"Kevin B." <db(at)ke5in(dot)com> writes:
> Select a.i, b.i
> from t as a
> left join t as b on a.i = b.i
> where a.n = 'a' and b.n = 'b' and b.i is null
This can't succeed since the b.n = 'b' condition is guaranteed to fail
when b.* is nulled out ...
regards, tom lane