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Nonsense. Venn diagrams might be incorrect to explain joins, but ever since I've seen this image[α] 10+ years ago, that's what I think of _every single time_ I'm writing one.

Mnemonic > Correct.

[α] http://www.codeproject.com/KB/database/Visual_SQL_Joins/Visu...

They don't show the projection, and I think it is far better to introduce those working with joins to projections, selections and the algebraic notion of a join.