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 2.6 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.