>> Is it possible that the sub-selects are somehow
>> affecting the result sets?
> I think you want to upgrade, 7.3beta at least gives your
> expected output on all the queries when I tried it. I don't
> currently have a 7.2.* system to test on here to try it
> with that series.
I think this may be the same problem as bug #526,
http://archives.postgresql.org/pgsql-bugs/2001-11/msg00168.php
If so, the fix is in 7.2.*.
regards, tom lane