Question

I have a very basic mysql table called memberships, that tracks which people belong to which groups. People can belong to many groups.

It contains 3 fields: id, person_id, group_id

I need to select all the person_ids that have group_ids of ’20’ and ’53’. The people must belong to ONLY those 2 groups.

Answer 1

SELECT DISTINCT a.person_id
FROM memberships a
JOIN memberships b
    ON a.person_id = b.person_id 
    AND b.group_id = 53
LEFT JOIN memberships c
    ON a.person_id = c.person_id 
    AND c.group_id NOT IN (20,53)
WHERE a.group_id  = 20 AND c.person_id IS NULL;

Although you could also go for a NOT EXISTS format.