testtrig 
   referencing NEW ROW AS newrow
   FOR EACH ROW WHEN (newrow.c2 IS NOT NULL)
   CALL "org.hsqldb.test.TriggerClass"

Java functions can be called from an SQL trigger. So it is possible to define the Java function to perform any external communication that are necessary for the trigger, and use SQL code for checks and alterations to data.

 CREATE TRIGGER t BEFORE UPDATE ON customer
   REFERENCING NEW AS newrow FOR EACH ROW
   BEGIN ATOMIC
     IF LENGTH(newrow.firstname) > 10 THEN
       CALL my_java_function(newrow.firstname, newrow.lastname);
     END IF;
   END

Trigger Creation

CREATE TRIGGER

trigger definition

<trigger definition> ::= CREATE TRIGGER <trigger name> <trigger action time> <trigger event> ON <table name> [BEFORE <other trigger name>] [ REFERENCING <transition table or variable list> ] <triggered action>

<trigger action time> ::= BEFORE | AFTER | INSTEAD OF

<trigger event> ::= INSERT | DELETE | UPDATE [ OF <trigger column list> ]

<trigger column list> ::= <column name list>

<triggered action> ::= [ FOR EACH { ROW | STATEMENT } ] [ <triggered when clause> ] <triggered SQL statement>

<triggered when clause> ::= WHEN <left paren> <search condition> <right paren>

<triggered SQL statement> ::= <SQL procedure statement> | BEGIN ATOMIC { <SQL procedure statement> <semicolon> }... END | [QUEUE <integer literal>] [NOWAIT] CALL <HSQLDB trigger class FQN>

<transition table or variable list> ::= <transition table or variable>...

<transition table or variable> ::= OLD [ ROW ] [ AS ] <old transition variable name> | NEW [ ROW ] [ AS ] <new transition variable name> | OLD TABLE [ AS ] <old transition table name> | NEW TABLE [ AS ] <new transition table name>

<old transition table name> ::= <transition table name>

<new transition table name> ::= <transition table name>

<transition table name> ::= <identifier>

<old transition variable name> ::= <correlation name>

<new transition variable name> ::= <correlation name>

Trigger definition is a relatively complex statement. The combination of <trigger action time> and <trigger event> determines the type of the trigger. Examples include BEFORE DELETE, AFTER UPDATE, INSTEAD OF INSERT. If the optional [ OF <trigger column list> ] is specified for an UPDATE trigger, then the trigger is activated only if one of the columns that is in the <trigger column list> is specified in the UPDATE statement that activates the trigger.

If a trigger is FOR EACH ROW, which is the default option, then the trigger is activated for each row of the table that is affected by the execution of an SQL statement. Otherwise, it is activated once only per statement execution. For FOR EACH ROW triggers, there is an OLD and NEW state for each row. For UPDATE triggers, both OLD and NEW states exist, representing the row before the update, and after the update. For DELETE, triggers, there is only an OLD state. For INSERT triggers, there is only the NEW state. If a trigger is FOR EACH STATEMENT, then a transient table is created containing all the rows for the OLD state and another transient table is created for the NEW state.

The [ REFERENCING <transition table or variable> ] is used to give a name to the OLD and NEW data row or table. This name can be referenced in the <SQL procedure statement> to access the data.

The optional <triggered when clause> is a search condition, similar to the search condition of a DELETE or UPDATE statement. If the search condition is not TRUE for a row, then the trigger is not activated for that row.

The <SQL procedure statement> is limited to INSERT, DELETE, UPDATE and MERGE statements.

The <HSQLDB trigger class FQN> is a delimited identifier that contains the fully qualified name of a Java class that implements the org.hsqldb.Trigger interface.

Early releases of HyperSQL version 2.0 do not allow the use of OLD TABLE or NEW TABLE in statement level triggers.

TRIGGERED SQL STATEMENT

triggered SQL statement

The <triggered SQL statement> has three forms.

The first form is a single SQL procedure statement. This statement can reference the OLD ROW and NEW ROW variables. For example, it can reference these variables and insert a row into a separate table.

The second form is enclosed in a BEGIN ... END block and can include one or more SQL procedure statements. In BEFORE triggers, you can include SET statements to modify the inserted or updated rows. In AFTER triggers, you can include INSERT, DELETE and UPDATE statements to change the data in other database tables. SELECT and CALL statements are allowed in BEFORE and AFTER triggers. CALL statements in BEFORE triggers should not modify data.

The third form specifies a call to a Java method.

Two examples of a trigger with a block are given below. The block can include elements discussed in the SQL-Invoked Routines chapter, including local variables, loops and conditionals. You can also raise an exception in such blocks in order to terminate the execution of the SQL statement that caused the trigger to execute.

/* the trigger throws an exception if a customer with the given last name already exists */
 CREATE TRIGGER trigone BEFORE INSERT ON customer 
   REFERENCING NEW ROW AS newrow
   FOR EACH ROW WHEN (newrow.id > 100)
   BEGIN ATOMIC
     IF EXISTS (SELECT * FROM CUSTOMER WHERE CUSTOMER.LASTNAME = NEW.LASTNAME) THEN
       SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'already exists';
     END IF;
   END
/* for each row inserted into the target, the trigger insert a row into the table used for logging */
 CREATE TRIGGER trig AFTER INSERT ON testtrig 
   BEFORE othertrigger  
   REFERENCING NEW ROW AS newrow
   FOR EACH ROW WHEN (newrow.id > 1)
   BEGIN ATOMIC
     INSERT INTO triglog VALUES (newrow.id, newrow.data, 'inserted');
     /* more statements can be included */
   END

TRIGGER EXECUTION ORDER

trigger execution order

<trigger execution order> ::= BEFORE <other trigger name>

HyperSQL extends the SQL Standard to allow the order of execution of a trigger to be specified by using [BEFORE <other trigger name>] in the definition. The newly defined trigger will be executed before the specified other trigger. If this clause is not used, the new trigger is executed after all the previously defined triggers of the same scope (BEFORE, AFTER, EACH ROW, EACH STATEMENT).

DROP TRIGGER

drop trigger statement

<drop trigger statement> ::= DROP TRIGGER <trigger name>

Destroy a trigger.

Chapter 10. Built In Functions

Fred Toussi

The HSQL Development Group

$Revision: 5310 $

Copyright 2010-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Overview

HyperSQL supports a wide range of built-in functions and allows user-defined functions written in SQL and Java languages. User-defined functions are covered in the SQL-Invoked Routines chapter. If a built-in function is not available, you can write your own using procedural SQL or Java.

Built-in aggregate functions such as SUM, MAX, ARRAY_AGG, GROUP_CONCAT are covered in the Data Access and Change chapter, which covers SQL in general. SQL expressions such as COALESCE, NULLIF and CAST are also discussed there.

The built-in functions fall into three groups:

  • SQL Standard Functions

    A wide rang of functions defined by SQL/Foundation are supported. SQL/Foundation functions that have no parameter are called without empty parentheses. Functions with multiple parameters often use keywords instead of commas to separate the parameters. Many functions are overloaded. Among these, some have one or more optional parameters that can be omitted, while the return type of some functions is dependent upon the type of one of the parameters. The usage of SQL Standard Functions (where they can be used) is covered more extensively in the Data Access and Change chapter

  • JDBC Open Group CLI Functions

    These functions were defined as an extension to the CLI standard, which is the basis for ODBC and JDBC and supported by many database products. JDBC supports an escape mechanism to specify function calls in SQL statements in a manner that is independent of the function names supported by the target database engine. For example SELECT {fn DAYOFMONTH (dateColumn)} FROM myTable can be used in JDBC and is translated to Standard SQL as SELECT EXTRACT (DAY_OF_MONTH FROM dateColumn) FROM myTable if a database engine supports the Standard syntax. If a database engine does not support Standard SQL, then the translation will be different. HyperSQL supports all the function names specified in the JDBC specifications as native functions. Therefore, there is no need to use the {fn FUNC_NAME ( ... ) } escape with HyperSQL. If a JDBC function is supported by the SQL Standard in a different form, the SQL Standard form is the preferred form to use.

  • HyperSQL Built-In Functions

    Many additional built-in functions are available for some useful operations. Some of these functions return the current setting for the session and the database. The General Functions accept arguments of different types and return values based on comparison between the arguments.

In the BNF specification used here, words in capital letters are actual tokens. Syntactic elements such as expressions are enclosed in angle brackets. The <left paren> and <right paren> tokens are represented with the actual symbol. Optional elements are enclosed with square brackets ( <left bracket> and <right bracket> ). Multiple options for a required element are enclosed with braces ( <left brace> and <right brace> ). Alternative tokens are separated with the vertical bar ( <vertical bar> ). At the end of each function definition, the standard which specifies the function is noted in parentheses as JDBC or HyperSQL, or the SQL/Foundation part of the SQL Standard.

String and Binary String Functions

In SQL, there are three kinds of string: character, binary and bit. The units are respectively characters, octets, and bits. Each kind of string can be in different data types. CHAR, VARCHAR and CLOB are the character data types. BINARY, VARBINARY and BLOB are the binary data types. BIT and BIT VARYING are the bit string types. In all string functions, the position of a unit of the string within the whole string is specified from 1 to the length of the whole string. In the BNF, <char value expr> indicates any valid SQL expression that evaluates to a character type. Likewise, <binary value expr> indicates a binary type and <num value expr> indicates a numeric type.

ASCII

ASCII ( <char value expr> )

Returns an INTEGER equal to the ASCII code value of the first character of <char value expr>. (JDBC)

BIT_LENGTH

BIT_LENGTH ( <string value expression> )

BIT_LENGTH can be used with character, binary and bit strings. It return a BIGINT value that measures the bit length of the string. (Foundation)

See also CHARACTER_LENGTH and OCTET_LENGTH.

CHAR

CHAR ( <UNICODE code> )

The argument is an INTEGER. Returns a character string containing a single character that has the specified <UNICODE code>, which is an integer. ASCII codes are a subset of the allowed values for <UNICODE code>. (JDBC)

CHARACTER_LENGTH

{ CHAR_LENGTH | CHARACTER_LENGTH } ( <char value expression> [ USING { CHARACTERS | OCTETS } ] )

The CHAR_LENGTH or CHARACTER_LENGTH function can be used with character strings, while OCTET_LENGTH can be used with character or binary strings and BIT_LENGTH can be used with character, binary and bit strings.

All functions return a BIGINT value that measures the length of the string in the given unit. CHAR_LENGTH counts characters, OCTET_LENGTH counts octets and BIT_LENGTH counts bits in the string. For CHAR_LENGTH, if [ USING OCTETS ] is specified, the octet count is returned, which is twice the normal length. (Foundation)

CONCAT

CONCAT ( <char value expr 1>, <char value expr 2> [, ...] )

CONCAT ( <binary value expr 1>, <binary value expr 2> [, ...] )

The arguments are character strings or binary strings. Returns a string formed by concatenation of the arguments. Minimum number of arguments is 2. Equivalent to the SQL concatenation expression <value expr 1> || <value expr 2> [ || ...] .

Handling of null values in the CONCAT function depends on the database property sql.concat_nulls ( SET DATABASE SQL SYNTAX CONCAT NULLS { TRUE || FALSE } ). By default, any null value will cause the function to return null. If the property is set false, then NULL values are replaced with empty strings.

(JDBC)

CONCAT_WS

CONCAT_WS ( <char value separator>, <char value expr 1>, <char value expr 2> [, ...] )

The arguments are character strings. Returns a string formed by concatenation of the arguments from the second argument, using the separator from the first argument. Minimum number of arguments is 3. Equivalent to the SQL concatenation expression <value expr 1> || <separator> || <value expr 2> [ || ...] . The function ignores null values and returns an empty string if all values are null. It returns null only if the separator is null.

This function is similar to a MySQL function of the same name.

(HyperSQL)

DIFFERENCE

DIFFERENCE ( <char value expr 1>, <char value expr 2> )

The arguments are character strings. Converts the arguments into SOUNDEX codes, and returns an INTEGER between 0-4 which indicates how similar the two SOUNDEX value are. If the values are the same, it returns 4, if the values have no similarity, it returns 0. In-between values are returned for partial similarity. (JDBC)

INSERT

INSERT ( <char value expr 1>, <offset>, <length>, <char value expr 2> )

Returns a character string based on <char value expr 1> in which <length> characters have been removed from the <offset> position and in their place, the whole <char value expr 2> is copied. Equivalent to SQL/Foundation OVERLAY( <char value expr1> PLACING < char value expr2> FROM <offset> FOR <length> ) . (JDBC)

INSTR

INSTR ( <char value expr 1>, <char value expr 2> [ , <offset> ] )

Returns as a BIGINT value the starting position of the first occurrence of <char value expr 2> within <char value expr 1>. If <offset> is specified, the search begins with the position indicated by <offset>. If the search is not successful, 0 is returned. Similar to the LOCATE function but the order of the arguments is reversed. (HyperSQL)

HEXTORAW

HEXTORAW( <char value expr> )

Returns a BINARY string formed by translation of hexadecimal digits and letters in the <char value expr>. Each character of the <char value expr> must be a digit or a letter in the A | B | C | D | E | F set. Each byte of the retired binary string is formed by translating two hex digits into one byte. (HyperSQL)

LCASE

LCASE ( <char value expr> )

Returns a character string that is the lower case version of the <char value expr>. Equivalent to SQL/Foundation LOWER (<char value expr>). (JDBC)

LEFT

LEFT ( <char value expr>, <count> )

Returns a character string consisting of the first <count> characters of <char value expr>. Equivalent to SQL/Foundation SUBSTRING(<char value expr> FROM 0 FOR <count>). (JDBC)

LENGTH

LENGTH ( <char value expr> )

Returns as a BIGINT value the number of characters in <char value expr>. Equivalent to SQL/Foundation CHAR_LENGTH(<char value expr>). (JDBC)

LOCATE

LOCATE ( <char value expr 1>, <char value expr 2> [ , <offset> ] )

Returns as a BIGINT value the starting position of the first occurrence of <char value expr 1> within <char value expr 2>. If <offset> is specified, the search begins with the position indicated by <offset>. If the search is not successful, 0 is returned. Without the third argument, LOCATE is equivalent to the SQL Standard function POSITION(<char value expr 1> IN <char value expr 2>). (JDBC)

LPAD

LPAD ( <char value expr 1>, <length> [, <char value expr 2> ] )

Returns a character string with the length of <length> characters. The string contains the characters of <char value expr 1> padded to the left with spaces. If <length> is smaller than the length of the string argument, the argument is truncated. If the optional <char value expr 2> is specified, this string is used for padding, instead of spaces. (HyperSQL)

LTRIM

LTRIM ( <char value expr> )

Returns a character string based on <char value expr> with the leading space characters removed. Equivalent to SQL/Foundation TRIM( LEADING ' ' FROM <char value expr> ). (JDBC)

OCTET_LENGTH

OCTET_LENGTH ( <string value expression> )

The OCTET_LENGTH function can be used with character or binary strings.

Return a BIGINT value that measures the length of the string in octets. When used with character strings, the octet count is returned, which is twice the normal length. (Foundation)

OVERLAY

OVERLAY ( <char value expr 1> PLACING <char value expr 2>

FROM <start position> [ FOR <string length> ] [ USING CHARACTERS ] )

OVERLAY ( <binary value expr 1> PLACING <binary value expr 2>

FROM <start position> [ FOR <string length> ] )

The character version of OVERLAY returns a character string based on <char value expr 1> in which <string length> characters have been removed from the <start position> and in their place, the whole <char value expr 2> is copied.

The binary version of OVERLAY returns a binary string formed in the same manner as the character version. (Foundation)

POSITION

POSITION ( <char value expr 1> IN <char value expr 2> [ USING CHARACTERS ] )

POSITION ( <binary value expr 1> IN <binary value expr 2> )

The character and binary versions of POSITION search the string value of the second argument for the first occurrence of the first argument string. If the search is successful, the position in the string is returned as a BIGINT. Otherwise zero is returned. (Foundation)

RAWTOHEX

RAWTOHEX( <binary value expr> )

Returns a character string composed of hexadecimal digits representing the bytes in the <binary value expr>. Each byte of the <binary value expr> is translated into two hex digits. (HyperSQL)

REGEXP_MATCHES

REGEXP_MATCHES ( <char value expr>, <regular expression> )

Returns true if the <char value expr> matches the <regular expression> as a whole. The <regular expression> is defined according to Java language regular expression rules. (HyperSQL)

REGEXP_SUBSTRING

REGEXP_SUBSTRING ( <char value expr>, <regular expression> )

Returns the first region in the <char value expr> that matches the <regular expression>. The <regular expression> is defined according to Java language regular expression rules. (HyperSQL)

REGEXP_SUBSTRING_ARRAY

REGEXP_SUBSTRING_ARRAY ( <char value expr>, <regular expression> )

Returns all the regions in the the <char value expr> that match the <regular expression>. The <regular expression> is defined according to Java language regular expression rules. Returns an array containing the matching regions (HyperSQL)

REPEAT

REPEAT ( <char value expr>, <count> )

Returns a character string based on <char value expr>, repeated <count> times. (JDBC)

REPLACE

REPLACE ( <char value expr 1>, <char value expr 2> [, <char value expr 3> ] )

Returns a character string based on <char value expr 1> where each occurrence of <char value expr 2> has been replaced with a copy of <char value expr 3>. If the function is called with just two arguments, the <char value expr 3> defaults to the empty string and calling the function simply removes the occurrences of <char value expr 2> from the first string.(JDBC)

REVERSE

REVERSE ( <char value expr> )

Returns a character string based on <char value expr> with characters in the reverse order. (HyperSQL)

RIGHT

RIGHT ( <char value expr>, <count> )

Returns a character string consisting of the last <count> characters of <char value expr>. (JDBC)

RPAD

RPAD ( <char value expr 1>, <length> [, <char value expr 2> ] )

Returns a character string with the length of <length> characters. The string begins with the characters of <char value expr 1> padded to the right with spaces. If <length> is smaller than the length of the string argument, the argument is truncated. If the optional <char value expr 2> is specified, this string is used for padding, instead of spaces. (HyperSQL)

RTRIM

RTRIM ( <char value expr> )

Returns a character string based on <char value expr> with the trailing space characters removed. Equivalent to SQL/Foundation TRIM(TRAILING ' ' FROM <character string>). (JDBC)

SOUNDEX

SOUNDEX ( <char value expr> )

Returns a four character code representing the sound of <char value expr>. The US census algorithm is used. For example the soundex value for Washington is W252. (JDBC)

SPACE

SPACE ( <count> )

Returns a character string consisting of <count> spaces. (JDBC)

SUBSTR

{ SUBSTR | SUBSTRING } ( <char value expr>, <offset>, <length> )

The JDBC version of SQL/Foundation SUBSTRING returns a character string that consists of <length> characters from <char value expr> starting at the <offset> position. (JDBC)

SUBSTRING

SUBSTRING ( <char value expr> FROM <start position> [ FOR <string length> ] [ USING CHARACTERS ] )

SUBSTRING ( <binary value expr> FROM <start position> [ FOR <string length> ] )

The character version of SUBSTRING returns a character string that consists of the characters of the <char value expr> from <start position>. If the optional <string length> is specified, only <string length> characters are returned.

The binary version of SUBSTRING returns a binary string in the same manner. (Foundation)

TRIM

TRIM ([ [ LEADING | TRAILING | BOTH ] [ <trim character> ] FROM ] <char value expr> )

TRIM ([ [ LEADING | TRAILING | BOTH ] [ <trim octet> ] FROM ] <binary value expr> )

The character version of TRIM returns a character string based on <char value expr>. Consecutive instances of <trim character> are removed from the beginning, the end or both ends of the<char value expr> depending on the value of the optional first qualifier [ LEADING | TRAILING | BOTH ]. If no qualifier is specified, BOTH is used as default. If [ <trim character> ] is not specified, the space character is used as default.

The binary version of TRIM returns a binary string based on <binary value expr>. Consecutive instances of <trim octet> are removed in the same manner as in the character version. If [ <trim octet> ] is not specified, the 0 octet is used as default. (Foundation)

TRANSLATE

TRANSLATE( <char value expr1>, <char value expr2>, <char value expr3> )

Returns a character string based on <char value expr1> source. Each character of the source is checked against the characters in <char value expr2>. If the character is not found, it is not modified. If the character is found, then the character in the same position in <char value expr3> is used. If <char value expr2> is longer than <char value expr3>, then those characters at the end that have no counterpart in <char value expr3> are dropped from the result. (HyperSQL)

 -- in this example any accented character in acolumn is replaced with one without an accent
 TRANSLATE( acolumn, 'ÁÇÉÍÓÚÀÈÌÒÙÂÊÎÔÛÃÕËÜáçéíóúàèìòùâêîôûãõëü', 'ACEIOUAEIOUAEIOUAOEUaceiouaeiouaeiouaoeu');

UCASE

UCASE ( <char value expr> )

Returns a character string that is the upper case version of the <char value expr>. Equivalent to SQL/Foundation UPPER( <char value expr> ) . (JDBC)

UPPER

UPPER ( <char value expr> )

Returns a character string that is the upper case version of the <char value expr> . (Foundation)

Numeric Functions

ABS

ABS ( <num value expr> | <interval value expr> )

Returns the absolute value of the argument as a value of the same type. (JDBC and Foundation)

ACOS

ACOS ( <num value expr> )

Returns the arc-cosine of the argument in radians as a value of DOUBLE type. (JDBC)

ASIN

ASIN ( <num value expr> )

Returns the arc-sine of the argument in radians as a value of DOUBLE type. (JDBC)

ATAN

ATAN ( <num value expr> )

Returns the arc-tangent of the argument in radians as a value of DOUBLE type. (JDBC)

ATAN2

ATAN2 ( <num value expr 1>, <num value expr 2> )

The <num value expr 1> and <num value expr 2> express the x and y coordinates of a point. Returns the angle, in radians, representing the angle coordinate of the point in polar coordinates, as a value of DOUBLE type. (JDBC)

CEILING

{ CEIL | CEILING } ( <num value expr> )

Returns the smallest integer greater than or equal to the argument. If the argument is exact numeric then the result is exact numeric with a scale of 0. If the argument is approximate numeric, then the result is of DOUBLE type. (JDBC and Foundation)

BITAND

BITAND ( <num value expr 1>, <num value expr 2> )

BITAND ( <bit value expr 1>, <bit value expr 2> )

BITANDNOT

BITANDNOT ( <num value expr 1>, <num value expr 2> )

BITANDNOT ( <bit value expr 1>, <bit value expr 2> )

BITNOT

BITNOT ( <num value expr 1> )

BITNOT ( <bit value expr 1> )

BITOR

BITOR ( <num value expr 1>, <num value expr 2> )

BITOR ( <bit value expr 1>, <bit value expr 2> )

BITXOR

BITXOR ( <num value expr 1>, <num value expr 2> )

BITXOR ( <bit value expr 1>, <bit value expr 2> )

These functions bit operations on two values, or in the case of BITNOT on a single values. The values are either integer values, or bit strings. The result is an integer value of the same type as the arguments, or a bit string of the same length as the argument. Each bit of the result is formed by performing the operation on corresponding bits of the arguments. The names of the function indicate NOT, OR, AND, XOR operations. The BITANDNOT performs NOT on the second argument, then performs AND on result and the first argument. (HyperSQL)

COS

COS ( <num value expr> )

Returns the cosine of the argument (an angle expressed in radians) as a value of DOUBLE type. (JDBC)

COT

COT ( <num value expr> )

Returns the cotangent of the argument as a value of DOUBLE type. The <num value expr> represents an angle expressed in radians. (JDBC)

DEGREES

DEGREES ( <num value expr> )

Converts the argument (an angle expressed in radians) into degrees and returns the value in the DOUBLE type. (JDBC)

EXP

EXP ( <num value expr> )

Returns the exponential value of the argument as a value of DOUBLE type. (JDBC and Foundation)

FLOOR

FLOOR ( <num value expr> )

Returns the largest integer that is less than or equal to the argument. If the argument is exact numeric then the result is exact numeric with a scale of 0. If the argument is approximate numeric, then the result is of DOUBLE type. (JDBC and Foundation)

LN

LN ( <num value expr> )

Returns the natural logarithm of the argument, as a value of DOUBLE type. (Foundation)

LOG

LOG ( <num value expr> )

Returns the natural logarithm of the argument, as a value of DOUBLE type. (JDBC)

LOG10

LOG10 ( <num value expr> )

Returns the base 10 logarithm of the argument as a value of DOUBLE type. (JDBC)

MOD ( <num value expr 1>, <num value expr 2> )

MOD

Returns the remainder (modulus) of <num value expr 1> divided by <num value expr 2>. The data type of the returned value is the same as the second argument. (JDBC and Foundation)

PI

PI ()

Returns the constant pi as a value of DOUBLE type. (JDBC)

POWER

POWER ( <num value expr 1>, <num value expr 2> )

Returns the value of <num value expr 1> raised to the power of <int value expr 2> as a value of DOUBLE type. (JDBC and Foundation)

RADIANS

RADIANS ( <num value expr> )

Converts the argument (an angle expressed in degrees) into radians and returns the value in the DOUBLE type. (JDBC)

RAND

RAND ( [ <int value expr> ] )

Returns a random value in the DOUBLE type. The optional [ <int value expr> ] is used as seed value. In HyperSQL each session has a separate random number generator. The first call that uses a seed parameter sets the seed for subsequent calls that do not include a parameter. (JDBC)

ROUND

ROUND ( <num value expr>, <int value expr> )

The <num value expr> is of the DOUBLE type or DECIMAL type. The function returns a DOUBLE or DECIMAL value which is the value of the argument rounded to <int value expr> places right of the decimal point. If <int value expr> is negative, the first argument is rounded to <int value expr> places to the left of the decimal point.

This function rounds values ending with .5 or larger away from zero for DECIMAL arguments and results. When the value ends with .5 or larger and the argument and result are DOUBLE, It rounds the value towards the closest even value.

The datetime version is discussed in the next section. (JDBC)

SIGN

SIGN ( <num value expr> )

Returns an INTEGER, indicating the sign of the argument. If the argument is negative then -1 is returned. If it is equal to zero then 0 is returned. If the argument is positive then 1 is returned. (JDBC)

SIN

SIN ( <num value expr> )

Returns the sine of the argument (an angle expressed in radians) as a value of DOUBLE type. (JDBC)

SQRT

SQRT ( <num value expr> )

Returns the square root of the argument as a value of DOUBLE type. (JDBC and Foundation)

TAN

TAN ( <num value expr> )

Returns the tangent of the argument (an angle expressed in radians) as a value of DOUBLE type. (JDBC)

TO_NUMBER

TO_NUMBER ( <char value expr> )

Performs a cast from character to DECIMAL number. The character string must consist of digits and can have a decimal point. Use the SQL Standard CAST expression instead of this non-standard function. (HyperSQL)

TRUNC

TRUNC ( <num value expr> [, <int value expr>] )

This is a similar to the TRUNCATE function when the first argument is numeric. If the second argument is omitted, zero is used in its place.

The datetime version is discussed in the next section. (HyperSQL)

TRUNCATE

TRUNCATE ( <num value expr> [, <int value expr>] )

Returns a value in the same type as <num value expr> but may reduce the scale of DECIMAL and NUMERIC values. The value is rounded by replacing digits with zeros from <int value expr> places right of the decimal point to the end. If <int value expr> is negative, ABS( <int value expr> ) digits to left of the decimal point and all digits to the right of the decimal points are replaced with zeros. Results of calling TRUNCATE with 12345.6789 with (-2, 0, 2, 4) are (12300, 12345, 12345.67, 12345.6789). The function does not change the number if the second argument is larger than or equal to the scale of the first argument.

If the second argument is not a constant (when it is a parameter or column reference) then the type of the return value is always the same as the type of the first argument. In this case, the discarded digits are replaced with zeros. (JDBC)

WIDTH_BUCKET

WIDTH_BUCKET ( <value expr 1> , <value expr 2>, <value expr 3>, <int value expr> )

Returns an integer value between 0 and <int value expr> + 1. The initial three parameters are of the same numeric or datetime type. The range, ( <value expr 2> , <value expr 3> ) is divided into <int value expr> equal sections (buckets). The returned integer value indicates the index of the bucket where <value expr 1> can be placed. If the <value expr 1> falls before or after the range, the return value is 0 or <value expr 1> + 1 respectively.

This function can be used with numeric or datetime values. Invalid arguments, including <int value expr> smaller than 1, or equal values for <value expr 2> and <value expr 3> will cause an exception. (Foundation)

An example is given below:

 WIDTH_BUCKET( 5, 10, 110, 10)
 0

 WIDTH_BUCKET( 23, 10, 110, 10)
 2

 WIDTH_BUCKET( 100, 10, 110, 10)
 10

 WIDTH_BUCKET( 200, 10, 110, 10)
 11

Date Time and Interval Functions

Functions to report the time zone.

Functions to Report the Time Zone.

TIMEZONE

TIMEZONE()

Returns the current time zone for the session. Returns an INTERVAL HOUR TO MINUTE value. (HyperSQL)

SESSION_TIMEZONE

SESSION_TIMEZONE()

Returns the default time zone for the current session. Returns an INTERVAL HOUR TO MINUTE value. (HyperSQL)

SESSIONTIMEZONE

SESSIONTIMEZONE()

Same as SESSION_TIMEZONE. (HyperSQL)

DATABASE_TIMEZONE

DATABASE_TIMEZONE()

Returns the time zone for the database engine. This is based on where the database server process is located. Returns an INTERVAL HOUR TO MINUTE value. (HyperSQL)

DBTIMEZONE

DBTIMEZONE()

Same as DATABASE_TIMEZONE. (HyperSQL)

Functions to Report the Current Datetime

CURRENT_DATE

CURRENT_DATE

CURRENT_TIME

CURRENT_TIME [ ( <time precision> ) ]

LOCALTIME

LOCALTIME [ ( <time precision> ) ]

CURRENT_TIMESTAMP

CURRENT_TIMESTAMP [ ( <timestamp precision> ) ]

LOCALTIMESTAMP

LOCALTIMESTAMP [ ( <timestamp precision> ) ]

These datetime functions return the datetime value representing the moment the function is called. CURRENT_DATE returns a value of DATE type. CURRENT_TIME returns a value of TIME WITH TIME ZONE type. LOCALTIME returns a value of TIME type. CURRENT_TIMESTAMP returns a value of TIMESTAMP WITH TIME ZONE type. LOCALTIMESTAMP returns a value of TIMESTAMP type. If the optional [ ( <time precision> ) ] or [ ( <timestamp precision> ) ] is used, then the returned value has the specified fraction of the second precision. (Foundation)

NOW

NOW ()

This function is equivalent to LOCALTIMESTAMP. It can be used as a no-arg function as the parens are optional. (HyperSQL)

CURDATE

CURDATE ()

This function is equivalent to CURRENT_DATE. (JDBC)

CURTIME

CURTIME ()

This function is equivalent to LOCALTIME. (JDBC)

SYSDATE

SYSDATE

This function is equivalent to LOCALTIMESTAMP. (HyperSQL)

SYSTIMESTAMP

SYSTIMESTAMP

This no-arg function is equivalent to CURRENT_TIMESTAMP and is enabled in ORA syntax mode only. (HyperSQL)

TODAY

TODAY

This no-arg function is equivalent to CURRENT_DATE. (HyperSQL)

Functions to Extract an Element of a Datetime

DAYNAME

DAYNAME ( <datetime value expr> )

This function is equivalent to EXTRACT ( DAY_NAME FROM ... ) Returns a string in the range of Sunday - Saturday. (JDBC)

DAYOFMONTH

DAYOFMONTH ( <datetime value expr> )

This function is equivalent to EXTRACT ( DAY_OF_MONTH FROM ... ) Returns an integer value in the range of 1-31. (JDBC)

DAYOFWEEK

DAYOFWEEK ( <datetime value expr> )

This function is equivalent to EXTRACT ( DAY_OF_WEEK FROM ... ) Returns an integer value in the range of 1-7. The first day of the week is Sunday. (JDBC)

DAYOFYEAR

DAYOFYEAR ( <datetime value expr> )

This function is equivalent to EXTRACT ( DAY_OF_YEAR FROM ... ) Returns an integer value in the range of 1-366. (JDBC)

DAYS

DAYS ( <datetime value expr> )

The <datetime value expr> is of DATE or TIMESTAMP type. This function returns the DAY number since the first day of the calendar. The first day is numbered 1. (HyperSQL)

HOUR

HOUR ( <datetime value expr> )

This function is equivalent to EXTRACT ( HOUR FROM ... ) Returns an integer value in the range of 0-23. (JDBC)

MINUTE

MINUTE ( <datetime value expr> )

This function is equivalent to EXTRACT ( MINUTE FROM ... ) Returns an integer value in the range of 0 - 59. (JDBC)

MONTH

MONTH ( <datetime value expr> )

This function is equivalent to EXTRACT ( MONTH FROM ... ) Returns an integer value in the range of 1-12. (JDBC)

MONTHNAME

MONTHNAME ( <datetime value expr> )

This function is equivalent to EXTRACT ( NAME_OF_MONTH FROM ... ) Returns a string in the range of January - December. (JDBC)

QUARTER

QUARTER ( <datetime value expr> )

This function is equivalent to EXTRACT ( QUARTER FROM ... ) Returns an integer in the range of 1 - 4. (JDBC)

SECOND

SECOND ( <datetime value expr> )

This function is equivalent to EXTRACT ( SECOND FROM ... ) Returns an integer or decimal in the range of 0 - 59, with the same precision as the <datetime value expr>. (JDBC)

SECONDS_SINCE_MIDNIGHT

SECONDS_SINCE_MIDNIGHT ( <datetime value expr> )

This function is equivalent to EXTRACT ( SECONDS_SINCE_MIDNIGHT FROM ... ) Returns an integer in the range of 0 - 86399. (HyperSQL)

UNIX_TIMESTAMP

UNIX_TIMESTAMP ( [ <datetime value expression> ] )

This function returns a BIGINT value. With no parameter, it returns the number of seconds since 1970-01-01. With a DATE or TIMESTAMP parameter, it converts the argument into number of seconds since 1970-01-01. The TIMESTAMP ( <num value expression> function returns a TIMESTAMP from a Unix timestamp. (HyperSQL)

WEEK

WEEK ( <datetime value expr> )

This function is equivalent to EXTRACT ( WEEK_OF_YEAR FROM ... ) Returns an integer in the range of 1 - 54. (JDBC)

YEAR

YEAR ( <datetime value expr> )

This function is equivalent to EXTRACT ( YEAR FROM ... ) Returns an integer in the range of 1 - 9999. (JDBC)

EXTRACT

EXTRACT ( <extract field> FROM <extract source> )

<extract field> ::= YEAR | MONTH | DAY | HOUR | MINUTE | DAY_OF_WEEK | WEEK_OF_YEAR | QUARTER | DAY_OF_YEAR | DAY_OF_MONTH |

TIMEZONE_HOUR | TIMEZONE_MINUTE | SECOND | SECONDS_SINCE_MIDNIGHT |

DAY_NAME | MONTH_NAME

<extract source> ::= <datetime value expr> | <interval value expr>

The EXTRACT function returns a field or element of the <extract source>. The <extract source> is a datetime or interval expression. The type of the return value is BIGINT for most of the <extract field> options. The exceptions is SECOND where a DECIMAL value is returned which has the same precision as the datetime or interval expression. The field values DAY_NAME or MONTH_NAME result in a character string. When MONTH_NAME is specified, a string in the range January - December is returned. When DAY_NAME is specified, a string in the range Sunday -Saturday is returned.

If the <extract source> is FROM <datetime value expr>, different groups of <extract source> can be used depending on the data type of the expression. The TIMEZONE_HOUR | TIMEZONE_MINUTE options are valid only for TIME WITH TIMEZONE and TIMESTAMP WITH TIMEZONE data types. The HOUR | MINUTE | SECOND | SECONDS_MIDNIGHT options, are valid for TIME and TIMESTAMP types. The rest of the fields are valid for DATE and TIMESTAMP types.

If the <extract source> is FROM <interval value expr>, the <extract field> must be one of the fields of the INTERVAL type of the expressions. The YEAR | MONTH options may be valid for INTERVAL types based on months. The DAY | HOUR | MINUTE | SECOND | SECONDS_MIDNIGHT options may be valid for INTERVAL types based on seconds. For example, DAY | HOUR | MINUTE are the only valid fields for the INTERVAL DAY TO MINUTE data type. (Foundation with HyperSQL extensions)

Functions for Datetime Arithmetic

ADD_MONTHS

ADD_MONTHS ( <datetime value expr>, <numeric value expr>)

This function is similar but different to simple addition a MONTH interval to a datetime value. The SQL Standard expression, <datetime value expr> + n MONTH, when used with the last day of a short month such as February, returns a date that has the same day of the month in the target month. The ADD_MONTHS function adjusts the target day to the last day of the target month. For all other days, the behaviour is the same. This function always returns a TIMESTAMP(0) value, regardless of the type of the argument. (HyperSQL)

The example below compares the output of the function and the expression.

 VALUES ADD_MONTHS ( DATE '2012-02-29' , 1), DATE '2012-02-29' + 1 MONTH

 C1                  C2         
 ------------------- ---------- 
 2012-03-31 00:00:00 2012-03-29 

LAST_DAY

LAST_DAY ( <datetime value expr> )

Returns the last day of the month for the given argument, preserving the year, month, hour, minute and secend fields of the date. The type of the result is always TIMESTAMP(0). (HyperSQL)

 VALUES LAST_DAY ( TIMESTAMP '2012-02-14 12:30:44')

 C1                  
 ------------------- 
 2012-02-29 12:30:44 

MONTHS_BETWEEN

MONTHS_BETWEEN ( <datetime value expr1> , <datetime value expr2> )

Returns the a number (not an INTERVAL) possibly with a fraction, representing the number of months between two days. If both dates have the same day of month, or are on the last day of the month, the result is an exact numeric. Otherwise, the fraction is calculated base on 31 days per month. You can cast the resulting value into INTERVAL MONTH and use it for datetime arithmetic. (HyperSQL)

 VALUES MONTHS_BETWEEN ( TIMESTAMP '2013-02-14 12:30:44', TIMESTAMP '2012-01-04 12:30:44')

 C1                                  
 ----------------------------------- 
 13.32258064516129000000000000000000 

TIMESTAMPADD

TIMESTAMPADD ( <tsi datetime field>, <numeric value expression>, <datetime value expr>)

TIMESTAMPDIFF

TIMESTAMPDIFF ( <tsi datetime field>, <datetime value expr 1>, <datetime value expr 2>)

<tsi datetime field> ::= SQL_TSI_FRAC_SECOND | SQL_TSI_SECOND | SQL_TSI_MINUTE | SQL_TSI_HOUR | SQL_TSI_DAY | SQL_TSI_WEEK | SQL_TSI_MONTH | SQL_TSI_QUARTER | SQL_TSI_YEAR

HyperSQL supports full SQL Standard datetime features. It supports adding integers representing units of time directly to datetime values using the arithmetic plus operator. It also supports subtracting one <datetime value expr> from another in the given units of date or time using the minus operator. An example of <datetime value expr> + <numeric value expression> <datetime field> is LOCALTIMESTAMP + 5 DAY. An example of ( <datetime value expr> - <numeric value expression> ) <datetime field> is (CURRENT_DATE - DATE '2008-08-8') MONTH which returns the number of calendar months between the two dates.

The two JDBC functions, TIMESTAMPADD and TIMESTAMPDIFF perform the same function as above SQL expressions. The field names are keywords and are different from those used in the EXTRACT functions. These names are valid for use only when calling these two functions. The return value for TIMESTAMPADD is of the same type as the datetime argument used. The return type for TIMESTAMPDIFF is always BIGINT, regardless of the type of arguments. The two datetime arguments of TIMESTAMPDIFF should be of the same type. The TIME type is not supported for the arguments to these functions.

TIMESTAMPDIFF is evaluated as <datetime value expr 2> - <datetime value expr 1>. (JDBC)

 TIMESTAMPADD ( SQL_TSI_MONTH, 3, DATE '2008-11-22' )

 TIMESTAMPDIFF ( SQL_TSI_HOUR, TIMESTAMP '2008-11-20 20:30:40', TIMESTAMP '2008-11-21 21:30:40' )

DATE_ADD

DATE_ADD ( <datetime value expr> , <interval value expr> )

DATE_SUB

DATE_SUB ( <datetime value expr> , <interval value expr> )

These functions are equivalent to arithmetic addition and subtraction, <datetime value expr> + <interval value expr> and <datetime value expr> - <interval value expr>. The functions are provided for compatibility with other databases. The supported interval units are the standard SQL interval unit listed in other chapters of this guide. The TIME type is supported for the argument to these functions. (HyperSQL)

 DATE_ADD ( DATE '2008-11-22', INTERVAL 3 MONTH )

 DATE_SUB ( TIMESTAMP '2008-11-22 20:30:40', INTERVAL 20 HOUR )

DATEADD

DATEADD ( <field>, <numeric value expr>, <datetime value expr> )

DATEDIFF

DATEDIFF ( <field>, <datetime value expr 1>, <datetime value expr 2> )

<field> ::= 'yy' | 'year' | 'mm' | 'month' | 'dd' | 'day' | 'hh' | 'hour' | 'mi' | 'minute' | 'ss' | 'second' | 'ms' | 'millisecond'

The DATEADD and DATEDIFF functions are alternatives to TIMESTAMPADD and TIMESTAMPDIFF, with fewer available field options. The field names are specified as strings, rather than keywords. The fields translate to YEAR, MONTH, DAY, HOUR, MINUTE, SECOND and MILLISECOND. DATEDIFF is evaluated as <datetime value expr 2> - <datetime value expr 1>. (HyperSQL}

DATEDIFF ( <datetime value expr 1>, <datetime value expr 2> )

This special form of DATEDIFF does not have a field parameter and return the number of days between two dates. This form is evaluated as <datetime value expr 1> - <datetime value expr 2>, which is different from the main form. This form is compatible with some other database engines. The TIME type is not supported for the arguments to these functions. (HyperSQL}

 DATEADD ( 'month', 3, DATE '2008-11-22' )

 DATEDIFF ( 'hour', TIMESTAMP '2008-11-22 20:30:40', TIMESTAMP '2008-11-22 00:30:40' )

ROUND

ROUND ( <datetime value expr> [ , <char value expr> ] )

The <datetime value expr> is of DATE, TIME or TIMESTAMP type. The <char value expr> is a format string for YEAR, MONTH, WEEK OF YEAR, DAY, HOUR, MINUTE or SECOND as listed in the table for TO_CHAR and TO_DATE format elements (see below). The datetime value is rounded up or down after the specified field and the rest of the fields to the right are set to one for MONTH and DAY, or zero, for the rest of the fields. For example rounding a timestamp value on the DAY field results in midnight the same date or midnight the next day if the time is at or after 12 noon. If the second argument is omitted, the datetime value is rounded to the nearest day. (HyperSQL)

TRUNC

TRUNC ( <datetime value expr> [ , <char value expr> ] )

Similar to the ROUND function, the <num value expr> is of DATE, TIME or TIMESTAMP type. The <char value expr> is a format string (such as 'YY' or 'MM') for YEAR, MONTH, WEEK OF YEAR, DAY, HOUR, MINUTE or SECOND as listed in the table for TO_CHAR and TO_DATE format elements (see below). The datetime value is truncated after the specified field and the rest of the fields to the right are set to one for MONTH and DAY, or zero, for the rest of the fields. For example applying TRUNC to a timestamp value on the DAY field results in midnight the same date. Examples of ROUND and TRUNC functions are given below. If the second argument is omitted, the datetime value is truncated to midnight the same date. (HyperSQL)

 ROUND ( TIMESTAMP'2008-08-01 20:30:40', 'YYYY' )

 TIMESTAMP '2009-01-01 00:00:00'

 TRUNC ( TIMESTAMP'2008-08-01 20:30:40', 'YYYY' )

 TIMESTAMP '2008-01-01 00:00:00'

Functions to Convert or Format a Datetime

TIMESTAMP

TIMESTAMP ( <num value expr> )

TIMESTAMP ( <char value expr> )

TIMESTAMP ( <char value expr>, <char value expr> )

TIMESTAMP ( <date value expr>, <time value expr> )

This function translates the arguments into a TIMESTAMP WIHOUT TIME ZONE value.

When the single argument is a numeric value, it is interpreted as a Unix timestamp in seconds.

When the single argument is a formatted date or timestamp string, it is translated to a TIMESTAMP.

When two arguments are used, the first argument is the date part and the second argument is the time part of the returned TIMESTAMP value. An example, including the result, is given below:

 TIMESTAMP ( '2008-11-22', '20:30:40' )

 TIMESTAMP '2008-11-22 20:30:40.000000'

TIMESTAMP_WITH_ZONE

TIMESTAMP_WITH_ZONE ( <num value expr> )

TIMESTAMP_WITH_ZONE ( <char value expr> )

This function translates the arguments into a TIMESTAMP WITH TIME ZONE value.

When the single argument is a numeric value, it is interpreted as a Unix timestamp in seconds.

When the single argument is TIMESTAMP, it is converted to TIMESTAMP WITH TIME ZONE.

The time zone of the returned value is the local time zone at the time of the timestamp argument. This accounts for daylight saving times. For example, if the local time zone was +4:00 at the time of the given Unix timestamp, the returned value is local timestamp at the time with time zone +4:00.

TO_CHAR

TO_CHAR ( <datetime value expr>, <char value expr> )

This function formats a datetime or numeric value to the format given in the second argument. The format string can contain pattern elements from the list given below, plus punctuation and space characters. An example, including the result, is given below:

 TO_CHAR ( TIMESTAMP'2008-02-01 20:30:40', 'YYYY BC MONTH, DAY HH' )

 2008 AD February, Friday 8

 TO_CHAR ( TIMESTAMP'2008-02-01 20:30:40', '"The Date is" YYYY BC MONTH, DAY HH' )

 The Date is 2008 AD February, Friday 8

The format is internally translated to a java.text.SimpleDateFormat format string. Separator characters (space, comma, period, hyphen, colon, semicolon, forward slash) can be included between the pattern elements. Unsupported format strings should not be used. You can include a string literal inside the format string by enclosing it in double quotes (see the second example above). (HyperSQL)

TO_DATE

TO_DATE ( <char value expr>, <char value expr> )

This function translates a formatted datetime sting to a TIMESTAMP(0) according to the format given in the second argument. See TO_TIMESTAMP below for further details.

TO_TIMESTAMP

TO_TIMESTAMP ( <char value expr>, <char value expr> )

This function translates a formatted datetime sting to a TIMESTAMP(6) according to the format given in the second argument. The format string can contain pattern elements from the list given below, plus punctuation and space characters. The pattern should contain all the necessary fields to construct a date, including, year, month, day of month, etc. The returned timestamp can then be cast into DATE or TIME types if necessary. An example, including the result, is given below:

 TO_TIMESTAMP ( '22/11/2008 20:30:40', 'DD/MM/YYYY HH:MI:SS' )

 TIMESTAMP '2008-11-22 20:30:40.000000'

The format strings that can be used for TO_DATE and TO_TIMESTAMP are more restrictive than those used for TO_CHAR, because the format string must contain the elements needed to build a full DATE or TIMESTAMP value. For example, you cannot use the 'WW', 'W', 'HH' or 'HH12' format elements with TO_DATE or TO_TIMESTAMP

The format is internally translated to a java.text.SimpleDateFormat format string. Unsupported format strings should not be used. With TO_CHAR, you can include a string literal inside the format string by enclosing it in double quotes. (HyperSQL)

The supported format components are all uppercase as follows:

Table 10.1. TO_CHAR, TO_DATE and TO_TIMESTAMP format elements

BC | B.C. | AD | A.D.Returns AD for common era and BC for before common era
RRRR

4-digit year

YYYY

4-digit year

IYYY

4-digit year, corresponding to ISO week of the year. The reported year for the last few days of the calendar year may be the next year.

YY

2 digit year

IY

2 digit year, corresponding to ISO week of the year

MM

Month (01-12)

MON

Short three-letter name of month

MONTH

Name of month

WW

Week of year (1-53) where week 1 starts on the first day of the year and continues to the seventh day of the year (not a calendar week).

W

Week of month (1-5) where week 1 starts on the first day of the month and ends on the seventh (not a calendar week).

IW

Week of year (1-52 or 1-53) based on the ISO standard. Week starts on Monday. The first week may start near the end of previous year.

DAY

Name of day.

DD

Day of month (01-31).

DDD

Day of year (1-366).

DY

Short three-letter name of day.

HH

Hour of day (00-11).

HH12

Hour of day (00-11).

HH24

Hour of day (00-23).

MI

Minute (00-59).

SS

Second (00-59).

FF

Fractional seconds.


Array Functions

Array functions are specialised functions with ARRAY parameters or return values. For the ARRAY_AGG aggregate function, see the Data Access and Change chapter.

CARDINALITY

CARDINALITY( <array value expr> )

Returns the element count for the given array argument. (Foundation)

MAX_CARDINALITY

MAX_CARDINALITY( <array value expr> )

Returns the maximum allowed element count for the given array argument. (Foundation)

POSITION_ARRAY

POSITION_ARRAY( <value expression> IN <array value expr> [ FROM <int value expr> ] )

Returns the position of the first match for the <value expression> in the array. By default the search starts from the beginning of the array. The optional <int value expr> specifies the start position. Positions are counted from 1. Returns zero if no match is found. (HyperSQL)

SORT_ARRAY

SORT_ARRAY( <array value expr> [ { ASC | DESC } ] [ NULLS { FIRST | LAST } ] )

Returns a sorted copy of the array. By default, sort is performed in ascending order and NULL elements are sorted first. (HyperSQL)

TRIM_ARRAY

TRIM_ARRAY( <array value expr>, <num value expr> )

Returns a new array that contains the elements of the <array value expr> minus the number of elements specified by the <num value expr>. Elements are discarded from the end of the array. (Foundation)

SEQUENCE_ARRAY

SEQUENCE_ARRAY( <value expr 1>, <value expr 2>, <value expr 3 )

Returns a new array that contains a sequence of values. The <value expr 1> is the lower bound of the range. The <value expr 2> is the upper bound of the range. The <value expr 3> is the increment. The elments of the array are within the inclusive range. The first element is <value expr 1> and each subsequent element is the sum of the previous element and the increment. If the increment is zero, only the first element is returned. When the increment is negative, the lower bound should be larger than the upper bound. The type of the arguments can be all number types, or a datetime range and an interval for the third argument (HyperSQL)

In the examples below, a number sequence and a date sequence are shown. The UNNEST table expression is used to form a table from the array.

 SEQUENCE_ARRAY(0, 100, 5)

 ARRAY[0,5,10,15,20,25,30,35,40,45,50,55,60,65,70,75,80,85,90,95,100] 

 SELECT * FROM UNNEST(SEQUENCE_ARRAY(10, 12, 1))

 C1 
 -- 
 10 
 11 
 12 

 SELECT * FROM UNNEST(SEQUENCE_ARRAY(CURRENT_DATE, CURRENT_DATE + 6 DAY, 1 DAY)) WITH ORDINALITY AS T(D, I) 

 D          I 
 ---------- - 
 2010-08-01 1 
 2010-08-02 2 
 2010-08-03 3 
 2010-08-04 4 
 2010-08-05 5 
 2010-08-06 6 
 2010-08-07 7

General Functions

General functions can take different types of arguments. Some General Functions accept a variable number of arguments.

Also see the Data Access and Change chapter for SQL expressions that are similar to functions, for example CAST and NULLIF.

CASEWHEN

CASEWHEN( <boolean value expr>, <value expr 2>, <value expr 3> )

If the <boolean value expr> is true, returns <value expr 2> otherwise returns <value expr 3>. Use a CASE WHEN expression instead for more extensive capabilities and options. CASE WHEN is documented in the Data Access and Change chapter. (HyperSQL)

COALESCE

COALESCE( <value expr 1>, <value expr 2> [, ...] )

Returns <value expr 1> if it is not null, otherwise returns <value expr 2> if not null and so on. The type of both arguments must be comparable. (Foundation)

CONVERT

CONVERT ( <value expr> , <data type> )

<data type> ::= { SQL_BIGINT | SQL_BINARY | SQL_BIT |SQL_BLOB | SQL_BOOLEAN | SQL_CHAR | SQL_CLOB | SQL_DATE | SQL_DECIMAL | SQL_DATALINK |SQL_DOUBLE | SQL_FLOAT | SQL_INTEGER | SQL_LONGVARBINARY | SQL_LONGNVARCHAR | SQL_LONGVARCHAR | SQL_NCHAR | SQL_NCLOB | SQL_NUMERIC | SQL_NVARCHAR | SQL_REAL | SQL_ROWID | SQL_SQLXML | SQL_SMALLINT | SQL_TIME | SQL_TIMESTAMP | SQL_TINYINT | SQL_VARBINARY | SQL_VARCHAR} [ ( <precision, length or scale parameters> ) ]

The CONVERT function is a JDBC escape function, equivalent to the SQL standard CAST expression. It converts the <value expr> into the given <data type> and returns the value. The <data type> options are synthetic names made by prefixing type names with SQL_. Some of the <data type> options represent valid SQL types, but some are based on non-standard type names, namely { SQL_LONGNVARCHAR | SQL_LONGVARBINARY |SQL_LONGVARCHAR | SQL_TINYINT }. None of the synthetic names can be used in any other context than the CONVERT function.

The definition of CONVERT in the JDBC Standard does not allow the precision, scale or length to be specified. This is required by the SQL standard for BINARY, BIT, BLOB, CHAR, CLOB, VARBINARY and VARCHAR types and is often needed for DECIMAL and NUMERIC. Defaults are used for precision.

HyperSQL also allows the use of real type names (without the SQL_ prefix). In this usage, HyperSQL allows the use of precision, scale or length for the type definition when they are valid for the type definition.

When MS SQL Server compatibility mode is on, the parameters of CONVERT are switched and only the real type names with required precision, scale or length are allowed. (JDBC)

DECODE

DECODE( <value expr main>, <value expr match 1>, <value expr result 1> [...,] [, <value expr default>] )

DECODE takes at least 3 arguments. The <value expr main> is compared with <value expr match 1> and if it matches, <value expr result 1> is returned. If there are additional pairs of <value expr match n> and <value expr result n>, comparison is repeated until a match is found the result is returned. If no match is found, the <value expr default> is returned if it is specified, otherwise NULL is returned. The type of the return value is a combination of the types of the <value expr result ... > arguments. (HyperSQL)

GREATEST

GREATEST( <value expr 1>, [<value expr ...>, ...] )

The GREATEST function takes one or more arguments. It compares the arguments with each other and returns the greatest argument. The return type is the combined type of the arguments. Arguments can be of any type, so long as they are comparable. (HyperSQL)

IFNULL

ISNULL

IFNULL | ISNULL ( <value expr 1>, <value expr 2> )

Returns <value expr 1> if it is not null, otherwise returns <value expr 2>. The type of the return value is the type of <value expr 1>. Almost equivalent to SQL Standard COALESCE(<value expr 1>, <value expr 2>) function, but without type modification. (JDBC)

LEAST

LEAST( <value expr 1>, [<value expr ...>, ...] )

The LEAST function takes one or more arguments. It compares the arguments with each other and returns the smallest argument. The return type is the combined type of the arguments. Arguments can be of any type, so long as they are comparable. (HyperSQL)

LOAD_FILE

LOAD_FILE ( <char value expr 1> [, <char value expr 2>] )

Returns a BLOB or CLOB containing the URL or file path specified in the first argument. If used with a single argument, the function returns a BLOB. If used with two arguments, the function returns a CLOB and the second argument is the character encoding of the file.

The file path is interpreted the same way as a TEXT TABLE source file location. The hsqldb.allow_full_path system property must be set true in order to access files outside the directory structure of the database files.

(HyperSQL)

NULLIF

NULLIF( <value expr 1>, <value expr 2> )

Returns <value expr 1> if it is not equal to <value expr 2>, otherwise returns null. The type of both arguments must be the same. This function is a shorthand for a specific CASE expression. (Foundation)

NVL

NVL( <value expr 1>, <value expr 2> )

Returns <value expr 1> if it is not null, otherwise returns <value expr 2>. The type of the return value is the type of <value expr 1>. For example, if <value expr 1> is an INTEGER column and <value expr 2> is a DOUBLE constant, the return type is cast into INTEGER. This function is similar to IFNULL. (HyperSQL)

NVL2

NVL2( <value expr 1>, <value expr 2>, <value expr 3> )

If <value expr 1> is not null, returns <value expr 2>, otherwise returns <value expr 3>. The type of the return value is the type of <value expr 2> unless it is null. (HyperSQL)

UUID

UUID ( [ { <char value expr> | <binary value expr> ] } )

With no parameter, this function returns a new UUID value as a 16 byte binary value. With a UUID hexadecimal string argument, it returns the 16 byte binary value of the UUID. With a 16 byte binary argument, it returns the formatted UUID character representation. (HyperSQL)

System Functions

CRYPT_KEY

CRYPT_KEY( <value expr 1>, <value expr 2> )

Returns a binary string representation of a cryptography key for the given cipher and cryptography provider. The cipher specification is specified by <value expr 1> and the provider by <value expr 2>. To use the default provider, specify null for <value expr 2>. (HyperSQL)

DIAGNOSTICS

DIAGNOSTICS ( ROW_COUNT )

This is a convenience function for use instead of the GET DIAGNOSTICS ... statement. The argument specifies the name of the diagnostics variable. Currently the only supported variable is the ROW_COUNT variable. The function returns the row count returned by the last executed statement. The return value is 0 after most statements. Calling this function immediately after executing an INSERT, UPDATE, DELETE or MERGE statement returns the row count for the last statement, as it is returned by the JDBC statement. (HyperSQL)

IDENTITY

IDENTITY ()

Returns the last IDENTITY value inserted into a row by the current session. The statement, CALL IDENTITY() can be made after an INSERT statement that inserts a row into a table with an IDENTITY column. The CALL IDENTITY() statement returns the last IDENTITY value that was inserted into a table by the current session. Each session manages this function call separately and is not affected by inserts in other sessions. The statement can be executed as a direct statement or a prepared statement. (HyperSQL)

DATABASE

DATABASE ()

Returns the file name (without directory information) of the database. (JDBC)

DATABASE_NAME

DATABASE_NAME ()

Returns the database name. This name is a 16 character, uppercase string. It is generated as a string based on the timestamp of the creation of the database, for example HSQLDB32438AEAFB. The name can be redefined by an admin user but the new name must be all uppercase and 16 characters long. This name is used in log messages with external logging frameworks. (HyperSQL)

DATABASE_VERSION

DATABASE_VERSION ()

Returns the full version string for the database engine. For example, 2.0.1. (JDBC)

USER

USER ()

Equivalent to the SQL function CURRENT_USER. (JDBC)

CURRENT_USER

CURRENT_USER

CURRENT_ROLE

CURRENT_ROLE

SESSION_USER

SESSION_USER

SYSTEM_USER

SYSTEM_USER

CURRENT_SCHEMA

CURRENT_SCHEMA

CURRENT_CATALOG

CURRENT_CATALOG

These functions return the named current session attribute. They are all SQL Standard functions.

The CURRENT_USER is the user that connected to the database, or a user subsequently set by the SET AUTHORIZATION statement.

SESSION_USER is the same as CURRENT_USER

SYSTEM_USER is the user that connected to the database. It is not changed with any command until the session is closed.

CURRENT_SCHEMA is default schema of the user, or a schema subsequently set by the SET SCHEMA command.

CURRENT_CATALOG is always the same within a given HyperSQL database and indicates the name of the catalog.

IS_AUTOCOMMIT

IS_AUTOCOMMIT()

Returns TRUE if the session is in autocommit mode. (HyperSQL)

IS_READONLY_SESSION

IS_READONLY_SESSION()

Returns TRUE if the session is in read only mode. (HyperSQL)

IS_READONLY_DATABASE

IS_READONLY_DATABASE()

Returns TRUE if the database is a read only database. (HyperSQL)

IS_READONLY_DATABASE_FILES

IS_READONLY_DATABASE_FILES()

Returns TRUE if the database is a read-only files database. In this kind of database, it is possible to modify the data, but the changes are not persisted to the database files. (HyperSQL)

ISOLATION_LEVEL

ISOLATION_LEVEL()

Returns the current transaction isolation level for the session. Returns either READ COMMITTED or SERIALIZABLE as a string. (HyperSQL)

SESSION_ID

SESSION_ID()

Returns the id of the session as a BIGINT value. Each session id is unique during the operational lifetime of the database. Id's are restarted after a shutdown and restart. (HyperSQL)

SESSION_ISOLATION_LEVEL

SESSION_ISOLATION_LEVEL()

Returns the default transaction isolation level for the current session. Returns either READ COMMITTED or SERIALIZABLE as a string. (HyperSQL)

DATABASE_ISOLATION_LEVEL

DATABASE_ISOLATION_LEVEL()

Returns the default transaction isolation level for the database. Returns either READ COMMITTED or SERIALIZABLE as a string. (HyperSQL)

TRANSACTION_SIZE

TRANSACTION_SIZE()

Returns the row change count for the current transaction. Each row change represents a row INSERT or a row DELETE operation. There will be a pair of row change operations for each row that is updated.

TRANSACTION_ID

TRANSACTION_ID()

Returns the current transaction ID for the session as a BIGINT value. The database maintains a global incremental id which is allocated to new transactions and new actions (statement executions) in different sessions. This value is unique to the current transaction. (HyperSQL)

ACTION_ID

ACTION_ID()

Returns the current action ID for the session as a BIGINT value. The database maintains a global incremental id which is allocated to new transactions and new actions (statement executions) in different sessions. This value is unique to the current action. (HyperSQL)

TRANSACTION_CONTROL

TRANSACTION_CONTROL()

Returns the current transaction model for the database. Returns LOCKS, MVLOCKS or MVCC as a string. (HyperSQL)

LOB_ID

LOB_ID( <column reference> )

Returns internal ID of a lob as a BIGINT value. Lob ID's are unique and never reused. The <column reference> is the name of the column (or variable, or argument) which is a CLOB or BLOB. Returns null if the value is null. (HyperSQL)

ROWNUM

ROWNUM()

ROW_NUMBER

ROW_NUMBER() OVER()

Returns the current row number (from 1) being processed in a select statement. This has the same semantics as the ROWNUM pseudo-column in Oracle syntax mode, but can be used in any syntax mode. The function is used in a SELECT of DELETE statement. The ROWNUM of a row is incremented as the rows are added to the result set. It is therefore possible to use a condition such as WHERE ROWNUM() < 10, but not ROWNUM() > 10 or ROWNUM = 10. The ROW_NUMBER() OVER() alternative performs the same function and is included for compatibility with other database engines.(HyperSQL)

Chapter 11. System Management

Fred Toussi

The HSQL Development Group

$Revision: 5321 $

Copyright 2002-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Mode of Operation and Tables

HyperSQL has many modes of operation and features that allow it to be used in very different scenarios. Levels of memory usage, speed and accessibility by different applications are influenced by how HyperSQL is deployed.

Mode of Operation

The decision to run HyperSQL as a separate server process or as an in-process database should be based on the following:

  • When HyperSQL is run as a server on a separate machine, it is isolated from hardware failures and crashes on the hosts running the application.

  • When HyperSQL is run as a server on the same machine, it is isolated from application crashes and memory leaks.

  • Server connections are slower than in-process connections due to the overhead of streaming the data for each JDBC call.

  • You can reduce client/server traffic using SQL Stored procedures to reduce the number of JDBC execute calls.

  • During development, it is better to use a Server with server.silent=false, which displays the statements sent to the server on the console window.

  • To improve speed of execution for statements that are executed repeatedly, reuse a parameterized PreparedStatement for the lifetime of the connections.

Database Types

There are three types of database, mem:, file: and res:. The mem: type is stored all in memory and not persisted to file. The file: type is persisted to file. The res: type is also based on files, but the files are loaded from the classpath, similar to resource and class files. Changes to the data in file: databases are persisted, unless the database is readonly, or files_readonly (using optional property settings). Changes to res: databases are not persisted.

Readonly Databases

A file: catalog can be made readonly permanently, or it can be opened as readonly. To make the database readonly, the property, value pair, readonly=true can be added to the .properties file of the database. The SHUTDOWN command must be used to close the database before making this change.

It is also possible to open a normal database as readonly. For this, the property can be included in the URL of the first connection to the database.

With readonly databases, it is still possible to insert and delete rows in TEMP tables.

RES and Files Readonly Databases

There is another option which allows MEMORY tables to be writeable, but without persisting the changes at SHUTDOWN. This option is activated with the property, value pair, files_readonly=true, which can be added to the .properties file of the database, or included in the URL of the first connection to the database.

A res: catalog, is a set of database files on the classpath (inside a jar or alongside class files). The database is opened with a URL in the form of jdbc:hsqldb:res:<database path>. These databases are always files_readonly and have the same restrictions as files_readonly file: catalogs.

CACHED tables and LOBS in these catalogs are readonly. It is not possible to create new LOBs in these catalogs, but you can use existing LOBs in new rows.

These options are useful for running application tests which operate on a predefined dataset.

Tables

TEXT tables are designed for special applications where the data has to be in an interchangeable format, such as CSV (comma separated values). TEXT tables should not be used for routine storage of data that changes a lot.

MEMORY tables and CACHED tables are generally used for data storage. The difference between the two is as follows:

  • The data for all MEMORY tables is read from the *.script file when the database is started and stored in memory. In contrast the data for cached tables is not read into memory until the table is accessed. Furthermore, only part of the data for each CACHED table is held in memory, allowing tables with more data than can be held in memory.

  • When the database is shutdown in the normal way, all the data for MEMORY tables is written out to the disk. In comparison, the data in CACHED tables that has changed is written out during operation and at shutdown.

  • The size and capacity of the data cache for all the CACHED tables is configurable. This makes it possible to allow all the data in CACHED tables to be cached in memory. In this case, speed of access is good, but slightly slower than MEMORY tables.

  • For normal applications it is recommended that MEMORY tables are used for small amounts of data, leaving CACHED tables for large data sets. For special applications in which speed is paramount and a large amount of free memory is available, MEMORY tables can be used for large tables as well.

  • You can change the type of the table with the SET TABLE <table name> TYPE { CACHED | MEMORY }statement.

Large Objects

HyperSQL 2.0 supports dedicated storage and access to BLOB and CLOB objects. These objects can have huge sizes. BLOB or CLOB is specified as the type of a column of the table. Afterwards, rows can be inserted into the table using a PreparedStatement for efficient transfer of large LOB data to the database. In mem: catalogs, CLOB and BLOB data is stored in memory. In file: catalogs, this data is stored in a single separate file which has the extension *.lobs. The size of this file can grow to huge, terabyte figures. By default, a minimum 32 KB is allocated to each LOB. You can reduced this if your LOBs are generally smaller.

LOB data should be stored in the database using a JDBC PreparedStatement object. The streaming methods send the LOB to the database in one operation as a binary or character stream. Inside the database, the disk space is allocated as needed and the data is saved as it is being received. LOB data should be retrieved from the database using a JDBC ResultSet method. When a streaming method is used to retrieve a LOB, it is retrieved in large chunks in a transparent manner. LOB data can also be retrieved as String or byte[], but these methods use more memory and may not be practical for large objects.

LOB data is not duplicated in the database when a lob is copied from one table to another. The disk space is reused when a LOB is deleted and is no longer contained in any table. This happens only at the time of a CHECKPOINT.

By using a dedicated LOB store, HyperSQL achieves consistently high speeds (usually over 20MB / s) for both storage and retrieval of LOBs.

There is an internal LOBS schema in the database to store the id's, sizes and addresses of the LOBs (but not the actual LOBS) in a few system tables. This schema is stored in the database as MEMORY tables. Therefore the amount of JVM memory should be increased when more than tens of thousands of LOBs are stored in the database. If your database contains more than a few hundreds of thousands of LOBs and memory use becomes an issue, you can change one or all LOB schema tables to CACHED tables. See statements below:

Example 11.1. Using CACHED tables for the LOB schema

 SET TABLE SYSTEM_LOBS.BLOCKS TYPE CACHED
 SET TABLE SYSTEM_LOBS.LOBS TYPE CACHED
 SET TABLE SYSTEM_LOBS.PARTS TYPE CACHED
 SET TABLE SYSTEM_LOBS.LOB_IDS TYPE CACHED

Deployment context

The files used for storing HyperSQL database data are all in the same directory. New files are always created and deleted by the database engine. Two simple principles must be observed:

  • The Java process running HyperSQL must have full privileges on the directory where the files are stored. This include create and delete privileges.

  • The file system must have enough spare room both for the 'permanent' and 'temporary' files. The default maximum size of the *.log file is 50MB. The *.data file can grow to up to 64GB (more if the default has been increased). The .backup file can be up to the size of the *.data file. The *.lobs file can grow to several terabytes. The temporary files created at the time of a SHUTDOWN can be equal in size to the *.script file and the .data file.

  • In desktop deployments, virus checker programs may interfere with the creation and modification of database files. You should exclude the directory containing the database files from virus checking.

ACID, Persistence and Reliability

HyperSQL 2.0 uses the same persistence mechanism as version 1.8, but with important enhancements. The code has proven reliable, as the last critical issue was fixed 2 years before the release of version 2.0.

There are further enhancements in version 2.2

  • More extensive locking mechanism has been added to code to support multithreaded access.

  • Incremental backup (an internal mechanism for crash protection) allows fast checkpoint and shutdown.

  • All files are synced at checkpoints and also just before closing.

  • The data file is enlarged in block increments

  • The NIO file access implementation has been improved

Persistence relies on the JVM, the operating system, and the computer hardware. A database system like HSQLDB can perform millions of read and write operations in an hour. As system hardware and software can go wrong, it is impossible to achieve zero failure rate. Therefore regular backups are recommended. HyperSQL 2.2 has built-in database backup and restore features, discussed elsewhere in this chapter.

A note regarding the NIO file access implementation: This implementation applies only to CACHED table data in the .data file. Other files are not accessed via NIO. There has been an issue with some JVM implementations of nio not releasing the file buffers after they were closed. HyperSQL uses a workaround which is recommended for Sun JVM's. This does not apply to other JVM's. In such environments, it is therefore recommended to test the CHECKPOINT DEFRAG operation and the shutting down and restarting the database inside the same Java process extensively with NIO. Use of NIO can be turned off if necessary.

Atomicity, Consistency, Isolation, Durability

Atomicity means a transaction either fails without changing the data, or succeeds. HyperSQL ensures atomicity both during operations and in the event of a system crash.

Consistency means all the implicit and explicit integrity constraints are always enforced. HyperSQL always enforces the constraints and at the same time does not allow unenforceable constraints (illegal forms of CHECK constraints) to be created.

Isolation means transactions do not interfere with each other. HyperSQL enforces isolation according to strict rules of the database isolation model (MVCC or LOCKS).

Durability means a committed transaction is protected in case of a system crash. HyperSQL ensures durability according to the setting for WRITE DELAY MILLIS. A zero delay setting results in an FileDescriptor#sync() call each time a transaction commits. A timed delay means the FileDescriptor#sync() call is executed in the given intervals and only the last transactions committed in the time interval may be lost. The default time interval is 0.5 second. The sync() call is also made at all critical points, including when a file is about to be closed. Durability of files requires a reliable JVM and disk storage system that stores the data safely with a sync() call. In practice, many systems are generally reliable in this respect.

System Operations

A database is opened when the first connection is successfully made. It remains open until the SHUTDOWN command is issued. If the connection property shutdown=true is used for the first connection to the database, the database is shutdown when the last connection is closed. Otherwise the database remains open and will accept the next connection attempt.

The SHUTDOWN command shuts down the database properly and allows the database to be reopened quickly. This command may take some seconds as it saves all the modified data in the .script and .data files. Variants of SHUTDOWN such as SHUTDOWN COMPACT and SHUTDOWN SCRIPT can be used from time to time to reduce the overall size of the database files. Another variant is SHUTDOWN IMMEDIATELY which ensures all changes to data are stored in the .log file but does not save the changes in .script and .data files. The shutdown is performed quickly but the database will take much longer to reopen.

During the lifetime of the database the checkpoint operation may be performed from time to time. The SET FILES LOG SIZE < value > setting and its equivalent URL property determine the frequency of automatic checkpoints. An online backup also performs a checkpoint when the backup is not a hot backup. A checkpoint can be performed by the user at any time using the CHECKPOINT statement. The main purpose of checkpoints is to reduce the total size of database files and to allow a quick restart in case the database is closed without a proper shutdown. The CHECKPOINT DEFRAG variant compacts the .data file the same way as SHUTDOWN COMPACT does. Obviously this variant takes much longer than a normal CHECKPOINT. A database setting allows a CHECKPOINT DEFRAG to be performed automatically when wasted space in the .data file exceeds the specified percentage.

In a multi-user application, automatic or user-initiated checkpoints are delayed until all other sessions have committed or rolled back. During a checkpoint, other sessions cannot access the database tables but can access the INFORMATION_SCHEMA system tables.

Backing Up Database Catalogs

The database engine saves the files containing all the data in a file catalog when a shutdown takes place. It automatically recovers from an abnormal termination and preserves the data when the catalog is opened next time. In an ideal operating environment, where there is no OS crash, disk failure, bugs in code, etc. there would be no need to backup a database. Backing up catalogs is an insurance policy against all sorts of misadventure that are not under the control of the database engine.

The data for each catalog consists of up to 5 files in the same directory with the endings such as *.properties, *.script, etc., as detailed in previous chapters.

HyperSQL 2 includes commands to backup the database files into a single .tar or .tar.gz file archive, or alternatively as copies of the database files. The backup can be performed by a command given in a JDBC session if the target database catalog is running, or on the command-line if the target catalog has been shutdown.

It is not recommended to backup the database file with an external file backup program while the database is running. The resulting backup will probably be inconsistent and not useful for restoring the database

Making Online Backups

To back up a running catalog, obtain a JDBC connection and issue a BACKUP DATABASE command in SQL. In its most simple form, the command format below will backup the database as a single .tar.gz file to the given directory. This type of backup performs a checkpoint immediately before backing up the files.

 BACKUP DATABASE TO <directory name> BLOCKING [ AS FILES ]

The directory name must end with a slash to distinguish it as a directory, and the whole string must be in single quotes like so: 'subdir/nesteddir/'.

Normal backup may take a long time with very large databases. Hot backup may be used in those situations. This type of backup does not perform a checkpoint and allows access to the database while backup is in progress.

 BACKUP DATABASE TO <directory name> NOT BLOCKING [ AS FILES ]

If you add AS FILES to the statements, the database files are backed up as separate files in the directory, without any gzip compression or tar archiving.

See the next section under Statements for details about the command and its options. See the sections below about restoring a backup.

Making Offline Backups

To back up an offline catalog, the catalog must be in shut down state. You will run a Java command like this. In this example, the database is named dbname and is in the dbdir directory. The backup is saved to a file named backup.tar in the tardir directory.

Example 11.2. Offline Backup Example

 java -cp hsqldb.jar org.hsqldb.lib.tar.DbBackupMain --save tardir/backup.tar dbdir/dbname


where tardir/backup.tar is a file path to the *.tar or *.tar.gz file to be created in your file system, and dbdir/dbname is the file path to the catalog file base name (in same fashion as in server.database.* settings and JDBC URLs with catalog type file:.

Examining Backups

You can list the contents of backup tar files with DbBackup on your operating system command line, or with any Pax-compliant tar or pax client (this includes GNU tar),

Example 11.3. Listing a Backup with DbBackup

 java -cp hsqldb.jar org.hsqldb.lib.tar.DbBackupMain --list tardir/backup.tar


You can also give regular expressions at the end of the command line if you are only interested in some of the file entries in the backup. Note that these are real regular expressions, not shell globbing patterns, so you would use .+script to match entries ending in "script", not *script.

You can examine the contents of the backup in their entirety by restoring the backup, as explained in the following section, to a temporary directory.

Restoring a Backup

You use DbBackup on your operating system command line to restore a catalog from a backup.

Example 11.4. Restoring a Backup with DbBackup

 java -cp hsqldb.jar org.hsqldb.lib.tar.DbBackupMain --extract tardir/backup.tar dbdir


where tardir/backup.tar is a file path to the *.tar or *.tar.gz file to be read, and dbdir is the target directory to extract the catalog files into. Note that dbdir specifies a directory path, without the catalog file base name. The files will be created with the names stored in the tar file (and which you can see as described in the preceding section). After restoring the database, you can connect to it as usual.

Encrypted Databases

HyperSQL supports encrypted databases. Encryption services use the Java Cryptography Extensions (JCE) and uses the ciphers installed with the JRE. HyperSQL itself does not contain any cryptography code.

Three elements are involved in specifying the encryption method and key. A cipher, together with its configuration is identified by a string which includes the name of the cipher and optional parameters. A provider is the fully qualified class name of the cipher provider. A key is represented as a hexadecimal string.

Creating and Accessing an Encrypted Database

First, a key must be created for the desired cipher and configuration. This is done by calling the function CRYPT_KEY(<cipher spec>, <provider>). If the default provider (the built-in JVM ciphers) is used, then NULL should be specified as the provider. The CRYPT_KEY function returns a hexadecimal key. The function call can be made in any HyperSQL database, so long as the provider class is on the classpath. This key can be used to create a new encrypted database. Calls to this function always return different keys, based on a generated random values.

As an example, a call to CRYPT_KEY('Blowfish', null) returned the string, '604a6105889da65326bf35790a923932'. To create a new database, the URL below is used:

jdbc:hsqldb:file:<database path>;crypt_key=604a6105889da65326bf35790a923932;crypt_type=blowfish

The third property name is crypt_provider. This is specified only when the provider is not the default provider.

HyperSQL works with any symmetric cipher that may be available from the JVM.

The files that are encrypted include the .script, .data, .backup and .log files. The .lobs file is not encrypted by default. The property crypt_lobs=true must be specified to encrypt the .lobs file. When this property is used, the blobs and clobs are both compressed and encrypted.

Speed Considerations

General operations on an encrypted database are performed the same as with any database. However, some operations are significantly slower than with the equivalent cleartext database. With MEMORY tables, there is no difference to the speed of SELECT statements, but data change statements are slower. With CACHED tables, the speed of all statements is slower.

Security Considerations

Security considerations for encrypted databases have been discussed at length in HSQLDB discussion groups. Development team members have commented that encryption is not a panacea for all security needs. The following issues should be taken into account:

  • Encrypted files are relatively safe in transport, but because databases contain many repeated values and words, especially known tokens such as CREATE, INSERT, etc., breaking the encryption of a database may be simpler than an unknown file.

  • Only the files are encrypted, not the memory image. Poking into computer memory, while the database is open, will expose the contents of the database.

  • HyperSQL is open source. Someone who has the key, can compile and use a modified version of the program that saves a full cleartext dump of an encrypted database

Therefore encryption is generally effective only when the users who have access to the crypt key are trusted.

Monitoring Database Operations

Database operations can be monitored at different levels using internal HyperSQL capabilities or add-ons.

External Statement Level Monitoring

Statement level monitoring allows you to gather statistics about executed statements. HyperSQL is supported by the monitoring tool JAMon (Java Application Monitor). JAMon is currently developed as the SourceForge project, jamonapi.

JAMon works at the JDBC level. It can monitor and gather statistics on different types of executed statements or other JDBC calls.

Early versions of JAMon were developed with HSQLDB and had to be integrated into HSQLDB at code level. The latest versions can be added on as a proxy in a much simpler fashion.

Internal Statement Level Monitoring

The internally-generated, individual sql log for the database can be enabled with the SET DATABASE EVENT LOG SQL LEVEL statement, described in this chapter. As all the executed statements are logged, there is an impact on speed. So you should only use this for debugging. Two levels of sql logging are supported.

Internal Event Monitoring

HyperSQL can log important internal events of the engine. These events occur during the operation of the engine, and are not always coupled with the exact type of statement being executed. Normal events such as opening and closing of files, or errors such as OutOfMemory conditions are examples of logged events.

HyperSQL supports two methods of logging. One method is specific to the individual database and is managed internally by HyperSQL. The other method is specific to JVM and is managed by a logging framework.

The internally-generated, individual log for the database can be enabled with the SET DATABASE EVENT LOG LEVEL statement, described in this chapter. This method of logging is very useful for desktop application deployment, as it provides an ongoing record of database operations.

Log4J and JDK logging

HyperSQL also supports log4J and JDK logging. The same event information that is passed to the internal log, is passed to external logging frameworks. These frameworks are typically configured outside HyperSQL. The log messages include the string "hsqldb.db." followed by the unique id (a 16 character string) of the database that generated the message, so they can be identified in a multi-database server context.

As the default JDK logging framework has several shortcomings, HyperSQL configures this logging framework for better operation. If you do not want HyperSQL to configure the JDK logging framework, you should include the system level property hsqldb.reconfig_logging=false in your environment.

Server Operation Monitoring

A Server or WebServer instance can be started with the property server.silent=false. This causes all the connections and their executed statements to be printed to stdout as the statements are submitted to the server.

Database Security

HyperSQL has extensive security features which are implemented at different levels and covered in different chapters of this guide.

  1. The server can use SSL and IP address access control lists. See the HyperSQL Network Listeners (Servers) chapter.

  2. You can define a system property to stop the database engine accessing the Java static functions that are on the classpath, apart from a limited set that you allow. See Securing Access to Classes in the SQL-Invoked Routines chapter.

  3. You can define a system property to allow access to files on the file system outside the database directory and its children. This access is only necessary if you use TEXT tables. See the Text Tables chapter.

  4. The database files can be encrypted. Discussed in this chapter.

  5. Within the database, the DBA privileges are required for system and maintenance jobs.

  6. You can define users and roles and grant them access on different database objects. Each user has a password and is granted a set of privileges. See the Access Control chapter.

  7. You can define a password complexity check function for new and changed passwords. This is covered below under Authentication Settings.

  8. You can use external authentication instead of internally stored password to authenticate users for each database. This is covered below under Authentication Settings.

HyperSQL security is multi-layered and avoids any loopholes to circumvent security. It is however the user's responsibility to enable the required level of security.

Security Defaults

The default setting are generally adequate for most embedded use of the database or for servers on the host that are accessed from the same machine. For servers accessed within a network, and especially for those accessed from outside the network, additional security settings must be used.

The default settings for server and web server do not use SSL or IP access control lists. These features are enabled programatically, or with the properties used to start the server.

The default settings allow a database user with the DBA role or with schema creation role to access static functions on the classpath. You can disable this feature or limit it to specific classes and methods. This can be done programatically or by setting a system property when you start a server.

If access to specific static functions is granted, then these functions must be considered as part of the database program and checked for any security flaws before inclusion in the classpath.

The default settings do not allow a user to access files outside the database directory. This access is for TEXT table source files. You can override this programatically or with a system property when you start a server.

The encryption of database file does not utilise any user-supplied information for encryption keys. This level of security is outside the realm of users and passwords.

The first user for a new database has the DBA role. This user name was always SA in older versions of HSQLDB, but not in the latest versions. The name of the first DBA user and its password can be specified when the database is created by the first connection to the database. These settings are then stored in the database.

The initial user with the DBA role should be used for admin purposes only. At least one additional role should be created for normal database use in the application and at least one additional user should be created and granted this role. The new role should not be given the DBA role. It can be given the CREATE_SCHEMA role, which allows it to create and access multiple schemas. Alternatively, the user with the DBA role can create the schemas and their objects and then grant specific privileges on the objects to the non-DBA role.

Authentication Control

Authentication is the mechanism that determines if a user can access the database at all. Once authentication is performed, the authorization mechanism is used to determine which database objects the particular user can access. The default authentication mechanism is password authentication. Each user is created with a password, which is stored in the database and checked each time a new database connection is created.

Password Complexity Check

HyperSQL allows you to define a function that checks the quality of the passwords defined in the database. The passwords are stored in the database. Each time a user connects, the user's name and password are checked against the stored list of users and passwords. The connection attempt is rejected if there is no match.

External Authentication

You can use an external authentication mechanism instead of the internal authentication mechanism. HyperSQL allows you to define a function that checks the combination of database unique name, user name, and password for each connection attempt. The function can use external resources to authenticate the user. For example, a directory server may be used. The password may be ignored if the external resource can verify the user's credential without it.

You can override external authentication for a user with the ALTER USER statement. See the Access Control chapter

Compatibility with Other RDBMS

HyperSQL is used more than any other database engine for application testing and development targeted at other databases. Over the years, this usage resulted in developers finding and reporting many obscure bugs in HyperSQL, which have all been fixed in the latest version. Also, HyperSQL 2 has been written to the SQL Standard and avoids the traps caused by superficial imitation of existing RDBMS.

HyperSQL has many property settings that relax conformance to the Standard in order to allow compatibility with other RDBMS, without breaking the core integrity of the database. These properties are modified with SET DATABASE SQL statements described in the SQL Conformance Settings section of this chapter

The SQL Standard has existed since 1989 and has been expanded over the years in several revisions. Also, the X-Open specification has defined a number of SQL functions which are implemented by most RDBMS.

Each RDBMS supports additional functions that are not covered by the standard. Some RDBMS use non-standard syntax for some operations. Fortunately, most popular RDBMS products have introduced better compatibility with the Standard in their recent versions, but there are still some portability issues. HyperSQL overcomes the potability issues using these strategies

  • An extensive set of functions cover the SQL Standard, X-Open, and most of the useful functions that other RDBMS may support.

  • Database properties, which can be specified on the URL or as SQL statements, relax conformance to the Standard in order to allow non-standard comparisons and assignments allowed by other RDBMS.

  • Specific SQL syntax compatibility modes allow syntax and type names that are supported by some popular RDBMS.

  • User-defined types and functions, including aggregate functions, allow any type or function that is supported by some RDBMS to be defined and used.

In the future, the supported compatibility modes with other RDBMS will be expanded further.

PostgreSQL Compatibility

PostgreSQL is fairly compatible with the Standard, but uses some non-standard features.

  • Use <set database sql syntax PGS statement> to enable the PostgreSQL's non-standard features. References to SERIAL, BIGSERIAL and TEXT data types, as well as sequence functions, are translated into HSQLDB equivalents.

  • Use MVCC if your application is multi-user

  • PostgreSQL functions are generally supported

  • For identity columns, PostgreSQL uses a non-standard linkage with an external identity sequence. In most cases, this can be converted to GENERATED BY DEFAULT AS IDENTITY. In those cases where the identity sequence needs to be shared by multiple tables, you can use a new HyperSQL 2.2 feature GENERATED BY DEFAULT AS SEQUENCE <sequence name>, which is the equivalent of the PostgreSQL implementation.

  • In CREATE TABLE statements, the SERIAL and BIGSERIAL types are translated into INTEGER or BIGINT, with GENERATED BY DEFAULT AS IDENTITY. Usage of DEFAULT NEXTVAL(<sequence name>) is supported so long as the <sequence name> refers to an existing sequence. This usage is translated into GENERATED BY DEFAULT AS SEQUENCE <sequence name>.

  • In SELECT and other statements, the NEXTVAL(<sequence name>) and LASTVAL() functions are supported and translated into HyperSQL's NEXT VALUE FOR <sequence name> and IDENTITY() expressions.

  • PostgreSQL uses a non-standard expression, SELECT 'A Test String' to return a single row table. The standard form is VALUES('A Test String'). In PGS syntax mode, this type of SELECT is supported.

  • HyperSQL supports SQL Standard ARRAY types. PostgreSQL also supports this, but not entirely according to the Standard.

  • SQL routines are portable, but some syntax elements are different and require changes.

  • You may need to use SET DATABASE SQL TDC { DELETE | UPDATE } FALSE statements, as PostgreSQL does not enforce the subtle rules of the Standard for foreign key cascading deletes and updates.

MySQL Compatibility

MySQL had many incompatibilities with the Standard. The latest versions have introduced much greater compatibly, but some of these features have to be turned on via properties. You should therefore check the current Standard compatibility settings of your MySQL database and use the available HyperSQL properties to achieve closer results. If you avoid the few anti-Standard features of MySQL you can port your databases to HyperSQL.

HyperSQL does not have the following non-standard limitations of MySQL.

  • With HyperSQL, an UPDATE statement can update UNIQUE and PRIMARY KEY columns of a table without causing an exception due to temporary violation of constraints. These constraints are checked at the end of execution, therefore there is no need for an ORDER BY clause in an UPDATE statement.

  • MySQL foreign key constraints are not enforced by the default MyISAM engine. Be aware of the possibility of data being rejected by HyperSQL due to these constraints.

  • With HyperSQL INSERT or UPDATE statements either succeed or fail due to constraint violation. MySQL has the non-standard IGNORE overrides to ignore violations, which is not accepted by HyperSQL.

  • Unlike MySQL, HyperSQL allows you to modify a table with an INSERT, UPDATE or DELETE statement which selects from the same table in a subquery.

Follow the guidelines below for converting MySQL databases and applications.

  • Use <set database sql syntax MYS statement> to enable support for AUTO_INCREMENT and TEXT data types. These type definitions are translated into HSQLDB equivalents.

  • Use MVCC with <set database transaction control statement> if your application is multi-user.

  • Avoid storing invalid values, for example invalid dates such as '0000-00-00' or '2001-00-00' which are rejected by HyperSQL.

  • Avoid the MySQL feature that trims spaces at the end of CHAR values.

  • In MySQL, a database is the same as a schema. In HyperSQL several schemas can exist in the same database and accessed transparently. In addition a HyperSQL server supports multiple separate databases.

  • In MySQL, older, non-standard, forms of database object name case-sensitivity make is difficult to port applications. Use the latest form which encloses case-sensitive names in double quotes.

  • MySQL functions are generally supported, including GROUP_CONCAT.

  • For fine control over type conversion, check the settings for <set database sql convert truncate statement>

  • If you use concatenation of possibly NULL values in your select statements, you may need to change the setting with the <set database sql concat nulls statement>

  • MySQL supports most SQL Standard types (except INTERVAL types), as well as non-standard types, which are also supported by HyperSQL. Supported types include SMALLINT, INT, BIGINT, DOUBLE, FLOAT, DECIMAL, NUMERIC, VARCHAR, CHAR, BINARY, VARBINARY, BLOB, DATE, TIMESTAMP (all Standard SQL) and TINYINT, DATETIME (non Standard).

  • MySQL uses a non-standard expression, SELECT 'A Test String' to return a single row table. The standard form is VALUES('A Test String'). In MYS syntax mode, this type of SELECT is supported.

  • SQL user-defined function and procedure syntax is very similar to SQL Standard syntax. A few changes may still be required.

Firebird Compatibility

Firebird generally follows the SQL Standard. Applications can be ported to HyperSQL without difficulty.

Apache Derby Compatibility

Apache Derby supports a smaller subset of the SQL Standard compared to HyperSQL. Applications can be ported to HyperSQL without difficulty.

  • Use MVCC with <set database transaction control statement> if your application is multi-user.

  • HyperSQL supports Java language functions and stored procedures with the standard syntax, which is similar to the way Derby supports these features.

Oracle Compatibility

Recent versions of Oracle support Standard SQL syntax for outer joins and many other operations. In addition, HyperSQL features a setting to support Oracle syntax and semantics for the most widely used non-standard features.

  • Use <set database sql syntax ORA statement> to enable support for some non-standard syntax of Oracle.

  • Use MVCC with <set database transaction control statement> if your application is multi-user.

  • Fine control over MVCC deadlock avoidance is provided by the <set database transaction rollback on conflict statement> and the corresponding hsqldb.tx_conflict_rollback connection property.

  • If your application relies on Oracle behaviour for nulls in multi-column UNIQUE constraints, use <set database sql unique nulls statement> to change the default.

  • If your application relies on Oracle behaviour for ordering of nulls in SELECT statements with ORDER BY, but without NULLS FIRST or NULLS LAST, use both <set database sql nulls last statement> and <set database sql nulls order statement> to change the defaults.

  • If you use the non-standard concatenation of possibly NULL values in your select statements, you may need to change the setting for <set database sql concat nulls statement>.

  • Many Oracle functions are supported, including no-arg functions such as SYSDATE and SYSTIMESTAMP and more complex ones such as TO_DATE and TO_CHAR.

  • Non-standard data type definitions such as NUMBER, VARCHAR2, NVARCHAR2, BINARY_DOUBLE, BINARY_FLOAT, LONG, RAW are translated into the closest HyperSQL / SQL Standard equivalent in ORA mode.

  • The DATE type is interpreted as TIMESTAMP(0) in ORA syntax mode.

  • The DUAL table and the expressions, ROWNUM, CURRVAL, NEXTVAL are supported in ORA syntax mode.

  • HyperSQL natively supports operations involving datetime and interval values. These features are based on the SQL Standard.

  • Many subtle automatic type conversions, syntax refinements and other common features are supported.

  • SQL routines are generally portable, but some changes may be required.

DB2 Compatibility

DB2 is highly compatible with the SQL Standard (except its lack of support for the INFORMATION_SCHEMA). Applications can be ported to HyperSQL without difficulty.

  • Use <set database sql syntax DB2 statement> to enable support for some non-standard syntax of DB2.

  • Use MVCC with <set database transaction control statement> if your application is multi-user.

  • HyperSQL supports almost the entire syntax of DB2 together with many of the functions. Even local temporary tables using the SESSION pseudo schema are supported.

  • The DB2 binary type definition FOR BIT DATA, as well as empty definition of column default values are supported in DB2 syntax mode.

  • Many DB2 functions are supported.

  • The DUAL table and the expressions, ROWNUM, CURRVAL, NEXTVAL are supported in DB2 syntax mode.

  • SQL routines are highly portable with minimal change.

MS SQLServer and Sybase Compatibility

SQLServer has some incompatibilities with the Standard syntax. The most significant is the use of square brackets instead of double quotes for case-sensitive column names.

  • Use <set database sql syntax MSS statement> to enable support for the CONVERT(<type definition>, <expression) function with switched order of arguments

  • Use MVCC with <set database transaction control statement> if your application is multi-user.

  • If you use the non-standard concatenation of possibly NULL values in your select statements, you may need to change the setting for <set database sql concat nulls statement>.

  • HyperSQL supports + for string concatenation. It also supports many functions supported by these dialects.

  • SQLServer uses a non-standard expression, SELECT 'A Test String' to return a single row table. The standard form is VALUES('A Test String'). In MSS syntax mode, this type of SELECT is supported.

  • SQL routines need quite a lot of changes.

Statements

System level statements are listed in this section. Statements that begin with SET DATABASE or SET FILES are for properties that have an effect on the normal operation of HyperSQL. The effects of these statements are also discussed in different chapters.

System Operations

These statements perform a system level action.

SHUTDOWN

shutdown statement

<shutdown statement> ::= SHUTDOWN [IMMEDIATELY | COMPACT | SCRIPT]

Shutdown the database. If the optional qualifier is not used, a normal SHUTDOWN is performed. A normal SHUTDOWN ensures all data is saved correctly and the database opens without delay on next use.

SHUTDOWN

Normal shutdown saves all the database files, then deletes the .log file (and the .backup file in the default mode). This does the same thing as CHECKPOINT, but closes the database when it completes. The database opens without delay on next used.

SHUTDOWN IMMEDIATELY

Saves the *.log file and closes the database files. This is the quickest form of shutdown. This command should not be used as the routine method of closing the database, because when the database is accessed next time, it may take a long time to start.

SHUTDOWN COMPACT

This is similar to normal SHUTDOWN, but reduces the *.data file to its minimum size. It can take much longer than normal SHUTDOWN. This shouldn't be used as routine.

SHUTDOWN SCRIPT

This is similar to SHUTDOWN COMPACT, but it does not rewrite the *.data and text table files. After SHUTDOWN SCRIPT, only the *.script and *.properties files remain. At the next startup, these files are processed and the *.data file is created if there are cached tables. This command in effect performs part of the job of SHUTDOWN COMPACT, leaving the other part to be performed automatically at the next startup.

This command produces a full script of the database which can be edited for special purposes prior to the next startup.

Only a user with the DBA role can execute this statement.

BACKUP DATABASE

backup database statement

<backup database statement> ::= BACKUP DATABASE TO <file path> [SCRIPT] {[NOT] COMPRESSED} {[NOT] BLOCKING} [AS FILES]

Backup the database to specified <file path> for archiving purposes.

The <file path> can be in two forms. If the <file path> ends with a forward slash, it specifies a directory. In this case, an automatic name for the archive is generated that includes the date, time and the base name of the database. The database is backed up to this archive file in the specified directory. The archive is in .tar.gz or .tar format depending on whether it is compressed or not.

If the <file path> does not end with a forward slash, it specifies a user-defined file name for the backup archive. The file extension must be either .tar.gz or .tar and this must match the compression option.

The default set of options is COMPRESSED BLOCKING.

If SCRIPT is specified, the backup will contain a *.script file, which contain all the data and settings of the database. This type of backup is suitable for smaller databases. With larger databases, this takes a long time. When the SCRIPT option is no used, the backup set will consist of the current snapshot of all database files.

If NOT COMPRESSED is specified, the backup is a tar file, without compression. Otherwise, it is in gzip format.

The qualifier, BLOCKING, means all database operations are suspended during backup. During backup, a CHECKPOINT command is silently executed. This mode is always used when SCRIPT is specified.

Hot backup is performed if NOT BLOCKING is specified. In this mode, the database can be used during backup. This mode should only be used with very large databases. A hot backup set is less compact and takes longer to restore and use than a normal backup set produced with the BLOCKING option. You can perform a CHECKPOINT just before a hot backup in order to reduce the size of the backup set.

If AS FILES is specified, the database files are copied to a directory specified by <file path> without any compression. The file path must be a directory. If the directory does not exist, it is created. The file path may be absolute or relative. If it is relative, it is interpreted as relative to the location of database files. When AS FILES is specified, SCRIPT or COMPRESSED options are not available. The backup can be performed as BLOCKING or NOT BLOCKING.

The HyperSQL jar also contains a program that creates an archive of an offline database. It also contains a program to expand an archive into database files. These programs are documented in this chapter under Backing up Database Catalogs.

Only a user with the DBA role can execute this statement.

CHECKPOINT

checkpoint statement

<checkpoint statement> ::= CHECKPOINT [DEFRAG]

Closes the database files, rewrites the script file, deletes the log file and reopens the database.

If DEFRAG is specified, also shrinks the *.data file to its minimum size. CHECKPOINT DEFRAG time depends on the size of the database and can take a long time with huge databases.

A checkpoint on a multi-user database waits until all other sessions have committed or rolled back. While the checkpoint is in progress other sessions are kept waiting. Checkpoint does not close any sessions.

Only a user with the DBA role can execute this statement.

SCRIPT

script statement

<script statement> ::= SCRIPT [<file name>]

Returns a script containing SQL statements that define the database, its users, and its schema objects. If <file name> is not specified, the statements are returned in a ResultSet, with each row containing an SQL statement. No data statements are included in this form. The optional file name is a single-quoted string. If <file name> is specified, then the script is written to the named file. In this case, all the data in all tables of the database is included in the script as INSERT statements.

Only a user with the DBA role can execute this statement.

Database Settings

These statements change the database settings.

SET DATABASE COLLATION

set database collation statement

<set database collation statement> ::= SET DATABASE COLLATION <collation name> [ NO PAD | PAD SPACE ]

Each database can have its own default collation. Sets the collation from the set of collations supported by HyperSQL. Once this command has been issued, the database can be opened in any JVM and will retain its collation.

All collations pad the shorter string with spaces when two strings are compared. If NO PAD is specified, comparison is performed without padding. The default system collation is named SQL_TEXT. To use the default without padding use SET DATABASE COLLATION SQL_TEXT NO PAD.

After you change the collation for a database that contains collated data, you must execute SHUTDOWN COMPACT or SHUTDOWN SCRIPT in order to recreate the indexes.

Only a user with the DBA role can execute this statement.

Collations are discussed in the Schemas and Database Objects chapter. Some examples of setting the database collation follow:

 -- this collation is an ascii collation with Upper Case Comparison (coverts strings to uppercase for comparison)
 SET DATABASE COLLATION SQL_TEXT_UCC

 -- this collation is case-insensitive English
 SET DATABASE COLLATION "English 1"
 -- this collation is case-sensitive French
 SET DATABASE COLLATION "French 2"  

SET DATABASE DEFAULT RESULT MEMORY ROWS

set database default result memory rows statement

<set database default result memory rows> ::= SET DATABASE DEFAULT RESULT MEMORY ROWS <unsigned integer literal>

Sets the maximum number of rows of each result set and internal temporary table that is held in memory. Temporary tables includes views, schema-based and session-based TEMPORARY tables, transient tables for subqueries, and INFORMATION_SCHEMA tables.

This setting applies to all sessions. Individual sessions can change the value with the SET SESSION RESULT MEMORY ROWS statement. The default is 0, meaning all result sets are held in memory.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.result_max_memory_rows.

SET DATABASE DEFAULT TABLE TYPE

set database default table type statement

<set database default table type> ::= SET DATABASE DEFAULT TABLE TYPE { CACHED | MEMORY }

Sets the type of table created when the next CREATE TABLE statement is executed. The default is MEMORY.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.default_table_type.

SET DATABASE EVENT LOG LEVEL

set database event log level statement

<set database event log level> ::= SET DATABASE EVENT LOG [ SQL ] LEVEL { 0 | 1 | 2 | 3 }

When the SQL option is not used, this statement sets the amount of information logged in the internal, database-specific event log. Level 0 means no log. Level 1 means only important (error) events. Level 2 means more events, including both important and less important (normal) events. Level 3 includes even more details. For readonly and mem: databases, if the level is set above 0, the log messages are directed to stderr.

The events are logged in a file with the extension .app.log alongside the main database files.

This is equivalent to the connection property hsqldb.applog.

When the SQL option is used, this statement logs the SQL statements as they are executed. Each log line contains the timestamp and the session number, followed by the SQL statement and JDBC arguments if any.

Levels 1, 2 and 3 are supported. Level 1 only logs commits and rollbacks, while Level 2 and 3 log all statements. Level 2 truncates long statements, while level 3 reports the full statement and parameter values.

The logged lines are stored in a file with the extension .sql.log alongside the main database files.

This is equivalent to the connection property hsqldb.sqllog.

Only a user with the DBA role can execute this statement.

In version 2.3, the equivalent URL properties, hsqldb.app_log and hsqldb.sql_log, can be used not only for a new database, but also when opening an existing file database to change the event log level.

An extract from an .sql.log file created with log Level 3 is shown below. The numbers after the timestamp (10 and 1) show the session number. The values for prepared statement parameters are shown in parentheses at the end of the statement.

Example 11.5. SQL Log Example

 2012-11-29 10:40:40.250 10 INSERT INTO TEST_CLOB VALUES (1,'Ut wisi enim ad minim veniam, quis nostrud exerci') 
 2012-11-29 10:40:40.250 1 INSERT INTO SYSTEM_LOBS.LOB_IDS VALUES(?, ?, ?, ?) (1,49,0,40)
 2012-11-29 10:40:40.250 1 COMMIT 
 2012-11-29 10:40:40.265 1 CALL SYSTEM_LOBS.ALLOC_BLOCKS(?, ?, ?) (1,0,1)
 2012-11-29 10:40:40.265 1 COMMIT 

SET DATABASE GC

set database gc statement

<set database gc statement> ::= SET DATABASE GC <unsigned integer literal>

An optional property which forces calls to System.gc() after the specified number of row operations. The default value for this property is 0, which means no System.gc() calls. Usual values for this property range from 10000 depending on the system and the memory allocation. This property may be useful in some in-process deployments, especially with older JVM implementations.

Only a user with the DBA role can execute this statement.

SET DATABASE UNIQUE NAME

set database unique name

<set database unique name statement> ::= SET DATABASE UNIQUE NAME <identifier>

Each HyperSQL catalog (database) has an engine-generated internal name. This name is a 16 character long string, beginning with HSQLDB and based on the time of creation of the database. The name is used for the log events that are sent to external logging frameworks. The new name must be exactly 16 characters long with no spaces.

Only a user with the DBA role can execute this statement.

SET DATABASE TRANSACTION CONTROL

set database transaction control statement

<set database transaction control statement> ::= SET DATABASE TRANSACTION CONTROL { LOCKS | MVLOCKS | MVCC }

Set the concurrency control system for the database. It can be issued only when all sessions have been committed or rolled back. This command and its modes is discussed in the Sessions and Transactions chapter.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.tx.

SET DATABASE TRANSACTION ROLLBACK ON CONFLICT

set database transaction rollback on conflict statement

<set database transaction rollback on conflict statement> ::= SET DATABASE TRANSACTION ROLLBACK ON CONFLICT { TRUE | FALSE }

When a transaction deadlock or conflict is about to happen, the current transaction is rolled back and an exception is raised. When this property is set false, the transaction is not rolled back. Only the latest statement that would cause the conflict is undone and an exception is raised. The property should not be changed unless the application can quickly perform an alternative statement and complete the transaction. It is provided for compatibility with other database engines which do not roll back the transaction upon deadlock. This command is also discussed in the Sessions and Transactions chapter.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.tx_conflict_rollback.

SET DATABASE DEFAULT ISOLATION LEVEL

set database default isolation level statement

<set database default isolation level> ::= SET DATABASE DEFAULT ISOLATION LEVEL { READ COMMITTED | SERIALIZABLE }

Sets the transaction isolation level for new sessions. The default is READ COMMITTED. Each session can also set its isolation level.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.tx_level.

SET DATABASE TEXT TABLE DEFAULTS

set database text table defaults statement

<set database text table defaults statement> ::= SET DATABASE TEXT TABLE DEFAULTS <character literal>

An optional property to override default text table settings. The string literal has the same format as the string used for setting the data source of a text table, but without the file name. See the Text Tables chapter.

Only a user with the DBA role can execute this statement.

SQL Conformance Settings

These statements modify the level of conformance to the SQL Standard in different areas. The settings that specify SQL SYNTAX are for compatibility with other database engines and are FALSE by default. For all the rest of the settings, TRUE means better conformance to the Standard (unless the Standard defines the behaviour as implementation dependent). The default value of a few of these settings is FALSE, due to widespread non-conforming statements that are already in use in user applications or statements generated by object relational tools. So long as it is practical, it is best to set the non-conforming defaults to TRUE in order to improve the quality of the database application.

SET DATABASE SQL SIZE

set database sql size statement

<set database sql size statement> ::= SET DATABASE SQL SIZE { TRUE | FALSE }

Enable or disable enforcement of column sizes for CHAR and VARCHAR columns. The default is TRUE, meaning table definition must contain VARCHAR(n) instead of VARCHAR.

SQL Standard requires enforcement.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.enforce_size.

SET DATABASE SQL NAMES

set database sql names statement

<set database sql names statement> ::= SET DATABASE SQL NAMES { TRUE | FALSE }

Enable or disable full enforcement of the rule that prevents SQL keywords being used for database object names such as columns and tables. The default is FALSE, meaning disabled.

SQL Standard requires enforcement. It is better to enable this check, in order to improve the quality and correctness of SQL statements.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.enforce_names.

SET DATABASE SQL REGULAR NAMES

set database sql regular names statement

<set database sql regular names statement> ::= SET DATABASE SQL REGULAR NAMES { TRUE | FALSE }

Enable or disable use of the underscore character at the beginning, or the dollar character anywhere in database object names such as columns and tables. The default is TRUE, meaning disabled.

SQL Standard does not allow the underscore character at the start of names, and does not allow the dollar character anywhere in a name. This setting can be changed for compatibility with existing database or for porting databases which include names that do not conform to the Standard.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.regular_names.

SET DATABASE SQL REFERENCES

set database sql references statement

<set database sql references statement> ::= SET DATABASE SQL REFERENCES { TRUE | FALSE }

This command can enable or disable full enforcement of the rule that prevents ambiguous column references in SQL statements (usually SELECT statements). A column reference is ambiguous when it is not qualified by a table name or table alias and can refer to more than one column in a JOIN list.

The property is FALSE by default.

SQL Standard requires enforcement. It is better to enable this check, in order to improve the quality and correctness of SQL statements. When false, the first matching table is used to resolve the column reference.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.enforce_refs.

SET DATABASE SQL TYPES

set database sql types statement

<set database sql types statement> ::= SET DATABASE SQL TYPES { TRUE | FALSE }

This command can enable or disable full enforcement of the rules that prevents illegal type conversions and parameters or nulls without type in SQL statements (usually SELECT statements). For example an INTEGER column or a DATE column cannot be compared to a character string or searched with a LIKE expression when the property is TRUE.

The property is FALSE by default.

SQL Standard requires enforcement. It is better to enable this check, in order to improve the quality and correctness of SQL statements.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.enforce_type.

SET DATABASE SQL TDC DELETE

set database sql tdc delete statement

<set database sql tdc delete statement> ::= SET DATABASE SQL TDC DELETE { TRUE | FALSE }

This command can enable or disable full enforcement of the SQL Standard rules that prevents triggered data change exceptions caused by ON DELETE CASCADE clauses of foreign key constraint.

When there are multiple constraints, a row may be updated by one constraint and deleted by another constraint in the same operation. This is not allowed by default. Changing this to false allows such violations of the Standard to pass without an exception.

The property is TRUE by default.

SQL Standard requires enforcement, therefore this property shouldn't be changed unless an application written for a non-conforming RDBMS needs it.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.enforce_tdc_delete.

SET DATABASE SQL TDC UPDATE

set database sql tdc update statement

<set database sql tdc update statement> ::= SET DATABASE SQL TDC UPDATE { TRUE | FALSE }

This command can enable or disable full enforcement of the SQL Standard rules that prevents triggered data change exceptions caused by multiple ON UPDATE or ON DELETE SET clauses of foreign key constraint. When there are multiple constraints, a field in a row may be updated by two constraints to different values in the same operation. This is not allowed by default. Changing this to FALSE allows such violations of the Standard to pass without an exception.

The property is TRUE by default.

SQL Standard requires enforcement, therefore this property shouldn't be changed unless an application written for a non-conforming RDBMS needs it.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.enforce_tdc_update.

SET DATABASE SQL TRANSLATE TTI TYPES

set database sql translate tti types statement

<set database sql translate tti types statement> ::= SET DATABASE SQL TRANSLATE TTI TYPES { TRUE | FALSE }

The JDBC Specification up to version 4.1 does not support some SQL Standard built-in types, therefore these types must be translated to a supported type when accessed through JDBC ResultSet and PreparedStatement methods.

If the property is true, the TIME / TIMESTAMP WITH TIME ZONE types and INTERVAL types are represented in JDBC methods of ResultSetMetaData and DatabaseMetaData as JDBC datetime types without time zone and the VARCHAR type respectively. The original type names are preserved.

The property is TRUE by default. If set to FALSE, the type codes for WITH TIME ZONE types will be SQL type codes as opposed to JDBC type codes.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property jdbc.translate_tti_types.

SET DATABASE SQL CONCAT NULLS

set database sql concat nulls statement

<set database sql concat nulls statement> ::= SET DATABASE SQL CONCAT NULLS { TRUE | FALSE }

When the property is TRUE, concatenation of a null value with a not-null value results in a null value. When the property is FALSE this type of concatenation result in the not-null value.

Setting this property FALSE results in concatenation behaviour similar to Oracle or MS SQL Server.

SQL Standard requires a NULL result..

The property is TRUE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.concat_nulls.

SET DATABASE SQL UNIQUE NULLS

set database sql unique nulls statement

<set database sql unique nulls statement> ::= SET DATABASE SQL UNIQUE NULLS { TRUE | FALSE }

When the property is TRUE, with multi-column UNIQUE constraints, it is possible to insert multiple rows for which one or more of the values for the constraint columns is NULL. When the property is FALSE, if there is any not-null value in the columns, then the set of values is compared to the existing rows and if there is a match, an exception is thrown. The setting FALSE, makes the behaviour more restrictive. For example, inserting (1, null) twice is possible by default, but not possible when the property is FALSE.

Setting this property FALSE results in UNIQUE constraint behaviour similar to Oracle.

SQL Standard requires the default (TRUE) behaviour.

The property is TRUE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.unique_nulls.

SET DATABASE SQL CONVERT TRUNCATE

set database sql convert truncate

<set database sql convert truncate statement> ::= SET DATABASE SQL CONVERT TRUNCATE { TRUE | FALSE }

When the property is TRUE, conversion from a floating point value (a DOUBLE value) to an integral type always truncates the fractional part. When the property is FALSE, rounding takes place instead of truncation. For example, assigning the value 123456E-2 to an integer column will result in 1234 by default, but 1235 when the property is FALSE.

Standard SQL considers this behaviour implementation dependent.

The property is TRUE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.convert_trunc.

SET DATABASE SQL AVG SCALE

set database sql avg scale

<set database sql avg scale> ::= SET DATABASE SQL AVG SCALE <numeric value>

By default, the result of division and the AVG and MEDIAN aggregate functions has the same type as the aggregated type of the values. This includes the scale. The scale specified with this property is used if it is larger than the scale of the operation. For example, the average of 5 and 10 is 7 by default, but 7.50 if the scale is specified as 2. The result of 7/3 is 2 by default but 2.33 if the scale is specified as 2.

Standard SQL considers this behaviour implementation dependent. Some databases use a default scale larger than zero.

The property is 0 by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.avg_scale.

SET DATABASE SQL DOUBLE NAN

set database sql double nan

<set database sql double nan> ::= SET DATABASE SQL DOUBLE NAN { TRUE | FALSE }

When the property is TRUE, division of a floating point value (a DOUBLE value) by zero raises an exception. When the property is FALSE, a Java Double.NaN, POSITIVE_INFINITY or NEGATIVE_INFINITY value is returned.

Standard SQL requires an exception to be raised.

The property is TRUE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.double_nan.

SET DATABASE SQL NULLS FIRST

set database sql nulls first

<set database sql nulls first> ::= SET DATABASE SQL NULLS FIRST { TRUE | FALSE }

When the property is TRUE, nulls appear before values in result sets with ORDER BY. When set FALSE, nulls appear after the values. Some databases, including PostgreSQL, Oracle and MS SQL Server, return nulls after the values.

The property is TRUE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.nulls_first.

SET DATABASE SQL NULLS ORDER

set database sql nulls order

<set database sql nulls order> ::= SET DATABASE SQL NULLS ORDER { TRUE | FALSE }

When NULLS FIRST or NULLS LAST is used explicitly in the ORDER BY clause, this property is ignored.

When the property is TRUE, nulls appear according to the value of NULL FIRST property as described above.

When set FALSE, nulls appear according to the value of NULLS FIRST property when DESC is not used in the ORDER BY clause. But if DESC is used, the position of nulls is reversed. Some databases, including MySQL and Oracle, return nulls in this manner when DESC is used in ORDER BY.

The property is TRUE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.nulls_order.

SET DATABASE SQL IGNORECASE

set database sql ignorecase

<set database sql ignorecase> ::= SET DATABASE SQL IGNORECASE { TRUE | FALSE }

This property is FALSE by default and should only be used in special circumstances where compatibility with a different database is required.

When the property is TRUE, all declarations of VARCHAR columns in tables or other database objects are converted to VARCHAR_IGNORECASE. This has a global effect on the database, unlike the SET IGNORECASE statement which applies only to the current session.

The property is FALSE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.ignore_case.

SET DATABASE SQL SYNTAX DB2

set database sql syntax DB2

<set database sql syntax DB2 statement> ::= SET DATABASE SQL SYNTAX DB2 { TRUE | FALSE }

This property, when set TRUE, enables support for some elements of DB2 syntax. Single-row SELECT statements (SELECT <expression list> without the FROM clause) are supported and treated as the SQL Standard equivalent, VALUES <expression list>. The DUAL table is supported, as well as the ROWNUM pseudo-column. BINARY type definitions such as VARCHAR(L) FOR BIT DATA are supported. Empty DEFAULT clauses in column definitions are supported.

The property is FALSE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.syntax_db2.

SET DATABASE SQL SYNTAX MSS

set database sql syntax MSS

<set database sql syntax MSS statement> ::= SET DATABASE SQL SYNTAX MSS { TRUE | FALSE }

This property, when set TRUE, enables support for some elements of SQLServer syntax. Single-row SELECT statements (SELECT <expression list> without the FROM clause) are supported and treated as the SQL Standard equivalent, VALUES <expression list>. The parameters of CONVERT() function are switched in this mode.

The property is FALSE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.syntax_mss.

SET DATABASE SQL SYNTAX MYS

set database sql syntax MYS

<set database sql syntax MYS statement> ::= SET DATABASE SQL SYNTAX MYS { TRUE | FALSE }

This property, when set TRUE, enables support for some elements of MySQL syntax. The TEXT data type is translated to LONGVARCHAR.

In CREATE TABLE statements, [NOT NULL | NULL] can be used immediately after the column type name and before the DEFAULT clause. AUTO_INCREMENT is translated to the GENERATED BY DEFAULT AS IDENTITY clause.

Single-row SELECT statements (SELECT <expression list> without the FROM clause) are supported and treated as the SQL Standard equivalent, VALUES <expression list>.

The property is FALSE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.syntax_mys.

SET DATABASE SQL SYNTAX ORA

set database sql syntax ORA

<set database sql syntax ORA statement> ::= SET DATABASE SQL SYNTAX ORA { TRUE | FALSE }

This property, when set TRUE, enables support for some elements of Oracle syntax. The DUAL table is supported, together with ROWNUM, NEXTVAL and CURRVAL syntax and semantics.

The non-standard types are translated to supported standard types. BINARY_DOUBLE and BINARY_FLOAT are translated to DOUBLE. LONG RAW and RAW are translated to VARBINARY with long or medium length limits. LONG and VARCHAR2 are translated to VARCHAR with long or medium length limits. NUMBER is translated to DECIMAL. Some extra type conversions and no-arg functions are also allowed in this mode.

The property is FALSE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.syntax_ora.

SET DATABASE SQL SYNTAX PGS

set database sql syntax PGS

<set database sql syntax PGS statement> ::= SET DATABASE SQL SYNTAX PGS { TRUE | FALSE }

This property, when set TRUE, enables support for some elements of PosgtreSQL syntax. The TEXT data type is translated to LONGVARCHAR, while the SERIAL data types is translated to BIGINT together with GENERATED BY DEFAULT AS IDENTITY.

Single-row SELECT statements (SELECT <expression list> without the FROM clause) are supported and treated as the SQL Standard equivalent, VALUES <expression list>.

The functions NEXTVAL(<sequence name string>), CURRVAL(<sequence name string>) and LASTVAL() are supported in this compatibility mode.

The property is FALSE by default.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property sql.syntax_pgs.

SET DATABASE REFERENTIAL INTEGRITY

set database referential integrity statement

<set database referential integrity statement> ::= SET DATABASE REFERENTIAL INTEGRITY { TRUE | FALSE }

This command enables or disables the enforcement of referential integrity constraints (foreign key constraints), check constraints apart from NOT NULL and execution of triggers. By default, all constraints are checked.

The only legitimate use of this statement is before importing large amounts of external data into tables that have existing FOREIGN KEY constraints. After import, the statement must be used again to enable constraint enforcement.

If you are not sure the data conforms to the constraints, run queries to verify all rows conform to the FOREIGN KEY constraints and take appropriate actions for the rows that do not conform.

A query example to return the rows in a foreign key table that have no parent is given below:

Example 11.6. Finding foreign key rows with no parents after a bulk import

 SELECT * FROM foreign_key_table LEFT OUTER JOIN primary_key_table 
   ON foreign_key_table.fk_col = primary_key_table.pk_col WHERE primary_key_table.pk_col IS NULL

Only a user with the DBA role can execute this statement.

Cache, Persistence and Files Settings

These statements control the memory and other settings for database persistence.

SET FILES BACKUP INCREMENT

set files backup increment statement

<set files backup increment statement> ::= SET FILES BACKUP INCREMENT { TRUE | FALSE }

Older versions of HSQLDB perform a backup of the .data file before its contents are modified and the whole .data file is saved in a compressed form when a CHECKPOINT or SHUTDOWN is performed. This takes a long time when the size of the database exceeds 100 MB or so (on an average 2010 computer, you can expect a backup speed of 20MB per second or more).

The alternative is backup in increments, just before some part of the .data file is modified. In this mode, no backup is performed at CHECKPOINT or SHUTDOWN. This mode is preferred for large databases which are opened and closed frequently.

The default mode is TRUE. If the old method of backup is preferred, the mode can be set FALSE.

Warning: The old, non-incremental setting, FALSE, shouldn't be used at all when the data file is larger than 4GB. If it is used, the data file is not fully backed up and can result in corruption. The zip compression method is used in this mode and it is limited to 4GB size.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.inc_backup.

SET FILES CACHE ROWS

set files cache rows statement

<set files cache rows statement> ::= SET FILES CACHE ROWS <unsigned integer literal>

Sets the maximum number of rows (of CACHED tables) held in the memory cache. The default is 50000 rows.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.cache_rows.

SET FILES CACHE SIZE

set files cache size statement

<set files cache size statement> ::= SET FILES CACHE SIZE <unsigned integer literal>

Sets maximum amount of data (of CACHED tables) in kilobytes held in the memory cache. The default is 10000 kilobytes. Note the amount of memory used is larger than this amount, which does not account for Java object size overheads.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.cache_size.

SET FILES DEFRAG

set files defrag statement

<set files defrag statement> ::= SET FILES DEFRAG <unsigned integer literal>

Sets the threshold for performing a DEFRAG during a checkpoint. The <unsigned integer literal> is the percentage of abandoned space in the *.data file. When a CHECKPOINT is performed either as a result of the .log file reaching the limit set by SET FILES LOG SIZE m, or by the user issuing a CHECKPOINT command, the amount of space abandoned since the database was opened is checked and if it is larger than specified percentage, a CHECKPOINT DEFRAG is performed instead of a CHECKPOINT.

The default is 0, which indicates no DEFRAG. Useful values are between 10 to 50.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.defrag_limit.

SET FILES LOG

set files log statement

<set files log statement> ::= SET FILES LOG { TRUE | FALSE }

Sets logging of database operations on or off. Turning logging off is for special usage, such as temporary cache usage. The default is TRUE.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.log_data.

SET FILES LOG SIZE

set files log size statement

<set files log size statement> ::= SET FILES LOG SIZE <unsigned integer literal>

Sets the maximum size in MB of the *.log file to the specified value. The default maximum size is 50 MB. If the value is zero, no limit is used for the size of the file. When the size of the file reaches this value, a CHECKPOINT is performed and the the *.log file is cleared to size 0.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.log_size.

SET FILES NIO

set files nio

<set files nio statement> ::= SET FILES NIO { TRUE | FALSE }

Sets the access method of the .data file. The default is TRUE and uses the Java nio classes to access the file via memory-mapped buffers.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.nio_data_file.

SET FILES NIO SIZE

set files nio size

<set files nio size statement> ::= SET FILES NIO SIZE <unsigned integer literal>

Sets The maximum size of .data file in megabytes that can use the nio access method. When the file gets larger than this limit, non-nio access methods are used. Values 64, 128, 256, 512, 1024 and larger multiples of 512 can be used. The default is 256MB.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.nio_max_size.

SET FILES WRITE DELAY

set files write delay statement

<set files write delay statement> ::= SET FILES WRITE DELAY {{ TRUE | FALSE } | <seconds value> | <milliseconds value> MILLIS}

Set the WRITE DELAY property of the database. The WRITE DELAY controls the frequency of file sync for the log file. When WRITE_DELAY is set to FALSE or 0, the sync takes place immediately at each COMMIT. WRITE DELAY TRUE performs the sync once every 0.5 seconds (which is the default). A numeric value can be specified instead.

The purpose of this command is to control the amount of data loss in case of a total system crash. A delay of 1 second means at most the data written to disk during the last second before the crash is lost. All data written prior to this has been synced and should be recoverable.

A write delay of 0 impacts performance in high load situations, as the engine has to wait for the file system to catch up.

To avoid this, you can set write delay down to 10 milliseconds.

Each time the SET FILES WRITE DELAY statement is executed with any value, a sync is immediately performed.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection properties hsqldb.write_delay and hsqldb.write_delay_millis.

SET FILES SCALE

set files scale

<set files scale statement> ::= SET FILES SCALE <scale value>

Changes the scale factor for the .data file. The default scale is 8 and allows 16GB of data storage capacity. The scale can be increased in order to increase the maximum data storage capacity. The scale values 8, 16, 32, 64, 128, 256, 512, 1024 are allowed. Scale value 1024 allows a maximum capacity of 2 TB.

This command can be used only when there is no data in CACHED tables. This is equivalent to the connection property hsqldb.cache_file_scale.

The scale factor indicates the size of the unit of storage of data in bytes. For example, with a scale factor of 128, a row containing a small amount of data will use 128 bytes. Larger rows may use multiple units of 128 bytes.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.cache_file_scale.

SET FILES LOB SCALE

set files lob scale

<set files lob scale statement> ::= SET FILES LOB SCALE <scale value>

Changes the scale factor for the .lobs file. The scale is interpreted in kilobytes. The default scale is 32 and allows 64TB of lob data storage capacity. The scale can be reduced in order to improve storage efficiency. If the lobs are a lot smaller than 32 kilobytes, reducing the scale will reduce wasted space. The scale values 1, 2, 4, 8, 16, 32 are allowed. For example if the average size of lobs is 4 kilobytes, the default scale of 32 will result in 28KB wasted space for each lob. Reducing the lob scale to 2 will result in average 1KB wasted space for each lob.

This command can be used only when there is no lob in the database.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.lob_file_scale.

SET FILES LOB COMPRESSED

set files lob compressed

<set files lob compressed statement> ::= SET FILES LOB COMPRESSED { TRUE | FALSE }

By default, lobs are not compressed for storage. When this setting is TRUE, all BLOB and CLOB values stored in the database are compressed. Compression reduces the storage size but increases the access time.

This command can be used only when there is no lob in the database.

Only a user with the DBA role can execute this statement.

This is equivalent to the connection property hsqldb.lob_compressed.

SET FILES SCRIPT FORMAT

set files script format

<set files script format statement> ::= SET FILES SCRIPT FORMAT { TEXT | COMPRESSED }

Changes the compression setting for database scripts. The default is text. Using COMPRESSED results in the storage of the .script file in gzip compressed form. Using this command causes a CHECKPOINT.

Only a user with the DBA role can execute this statement.

SET TABLE TYPE

set table type

<set table type statement> ::= SET TABLE <table name> TYPE { MEMORY | CACHED }

Changes the storage type of an existing table between CACHED and MEMORY types.

Only a user with the DBA role can execute this statement.

Authentication Settings

Two settings are available for authentication control.

When the default password authentication is used, the passwords can be checked for complexity according to administrative rules

SET DATABASE PASSWORD CHECK FUNCTION

set database password check function

<set database password check function statement> ::= SET DATABASE PASSWORD CHECK FUNCTION { <routine body> | NONE }

The routine body is the body of a function that has a VARCHAR parameter and returns a BOOLEAN. This function checks the PASSWORD submitted as parameter and returns TRUE if it conforms to complexity checks, or FALSE, if it does not.

The <routine body> can be an SQL block or an external Java function reference. This is covered in the SQL-Invoked Routines chapter

To disable this mechanism, the token NONE can be specified instead of the <routine body>.

Only a user with the DBA role can execute this statement.

In the examples below, an SQL function and a Java function are used.

 SET DATABASE PASSWORD CHECK FUNCTION
   BEGIN ATOMIC
     IF CHAR_LENGTH(PASSWORD) > 6 THEN
       RETURN TRUE;
     ELSE 
       RETURN FALSE;
     END IF;
   END

 SET DATABASE PASSWORD CHECK FUNCTION EXTERNAL NAME 'CLASSPATH:org.anorg.access.AccessClass.accessMethod'

 // the Java method is defined like this
 public static boolean accessMethod(String param) {
     return param != null && param.length > 6;
 }

It is possible to replace the default password authentication completely with a function that uses external authentication servers, such as LDAP. This function is called each time a user connects to the database.

SET DATABASE AUTHENTICATION FUNCTION

set database authentication function

<set database authentication function statement> ::= SET DATABASE AUTHENTICATION FUNCTION { <external body reference> | NONE }

The routine body is an external Java function reference. This function has three String parameters. The first parameter is the unique name of the database, the second parameter the user name, and the third parameter the password.

External authentication can be used in two different patterns. In the first pattern, user names must be stored in the database. In the second pattern, user names shouldn't be stored in the database and any names that are stored in the database are ignored.

In both patterns, the username and password are checked by the authentication function. If the function throws a runtime exception then authentication fails.

In the first pattern, the function always returns null if authentication is successful.

In the second pattern, the function returns a list of role names that have been granted to the user. These roles must match the ROLE objects that have been defined in the database.

The Java function should return an instance of org.hsqldb.jdbc.JDBCArrayBasic constructed with a String[] argument that contains the role names.

Only a user with the DBA role can execute this statement.

 SET DATABASE AUTHENTICATION FUNCTION EXTERNAL NAME 'CLASSPATH:org.anorg.access.AccessClass.accessExernalMethod'

 // the Java method is defined like this
 public static java.sql.Array accessExternalMethod(String database, String user, String password) {
     if (externalCheck(database, user, password) {
         return null;
     }
     throw new RuntimeException("failed to authenticate");
 }

Chapter 12. Properties

Fred Toussi

The HSQL Development Group

$Revision: 5260 $

Copyright 2002-2012 Fred Toussi. Permission is granted to distribute this document without any alteration under the terms of the HSQLDB license. Additional permission is granted to the HSQL Development Group to distribute this document with or without alterations under the terms of the HSQLDB license.

2014-02-13 18:21:41-0500

Connection URL

The normal method of accessing a HyperSQL catalog is via the JDBC Connection interface. An introduction to different methods of providing database services and accessing them can be found in the SQL Language chapter. Details and examples of how to connect via JDBC are provided in our JavaDoc for JDBCConnection.

A uniform method is used to distinguish between different types of connection. The common driver identifier is jdbc:hsqldb: followed by a protocol identifier (mem: file: res: hsql: http: hsqls: https:) then followed by host and port identifiers in the case of servers, then followed by database identifier. Additional property / value pairs can be appended to the end of the URL, separated with semicolons.

Table 12.1. Memory Database URL

Driver and ProtocolHost and Port ExampleDatabase Example
jdbc:hsqldb:mem:
not available
accounts

Lowercase, single-word identifier creates the in-memory database when the first connection is made. Subsequent use of the same Connection URL connects to the existing DB.

The old form for the URL, jdbc:hsqldb:. creates or connects to the same database as the new form for the URL, jdbc:hsqldb:mem:.


Table 12.2. File Database URL

Driver and ProtocolHost and Port ExampleDatabase Example
jdbc:hsqldb:file:
not available
accounts
/opt/db/accounts
C:/data/mydb

The file path specifies the database files. It should consist of a relative or absolute path to the directory containing the database files, followed by a '/' and the database name. In the above examples the first one refers to a set of mydb.* files in the directory where the javacommand for running the application was issued. The second and third examples refer to absolute paths on the host machine: For example, files named accounts.* in the directory /opt/db for the accounts database.


Table 12.3. Resource Database URL

Driver and ProtocolHost and Port ExampleDatabase Example
jdbc:hsqldb:res:
not available
/adirectory/dbname
Database files can be loaded from one of the jars specified as part of the Java command the same way as resource files are accessed in Java programs. The /adirectory above stands for a directory in one of the jars.

Table 12.4. Server Database URL

Driver and ProtocolHost and Port ExampleDatabase Example
jdbc:hsqldb:hsql:
jdbc:hsqldb:hsqls:
jdbc:hsqldb:http:
jdbc:hsqldb:https:
//localhost
//192.0.0.10:9500
//dbserver.somedomain.com
/an_alias
/enrolments
/quickdb

The host and port specify the IP address or host name of the server and an optional port number. The database to connect to is specified by an alias. This alias is a lowercase string defined in the server.properties file to refer to an actual database on the file system of the server or a transient, in-memory database on the server. The following example lines in server.properties or webserver.properties define the database aliases listed above and accessible to clients to refer to different file and in-memory databases.

The old form for the server URL, e.g., jdbc:hsqldb:hsql//localhost connects to the same database as the new form for the URL, jdbc:hsqldb:hsql//localhost/ where the alias is a zero length string.


Variables In Connection URL

Two types of variables are allowed for file: database URLs. These properties have an effect only if used for the first connection to the database (the connection which opens the database). When running a server, these variables have no effect on the connection URL but can be appended to the database path URL in server.properties or the server command line.

If the database part of a file: database begins with ~/ or ~\ the tilde character is replaced with the value of the system property "user.home" resulting in the database being created or accessed in this directory, or one of its subdirectories. In the examples below, the database files for mydb and filedb are located in the user's home directory.

 jdbc:hsqldb:file:~/mydb
 jdbc:hsqldb:file:~/filedb;shutdown=true

If the database URL contains a string in the form of ${propname} then the sequence of characters is replaced with the system property with the given name. For example you can use this in the URL of a database that is used in a web application and define the system property, "propname" in the web application properties. In the example below, the string ${mydbpath} is replaced with the value of the property, mydbpath

 jdbc:hsqldb:file:${mydbpath}

Connection properties

Each JDBC Connection to a database can specify connection properties. The properties user and password are always required. The following optional properties can also be used.

Connection properties are specified either by establishing the connection via the method call below, or the property can be appended to the full Connection URL. The first three of these properties can be used for any connection, including connection to a Server but the other three have an effect only with the first connection to a file: or mem: database, or when appended to the database path URL in server.properties or the server command line.

 DriverManager.getConnection (String url, Properties info);

Table 12.5. User and Password

NameDefaultDescription
userSAuser name

Standard property. This property is case sensitive. Example below:

 jdbc:hsqldb:file:enrolments;user=aUserName;ifexists=true
passwordempty stringpassword for the user

Standard property. This property is case sensitive. Example below:

 jdbc:hsqldb:file:enrolments;user=aUserName;password=3xLVz

For compatibility with other engines, a non-standard form of specifying user and password is also supported. In this form, user name and password appear at the end of the URL string, prefixed respectively with the question mark and the ampersand:

 jdbc:hsqldb:file:enrolments;create=false?user=aUserName&password=3xLVz

Table 12.6. Closing old ResultSet when Statement is reused

NameDefaultDescription
close_resultfalseclosing the old result set when a new ResultSet is created by a Statement

This property is used for compatibility with the JDBC specification. When true (the JDBC specification), a ResultSet that was previously returned by executing a Statement or PreparedStatement is closed as soon as the Statement is executed again.

The default is false as previous versions of HSQLDB did not close old result set. The user application should close old result sets when they are no longer needed and should not rely on auto-closing side effect of executing the Statement.

The default is false. When the property is true, the old ResultSet is closed when a Statement is re-executed. Example below:

 jdbc:hsqldb:hsql://localhost/enrolments;close_result=true

When a ResultSet is used inside a user-defined stored procedure, the default, false, is always used for this property.


Table 12.7. Column Names in JDBC ResultSet

NameDefaultDescription
get_column_nametruecolumn name in ResultSet

This property is used for compatibility with other JDBC driver implementations. When true (the default), ResultSet.getColumnName(int c) returns the underlying column name. This property can be specified differently for different connections to the same database.

The default is true. When the property is false, the above method returns the same value as ResultSet.getColumnLabel(int column) Example below:

 jdbc:hsqldb:hsql://localhost/enrolments;get_column_name=false

When a ResultSet is used inside a user-defined stored procedure, the default, true, is always used for this property.


Table 12.8. Creating New Database

NameDefaultDescription
ifexistsfalseconnect only if database already exists

Has an effect only with mem: and file: database. When true, will not create a new database if one does not already exist for the URL.

When the property is false (the default), a new mem: or file: database will be created if it does not exist.

Setting the property to true is useful when troubleshooting as no database is created if the URL is malformed. Example below:

 jdbc:hsqldb:file:enrolments;ifexists=true
createtruecreate the database if it does not exist

Similar to the ifexists property, but with opposite meaning.

Has an effect only with mem: and file: database. When false, will not create a new database if one does not already exist for the URL.

When the property is true (the default), a new mem: or file: database will be created if it does not exist.

Setting the property to true is useful when troubleshooting as no database is created if the URL is malformed. Example below:

 jdbc:hsqldb:file:enrolments;create=false

Table 12.9. Automatic Shutdown

NameDefaultDescription
shutdownfalseshut down the database when the last connection is closed

If this property is true, when the last connection to a database is closed, the database is automatically shut down. The property takes effect only when the first connection is made to the database. This means the connection that opens the database. It has no effect if used with subsequent connections.

This command has two uses. One is for test suites, where connections to the database are made from one JVM context, immediately followed by another context. The other use is for applications where it is not easy to configure the environment to shutdown the database. Examples reported by users include web application servers, where the closing of the last connection coincides with the web app being shut down.

 jdbc:hsqldb:file:enrolments;shutdown=true

In addition, when the first connection to an in-process file: or mem: database creates a new database all the user-defined database properties can be specified as URL properties. See the next section for details.

Database Properties in Connection URL and Properties

Each database has several default settings (properties) that are listed in the System Management chapter. These properties can be changed via SQL commands after a connection is made to the database. It is possible to specify most of these properties in the connection properties or as part of the URL string when the first connection is made to a new file: or mem: database. This allows the properties to be set without using any SQL commands. The corresponding SQL command is given for each property. For a server, these properties can be appended to the database path URL in server.properties or the server command line.

Note the prefered method of setting database properties is by using a set of SQL statements. These statements can be used both for a new database or an existing database, unlike URL properties that are generally effective for new databases only.

If the properties are used for connection to an existing database, they are ignored.

The exceptions are the following property settings that are allowed for the first connection to an existing database (the connection which reopens the database): readonly=true, files_readonly=true, hsqldb.lock_file=false, hsqldb.sqllog=1-3, hsqldb.applog=1-3. These specific property / value pairs override the existing database properties. For example a normal database is opened as readonly, or the lock file is not created, or the sql log level is set to a value between 1 and 3.

Management of properties has changed since version 1.8. The old SET PROPERTY statement does not change a property and is ignored. The statement is retained to simplify application upgrades.

In the example URL below, two properties are set for the first connection to a new database.

 jdbc:hsqldb:file:enrolments;hsqldb.cache_rows=10000;hsqldb.nio_data_file=false

In the table below, database properties that can be used as part of the URL or in connection properties are listed. For each property that can also be set with an SQL statement, the statement is also given. These statements are described more extensively in the System Management chapter.

Table 12.10. Validity Check Property

NameDefaultDescription
check_propsfalsechecks the validity of the connection properties

If the property is true, every database property that is specified on the URL or in connection properties is checked and if it is not used correctly, an error is returned.

this property cannot be set with an SQL statement

SQL Conformance Properties

Table 12.11. SQL Keyword Use as Identifier

NameDefaultDescription
sql.enforce_namesfalseenforcing SQL keywords

This property, when set true, prevents SQL keywords being used for database object names such as columns and tables.

SET DATABASE SQL NAMES { TRUE | FALSE }

Table 12.12. SQL Keyword Starting with the Underscore or Containing Dollar Characters

NameDefaultDescription
sql.regular_namestrueenforcing SQL keywords

This property, when set true, prevents database object names such as columns and tables beginning with the underscore or containing the dollar character.

SET DATABASE SQL REGULAR NAMES { TRUE | FALSE }

Table 12.13. Reference to Columns Names

NameDefaultDescription
sql.enforce_refsfalseenforcing column reference disambiguation

This property, when set true, causes an error when an SQL statement (usually a select statement) contains column references that can be resolved by more than one table name or alias. In effect forces such column references to have a table name or table alias qualifier.

SET DATABASE SQL REFERENCES { TRUE | FALSE }

Table 12.14. String Size Declaration

NameDefaultDescription
sql.enforce_sizetruesize enforcement of string columns

Conforms to SQL standards for size and precision of data types. When true, all VARCHAR column type declarations require a size. When the property is false and there is no size in the declaration, a default size is used. Note that all other types accept a declaration without a size, which is interpreted as a default size.

SET DATABASE SQL SIZE { TRUE | FALSE }

Table 12.15. Type Enforcement in Comparison and Assignment

NameDefaultDescription
sql.enforce_typesfalseenforcing type compatibility

This property, when set true, causes an error when an SQL statements contains comparisons or assignments that are non-standard due to type mismatch. Most illegal comparisons and assignments will cause an exception regardless of this setting. This setting applies to a small number of comparisons and assignments that are possible, but not standard conformant, and were allowed in previous versions of HSQLDB.

SET DATABASE SQL TYPES { TRUE | FALSE }

Table 12.16. Foreign Key Triggered Data Change

NameDefaultDescription
sql.enforce_tdc_deletetrueenforcing triggered data change violation for deletes

The ON DELETE and ON UPDATE clauses of constraints cause data changes in rows in different tables or the same table. When there are multiple constraints, a row may be updated by one constraint and deleted by another constraint in the same operation. This is not allowed by default. Changing this property to false allows such violations of the Standard to pass without an exception. Used for porting from database engines that do not enforce the constraints.

SET DATABASE SQL TDC DELETE { TRUE | FALSE }
 
sql.enforce_tdc_updatetrueenforcing triggered data change violation for updates

The ON DELETE and ON UPDATE clauses of foreign key constraints cause data changes in rows in different tables or the same table. With multiple constraint, a field may be updated by two constraints and set to different values. This is not allowed by default. Changing this property to false allows such violations of the Standard to pass without an exception. Used for porting from database engines that do not enforce the constraints properly.

SET DATABASE SQL TDC UPDATE { TRUE | FALSE }

Table 12.17. Use of LOB for LONGVAR Types

NameDefaultDescription
sql.longvar_is_lobfalsetranslating lon