Question

Given this data:

users
  id    name
  ==    ====
  1     Alice
  2     Bob
  3     Carl

organizations
  id    name
  ==    ====
  1     Aaa
  2     Bbb

memberships
  id    organization_id   user_id
  ==    ===============   =======
  1     1                 1
  2     2                 1
  3     1                 2

I want to find users that do not belong to a particular organization X:

• users with no membership records at all

  AND

• users with membership records, but not for organization X.

For example, I want users that are not in organization 2. I am expecting:

users.id
========
2
3

Attempting with this join isn’t working:

SELECT *
FROM users left join memberships on memberships.user_id = users.id
where memberships.id is null or memberships.organization_id <> 1

It’s returning users 1,2,3 since 1 matches on the second where condition.

Is there an efficient way to do this?

Answer 1

Restricting your JOIN to organization of 2 and then testing for null is the one way to do what you’re looking for e.g.

SELECT * 
FROM   users 
       LEFT JOIN memberships 
         ON memberships.user_id = users.id 
            AND memberships.organization_id = 2 
WHERE  memberships.id IS NULL 

You can also use NOT IN

SELECT * 
FROM   users 
WHERE id NOT IN (SELECT user_id from memberships where organization_id = 2 )