Lynn Beighley's
Head First SQL is pretty good if you want to get a good understanding of the basic principles and build from there - although not everybody gets along with the chatty style and slightly too-cute jokes of the Head First books, I have found them pretty effective for stuff I wasn't already familiar with or hadn't done in a while.
After that, just throw caution to the wind and try Joe Celko's
SQL for Smarties (Third Edition), although be aware that it focuses on getting things done using standard SQL rather than the proprietary extensions database vendors (especially Microsoft) graft on to make life easier for people who don't understand sets and declarative programming and are too lazy to learn. In other words, it shows you how to do it the
right way
As you're working specifically with MySQL, I can also recommend Schwartz et al.'s
High Performance MySQL (Second Edition) which, balancing the purist approach of Celko's book, is full of info about what's actually going on under the hood and how to get the best from it.