CodeProject: Visual Representation of SQL Joins. Free source code and programming help
http://www.codeproject.com/KB/database/Visual_SQL_Joins.aspx
When Did You Join Twitter?
http://whendidyoujointwitter.appspot.com/
In the Woods - A Closer Look at SQL Joins
http://blog.themeforest.net/tutorials/a-closer-look-at-sql-joins/

JOINs em Mysql