HSQLDB (Hyper SQL Database) is a relational database management system written in Java. It has a JDBC driver and supports a large subset of SQL-92 and SQL:2008 standards. It offers a fast, small database engine which offers both in-memory and disk-based tables. Both embedded and server modes are available for purchase.
It includes tools such as a minimal Web server, command line and GUI management tools (can be run as applets), and a number of demonstration examples.
For applications that require more speed with very large data sets, or need to store more data in a memory database, the commercial product HyperXtremeSQL is available from http://hyperxtreme.co.uk. This product is fully compatible with HSQLDB queries and other statements.