Just another way to do it. It should perform better than the JOIN, but I'm not sure about INTERSECT.
select product_id
from tablea a
WHERE brand_id=1
AND EXISTS(
select product_id
from tablea b
WHERE brand_id=2
AND a.product_id = b.product_id)
Luis C.
General Disclaimer:
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How to post data/code on a forum to get the best help: Option 1 / Option 2