SQL is not a difficult subject - you can spend a couple of hours reading an online tutorial and make a legitimate claim of knowing/understanding SQL. The difficult part, however, is to use SQL to solve actual problems, which takes lots of practice. I think the same thing can be said about any programming language. For example, one can learn Java in CS201, but it doesn't instantly make them a great programmer. So to prepare for CS122 (and be good at SQL), my suggestion is to complete all the exercises in the SQL Essentials book.

With that said, if you really like reading books, Learning SQL is a pretty good book with some exercises. I've also heard some good things about Joe Celko's SQL for Smarties.