SQL Injection attacks are a well know exploit of insecure database systems. If you don't know about SQL Injection, you can change that by visiting Wikipedia.

If you feel like injecting some SQL into your site, you should visit the SQL Injection Cheat Sheet page for all of the approaches. As the author mentions, only MySQL, Microsoft SQL Server, (some) ORACLE, and (some) PostgreSQL are supported and that most of the samples are not correct for every situation.

Happy hacking!