SQL Queries for Mere Mortals
It gives real practical information and examples, without putting on vendor blinders, nor going too deep into theory (which gets broken by SQL, anyway).
Also, if you actually work with databases (or plan to), check out www.dbdebunk.com/. It's a lot of reading, but good issues...and Pascal is right, every single time.
Overall, SQL is easy to learn. I had never written a single SQL statement before late February of this year. A few bugs came up in a web app my company supports, and nobody else had time. I was fixing it on the DB side by the next day. It's not just easy to learn in comparison to other things, but plain easy to learn in general.
There will be more to learn as you go on, with specific DB software, but getting started takes very little time or effort. If you passed high school geometry, and have had any logic or discrete math courses in school, then you have all the groundwork in the back of your brain, already.