select
   a.id
from
   master as m,
   aux as a
where
   a.gid = m.groupid
and
   a.sid = m.sectionid
and
   not in m.groupid
and
   not in m.sectionid

id    groupid    sectionid
1     A Group    21
2     A Group    23
3     B Group    55
4     C Group    999
5     D Group    52A
6     D Group    53

id    gid        sid
1     A Group    21
2     A Group    22
3     A Group    23
4     B Group    55
5     B Group    55A
6     C Group    999
7     D Group    52A
8     D Group    53
9     D Group    56

id    groupid    sectionid
1     A Group    21
2     A Group    23
3     B Group    55
4     C Group    999
5     D Group    52A
6     D Group    53
7     A Group    22
8     B Group    55A
9     D Group    56