HSQLDB
HSQLDB (HyperSQL DataBase) is the SQL relational database engine written in Java. It offers a multithreaded and transactional database engine with in-memory and disk-based tables and supports embedded and server modes. It includes a powerful command line SQL tool and simple GUI query tools. HSQLDB supports range of SQL Standard features seen in any open source database engine: SQL: 2011 core language features and an extensive list of SQL: 2011 optional features.
WHAT’S NEW IN VERSION 2.3