Venn diagrams are almost, but not quite, completely wrong. They hint at the idea of including failed joins vs only successful joins, but this is only a small pa

Venn diagrams are almost, but not quite, completely wrong. They hint at the idea of including failed joins vs only successful joins, but this is only a small part of the full logic behind Cartesian product + filter - even if it's the most commonly used pattern in practice in most database models that ultimately get used in an object oriented programming language. I think it's OK to use the venn diagram only as an explanatory tool, briefly, and informally. If you raise it to the level of a complete description of semantics, then it's very inadequate. Any step off the beaten path, where the cardinality of the join is no longer 1:1 or 1:n, and you're back in the weeds again.