An arguement of use / not use RDBMS http://www.cincomsmalltalk.com/blog/blogView?showComments=true&entry=3366636502 .

Hsql is a very nice tool for development: fast, support most sql standard, easy to setup. In addition, there is a one really nice feature. It will store the INSERT, UPDATE and DELETE statement in a script file to persist the database status. This make debug a lot simpler. Highly recommend!

http://hsqldb.sourceforge.net/

Besides, a nice introduction of SQLite and Java Berkeley DB - http://www-900.ibm.com/developerWorks/cn/java/l-embed-db/index.shtml

A solution of close don't commit data to disk: http://blog.taragana.com/index.php/archive/hsqldb-database-connection-close-doesnt-write-to-disk-solution/ but for me this is feature than bug, as I can easily keep my data back to the copy I want.

A document base DB, it is not hold a table ( or relation ) , it hold a Document , what I understand it is a map - http://www.couchdbwiki.com/index.php?title=Main_Page
