Apache Derby, an Apache DB subproject, is an open source relational database implemented entirely in Java and available under the Apache License, Version 2.0. Some key advantages include:

• Derby has a small footprint -- about 2 megabytes for the base engine and embedded JDBC driver.
• Derby is based on the Java, JDBC, and SQL standards.
• Derby provides an embedded JDBC driver that lets you embed Derby in any Java-based solution.
• Derby also supports the more familiar client/server mode with the Derby Network Client JDBC driver and Derby Network Server.
• Derby is easy to install, deploy, and use.