sigh... this is totally wrong... as Jeff himself kinda realized with his comment "There's also a cartesian product or cross join, which as far as I can tell, ca

sigh... this is totally wrong... as Jeff himself kinda realized with his comment "There's also a cartesian product or cross join, which as far as I can tell, can't be expressed as a Venn diagram" SQL does have set operations, and the Venn diagram treatment is much less terrible [though still not quite accurate] at explaining SQL set semantics. The biggest flaw with the Venn diagram thing is that it doesn't capture the productive aspect of joins. A inner join B on (some-condition) will produce ALL combinations of A records pasted to B records where some-condition holds. The Venn diagram approach can't distinguish between a single A-pasted-to-B record and 100 thousand combinations of A-pasted-to-different-Bs. All it can say is that the resulting combinations consist of some set of A and B records. Venn diagrams don't accurately describe the records resulting from a join at all. At best, they rule out records that can't occur. So they're not so great as an explanatory device.