A Useful Site for Learning SQL

Recently, I start to get myself familiar with SQL (Standard Query Language). SQL is an easily-pick-up language compared with other languages. After reading some books for several days, it's not hard for me to decode the syntax lines. Therefore, I accidentally found a pretty useful website with a lot of tutorials, especially for beginners. It's SQLZoo. That also contains the links to its sister sites for learning Linux and XML. I haven't visited them yet, but I think they will be useful as well.