I've been needing to "brush up" on my SQL and RDBMS skills (let's just say it's been a while :-). I found the following books from O'Reilly and Addison-Wesley to be extremely useful, in order of increasing usefulness:
The second two books above are from O'Reilly. I really like the two O'Reilly SQL books! I refer to them the most right now. The SQL Cookbook updated a lot of my existing knowledge and added to it, and the Art of SQL is fantastic! I just love the way it uses Sun Tzu's "Art of War" to apply to winning the "war" on DBMS and DBM complexity!