मुझे लगता है कि यह भी काम करना चाहिए
select distinct g.GroupID, c.ClassID
from @Groups g
left join @Classes c on g.TagID = c.TagID
where not exists (
select *
from @Groups g2
where g2.GroupID = g.GroupID
and g2.TagID not in (
select TagID
from @Classes c2
where c2.ClassID = c.ClassID
)
) or c.ClassID is null