I thought Ligaya Turmelle's post on SQL joins was a great primer for novice developers. Since SQL joins appear to be set-based, the use of Venn diagrams to explain them seems, at first blush, to be a natural fit. However, like the commenters to her post, I found that the Venn diagrams didn't quite match the SQL join syntax reality in my testing.
This is a companion discussion topic for the original blog entry at: http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html