Oracle 9i

Under Oracle 9.2.0.1.0, the following statement fails with the nonsensical error 'ORA-00979: not a GROUP BY expression':

select (select max(dummy) from dual where dummy=a.dummy)
from dual a
, ( select dummy from dual group by dummy ) b
where a.dummy = b.dummy
and a.dummy = 'X'

Under Oracle 8.1.7.0.0, the statement works.