In a recent blog post by Kris Wenzel, he wrote about database joins:
One of the biggest issues beginning SQL writers have is being able to write queries that use more than one table. In this series of articles we are going to show you how to write a query that combines, or joins, data from more than one table. Once you have gone through the examples you will understand how to write the basic commands to make this happen and why data is separated in the first place.
This first article introduces the concept of joining tables. The focus is going to be more on the type of joins, not necessarily their syntax. The later articles focus on the various types of joins. Through the narrative and examples you’ll become very comfortable with each one.
He has an interesting series of articles on database joins that you should read.