Just wanted to second that. I found http://www.sqlcourse.com/ very useful, and totally "why didn't someone else think of this?" to be able to type code into the page and click on 'submit' and see what happened. If this is the way you like to learn then it's pretty good.
($_='kkvvttuubbooppuuiiffssqqffssmmiibbddllffss')
=~y~b-v~a-z~s; print