Zitat von Mysql Doku

Join Processing Changes in MySQL 5.0.12
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The way MySQL that determines the result columns of NATURAL or USING join operations (and thus the result of the entire FROM clause).
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SELECT ... FROM t1, t2, t3
WHERE t1.b = t2.b AND t2.c = t3.c;

That join is missing one more equi-join predicate (t1.a = t3.a). As a result, it produces one row, not the empty result that it should. The correct equivalent query is this:

SELECT ... FROM t1, t2, t3
WHERE t1.b = t2.b AND t2.c = t3.c AND t1.a = t3.a;