Firebird RDBMS offers ANSI SQL features & runs on Linux, Windows & several Unix platforms. Features excellent concurrency & performance & powerful stored procedure & trigger language.
More information and downloads at https://firebirdsql.org/, issue tracker is https://github.com/FirebirdSQL/firebird/issues
WhiteBear is a set of foundation classes for database engines written in Java. It supports object and relational database, is highly portable and may be used in embedded or server environment. Project has been created from scratch and plan to includes storage engine, server, SQL interpreter, ODBC, JDBC and JDO interface