What about PostgreSQL (www.postgresql.org) ? Or the small java DBs like hsql
?

David

GPL is a concern for the software and the way that it is further
developed or integrated into packages. You will have no problem
creating a commercial solution to use it as long as you acknowledge
the source. I suggest you - or better yet your legal advisor - read
the documentation for the GPL before you go for a proprietary solution
(which is under far more restrictive licence terms than the GPL).

There are more SQL implementations than you can shake a stick at.
Surely some will fit your bill for size and cost.  See
http://mindprod.com/jgloss/sql.html

--
Canadian Mind Products, Roedy Green.
Coaching, problem solving, economical contract programming.
See http://mindprod.com/jgloss/jgloss.html for The Java Glossary.

I don't see how GPL would restrict you from using MySQL. You can use
it for free. Your customers can use it for free, also when using it
with your software. You just can't distribute MySLQ. You can tell
your customers where to get it and help setting it up.

A quote from http://www.mysql.com/products/licensing.html :
"As long as you never distribute (internally or externally) the MySQL
Software in any way, you are free to use it for powering your application,
irrespective of whether your application is under GPL or other OSI approved
license or not."

-Renni