Hsqldb is an sql relational database engine written in java. it has a jdbc driver and supports a rich subset of sql-92 (bnf tree format) plus sql:1999 and sql:2003 enhancements. it offers a small, fast database engine that offers both in-memory and disk-based tables. embedded and server modes are available. additionally, it includes tools such as a minimal web server, in-memory query and management tools (can be run as applets), and a number of demonstration examples.

web site: http://hsqldb.org/