The SQL standard is a non-free ANSI/ISO standard, and it is really hard to find good drafts.
Large comparison table between implementations: http://troels.arvin.dk/db/rdbms/
MySQL is generally compatible with the SQL standard for DBMS, but it is well known that there are many differences between how different DBMS implement SQL, so it is not safe to assume that using SQL only will lead to very high portability.
Other important implementations of SQL-like languages are:
- http://www.sqlite.org/sqllogictest/doc/trunk/about.wiki can be used to test and compare multiple SQL databases