All right, so I had an extra day to kill last week, and figured I might as well become expert in SQL. After all, I found a book that said that I could teach myself SQL in 24 hours, so I thought "hey, why not?".
Obviously, I'm not being serious there, but I actualy did try this book out a few years back. In my opinion it offers a pretty decent overall view of the basics of database theory, design, and of course the various components of the SQL language. The book gets its title because it is broken up into 24 chapters, with each requiring about an hour to complete.
If you don't have an interest in learning about databases or SQL, the material may seem pretty dry, but I found that the book did a pretty decent job of showing examples in each chapter, and also pointing out the differences between various SQL implementations.
All in all, it took me a little over a year* to finally read through the entire book, so the whole "teach youself in 24 hours" thing may be a little optimistic, but I thought it offered a nice foundation to build upon later. If you're looking for a beginner's guide to SQL, this one is pretty good.
(* It only took me a year to finish the book because the information that I originally needed to know was contained in just a few chapters, and the others were just "extra" knowledge.)