MySQL: Get all users not a part of a group - php

Question

I'm trying to get my head around this query but one of the records still keeps popping up. In summary, my intention is to show group members on the left and names from a phonebook which are not included in the group on the right. That way the user can select them and add them to the group.
user_id firstname group_id grpname
------- --------- -------- ----------
1 Luker 3 Abc
2 John 1 Some Group
3 Sam 2 Awesome Group
4 Mitch 1 Some Group
4 Mitch 2 Awesome Group
5 Rocky (NULL) (NULL)
6 Pops (NULL) (NULL)
The unique thing is that if one of the users is a part of multiple groups (user_id 4), their name should not be shown in the phonebook at all since it would already be placed in the list of existing members.
-- Query for group_id 2
SELECT user.id user_id, user.firstname, grp.id group_id, grp.grpname FROM agi_user user
LEFT JOIN agi_group_user gu ON user.id = gu.user_id
LEFT JOIN agi_groups grp ON gu.group_id = grp.id
WHERE grp.id IS NULL OR grp.id != 2
GROUP BY user.id
but for some reason the user Mitch still keeps popping up.
user_id firstname group_id grpname
------- --------- -------- ----------
1 Luker 3 Abc
2 John 1 Some Group
4 Mitch 1 Some Group
5 Rocky (NULL) (NULL)
6 Paps (NULL) (NULL)
Edit:: The output I need is
user_id firstname group_id grpname
------- --------- -------- ----------
1 Luker 3 Abc
2 John 1 Some Group
5 Rocky (NULL) (NULL)
6 Paps (NULL) (NULL)
Basically, to I want to get all users not a part of that group including NULL. But since one user is a part of multiple groups, her record still comes up when it shouldn't because that other group probably has a group_id of 3 or 4 (or any other besides 2).

Answer 1

You can use NOT EXIST to check if there are no group/user rows at all for group 2 and the given user:
SELECT
user.id user_id,
user.firstname,
grp.id group_id,
grp.grpname
FROM
agi_user user
LEFT JOIN agi_group_user gu ON user.id = gu.user_id
LEFT JOIN agi_groups grp ON gu.group_id = grp.id
WHERE
NOT EXISTS (
SELECT 'x' FROM agi_group_user x
WHERE
x.user_id = user.user_id and
x.group_id = 2)
GROUP BY
user.id
Note that if a user is not in group 2, but is in two other groups, he will show up 1 time in this result and only one of the groups he is in is returned. To fix that, you can add grp.id to the GROUP BY clause, or maybe you can use GROUP_CONCAT to return a list of group names in a single field.
Alternatively, this should work too, and in MySQL it might even perform better since it sucks at subqueries. Personally I think it is semantically less clear what is going on, though.
It joins the user/groups table a second time, but adds the group_id (2) to the join. If there are no rows returned for this table, then the user is not in group 2. The comment regarding GROUP_CONCAT applies to this query as well.
SELECT
user.id user_id,
user.firstname,
grp.id group_id,
grp.grpname
FROM
agi_user user
LEFT JOIN agi_group_user gu ON user.id = gu.user_id
LEFT JOIN agi_groups grp ON gu.group_id = grp.id
LEFT JOIN agi_group_user x ON user.id = x.user_id and x.group_id = 2
WHERE
x.group_id IS NULL
GROUP BY
user.id

Answer 2

If I understood correctly, you want this: "All users that are not in group 2"
This is called an anti-semijoin (or just anti-join) and can be done with a LEFT JOIN /IS NULL query:
SELECT u.id user_id, u.firstname
FROM agi_user AS u
LEFT JOIN agi_group_user AS gu
ON u.id = gu.user_id
AND gu.group_id = 2
WHERE gu.group_id IS NULL ;
or with a NOT EXISTS subquery:
SELECT u.id user_id, u.firstname
FROM agi_user AS u
WHERE NOT EXISTS
( SELECT 1
FROM JOIN agi_group_user AS gu
WHERE u.id = gu.user_id
AND gu.group_id = 2
) ;