Question

Is what i have:

SELECT users.*
            FROM users INNER JOIN roles_users ON users.id = roles_users.user_id 
            WHERE roles_users.role_id IN (1)
            GROUP BY users.id
            HAVING COUNT(*) = 1

I would like to grab all the users that ONLY has one row, with role_id 1.

The above query grabs all the users that have a row with role_id 1, so it shows also those who have more rows with role_id 2, role_id 3 for example.

So how can i do WHERE roles_users.role_id IN (1) AND NOT in 2,3,4.. something?

Answer 1

Add an anti-join to exclude those users that also have other roles

SELECT users.*
FROM users
INNER JOIN roles_users ON users.id = roles_users.user_id 
-- replace by IN (1, 4) if needed
WHERE roles_users.role_id IN (1)
AND NOT EXISTS (SELECT 1 FROM roles_users
                 WHERE roles_users.user_id = users.id
                    -- replace by NOT IN (1, 4) if needed
                   AND roles_users.role_id NOT IN (1))
GROUP BY users.id
HAVING COUNT(*) = 1

The above is quite MySQL-specific as normally, you shouldn’t select columns that are not part of the GROUP BY clause. Besides, maybe it is not best performing, as roles_users is joined two times to users. I don’t know if MySQL can optimise this.