HSQLDB is a relational database engine written in Java, with a JDBC driver, supporting a large subset of ANSI-92 SQL. A small, fast engine with both in memory and disk based tables. This product is the continuation of HypersonicSQL. Active since 2001.

100% Java
A full RDBMS (Relational Database Management System), with the Object capabilities of Java
Switchable source code to support JDK 1.1.x, 1.2.x, 1.3.x, 1.4.x and above
Super fast startup and SELECT, INSERT, DELETE and UPDATE operations
Standard SQL (Structured Query Language) syntax
Inner and outer joins, SELECT queries as correlations in joins
Scalar (single value) SELECTS, correlated subqueries including IN, EXISTS, ANY, ALL
Views, Temp tables and sequences
Primary key, unique and check constraints on single or multiple columns
Indexes on single or multiple columns
ORDER BY, GROUP BY and HAVING
COUNT, SUM, MIN, MAX, AVG and statistical aggregate functions (also in expressions and function arguments)
Full support for SQL expressions such as CASE .. WHEN .. ELSE .. , NULLIF etc.
SQL standard autoincrement column support plus sequences
Transaction COMMIT, ROLLBACK and SAVEPOINT support
Referential Integrity (foreign keys) with full cascading options (delete, update, set null, set default)
Multiple schemata per database
Java stored procedures and functions
Triggers
Database security with passwords, user rights and roles with GRANT and REVOKE
Extensive set of ALTER TABLE commands allowing change of column type
In-memory tables for fastest operation
Disk based tables for large data sets
Text tables with external file data sources such as CSV files
In-memory (like applets), embedded (into Java applications) and Client-Server operating modes
Three client server protocols: HSQL, HTTP and HSQL-BER - can run as an HTTP web server - all with SSL option
Can be used in applets, read-only media (CD), inside jars, webstart and embedded applications
Multiple databases per JVM
Disk tables (CACHED TABLE) up to 8GB and text tables up to 2GB each
Size of each string and binary data item only limited by memory
Full support for PreparedStatement objects to speed up query processing
95% JDBC interface support with batch statement and scrollable ResultSet functionality
All JDBC 1 data types supported, plus 'Object' , Boolean, Blob and Clob
Full JDBC 2 DatabaseMetaData and ResultSetMetaData support
Database dump as SQL script with or without data
Powerful and compact java command line and GUI tools for database management