HSQLDB (HyperSQL DataBase) is the leading SQL relational database engine written in Java. It offers a small, fast 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 the widest 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. It supports nearly full Advanced ANSI-92 SQL (BNF format). Many extensions to the Standard, including syntax compatibility and features of other popular database engines, are also supported. Version 2.2 is fully multithreaded and supports high performance 2PL and MVCC (multiversion concurrency control) transaction control models. See the list of new features in version 2.2. HSQLDB has been constantly developed over 11 years and is used as a database and persistence engine in over 1700 Open Source Software projects and many commercial products. The latest versions are extremely stable and reliable. It is known for its small size, ability to execute completely or partly in memory, its flexibility and speed.