A Visual Guide to SQL Joins
Your SQL database can be huge. So many tables, so much data, combining it can be hard. Even once you've sorted out what data you want it can be a real task to visualize what joins you need so that you only grab the data you want. The more efficient the join the better. I found this nifty image on SQL joins that I think does a great job at showing how the various joins work and does a great job explaining how to use them.
I hope this helps someone else as I know I am always having a hard time figuring out which joins to use and when.