left join i tags tabella e includi l'ID cercato nella clausola di join e controlla i conteggi in having .
SELECT u.id,u.name,GROUP_CONCAT(ut.tagid) as tags
FROM users u
LEFT JOIN usertag as ut ON u.id = ut.userid
LEFT JOIN tags t ON t.id=ut.tagid AND t.ID IN (10,20,30) --change this as needed
GROUP BY u.id,u.name
HAVING COUNT(ut.tagid) >= COUNT(t.id) AND COUNT(t.id) = 3 --change this number to the number of tags
Un'altra opzione è usare FIND_IN_SET se ci sono valori limitati. Ad esempio,
SELECT * FROM (
SELECT u.*, GROUP_CONCAT(ut.tagid) as tags
FROM users as u
LEFT JOIN usertag as ut ON u.id = ut.userid
GROUP BY u.id
) T
WHERE FIND_IN_SET('10',tags) > 0 AND FIND_IN_SET('20',tags) > 0
