SELECT product_id FROM
(
SELECT product_id FROM T WHERE av_id = 3
UNION ALL
SELECT product_id FROM T WHERE av_id = 11
UNION ALL
SELECT product_id FROM T WHERE av_id = 15
) AS F
GROUP BY product_id
HAVING COUNT(*) = 3
SELECT product_id FROM
(
SELECT product_id FROM T WHERE av_id = 3
UNION ALL
SELECT product_id FROM T WHERE av_id = 2
) AS F
GROUP BY product_id
HAVING COUNT(*) = 2