Most SQL databases follow the ANSI SQL standards to a degree, but
The standard is ambiguous, leaving some areas open to interpretation (eg: how different operat
I'm not sure how comprehensive this list is, but maybe this will help - http://troels.arvin.dk/db/rdbms/
Except of already mentioned some comparison you can find in Wikipedia
Also similar question was already posted on Stackoverflow where you can fin a couple of useful links.