The O'Reilly "SQL Pocket Guide" is very well written, and, IMO, one of the best tools for learning SQL. It's extremely concise, and yet packed chock full of information. It's missing some things, sure, but once you have a strong foundation, the rest you can get from RDBMS docs. The fact that this is such a short book, and not a reference guide but an actual guide to using SQL, makes this book approachable and unintimidating. I highly recommend it.