Need some magic with alternative rows

Hi,
I need some magic with alternative rows.

I've got a table with rows relating another table.
mytab (id, name, group_nr, class_nr, fk)

SELECT * FROM mytab
WHERE (fk = 994010)
OR ((class_fk = 40) AND (fk = 0))

994001, Tick, 1, 40, 994010
4001, Reporting, 1, 40, 0
4002, LEADS, 2, 40, 0
994003, Track, 3, 40, 994010
4003, Details, 3, 40, 0

The rows with fk=0 are defaults that should only be used when there is
no specific reference to the other table.

Is there a way to remove the default-rows if there exists a fk<>0 row
that has the same group_nr ?
I'd like to get just:

994001, Tick, 1, 40, 994010
4002, LEADS, 2, 40, 0
994003, Track, 3, 40, 994010

Regards
Andreas