Hi Stanley! Thank you for your detailed answer. I'm currently mainly looking for an alternative to hsqldb. I'm maintaining a persistence framework (cope). We don't use hsqldb for production, but for testing a VM-contained database is just so convenient.
Until recently, hsqldb did the job. But then I realized, that for proper implementation of queries across inheritance I need joins with parenthesis. Something like "select * from t t1 join (t t2 join t t3 on t2.a=t3.a) on t1.a=t2.a" Derby can do this, MySQL, Oracle and PostgreSQL as well, but not Hsqldb. Also hsqldb does not support transaction isolation (although the jdbc driver says so). So I hope, that derby and hsqldb complement each other, so I can have a full test of the framework just with hsqldb and derby combined. Best regards, Ralf.
