HSQLDB is a relational database written in Java. It's small and very easy to use. I wouldn't run my ecommerce sites on it, but it's perfect for small projects.
Hi, You could also use Oracle SQL Developer as an IDE for learning SQL. Even though the saying goes like "IDE's destroy your knowledge of coding", it is actually the other way around. When IDE's keep auto completing the correct code, and you try out a few samples repeatedly, the code syntaxes stay rock solid in your mind.
I recommend using Oracle SQL Developer with Oracle Database XE edition.
SCJP, SCWCD, SCBCD, IBM XML, IBM OOAD, SCEA 5, ITAC - Distinguished IT Architect.