Also don’t forget the Udemy - MySQL Database for Beginners that was mentioned in the Lifehacker University Fall Course earlier this month.

http://pgexercises.com/ is another great one that I’m quite fond of, since you can run your botched queries as well and see the results. The site is aimed specifically at Postgres, but most of the syntax should be applicable for all SQL dialects.

Mode SQL school is another good resource for learning SQL.