Thanks Fabrice, I like such brain teasers!

SELECT SET_ID
FROM SET S
WHERE NOT EXISTS (SELECT 1 FROM SET_ITEM SI
WHERE SI.SET_ID = S.SET_ID
AND NOT EXISTS (SELECT 1 FROM SET_ITEM SI2
WHERE SI2.SET_ID = :S
AND SI2.ITEM_ID = SI.ITEM_ID
AND SI2.QUANTITY = SI.QUANTITY)
AND NOT EXISTS (SELECT 1 FROM SET_ITEM SI
WHERE SI.SET_ID = :S
AND NOT EXISTS (SELECT 1 FROM SET_ITEM SI2
WHERE SI2.SET_ID = S.SET_ID
AND SI2.ITEM_ID = SI.ITEM_ID
AND SI2.QUANTITY = SI.QUANTITY)

The first exists checks that the SET_ID doesn't contain any records that
doesn't exist for :S, whereas the second exists should cater for the opposite.

For the supersets just use the second half of the where clause.

Note that I've never tried using exists within exists, that I haven't
actually executed (nor prepared) the above statement, and that I do not
know at what speed such a query would execute.

Tell me if it works - if not, I would recommend you to write a stored
procedure.

HTH,
Set (that's what I call myself in English, not the name of your table)