Question

(Not my real MySQL schema, but illustrates what needs done.)

Users can belong to many groups, and groups have many users.

users:
  id INT
  validated TINYINT(1)

groups:
  id INT
  name VARCHAR(20)

groups_users:
  group_id INT
  user_id INT

I need to find groups that contain both validated and unvalidated users (validated being 1 or 0, respectively), in order to perform a specific manual maintenance task. There are thousands of users, all belong to at least one group, but a group usually only has 2-5 users.

This is a live production server, so I could probably craft a query myself, but the last one I tried took a matter of minutes before I killed it. (I’m not one of those brilliant SQL wizards.) I suppose I could take the server down for maintenance, but, if possible, a query that gets this job done in a matter of seconds would be fantastic.

Thanks!

Answer 1

For each user, select all users with which he is in the same group. Then only select the users which validated differs.

SELECT users u1 LEFT JOIN groups_users g1 ON id=user_id LEFT JOIN groups_users g2 ON group_id=group_id LEFT JOIN users u2 ON user_id=id WHERE u1.validated=1 AND u2.validated=0;