Question

Table 1: Users

| profile_id | name    |
------------------------
| 1          | Joe     |
| 2          | Jane    |
| 3          | Jill    |
| 4          | Jeffery |

Table 2: User and Role to Team Lookup

| team_id | profile_id | role   |
---------------------------------
| 1       | 1          | coach  |
| 1       | 2          | player |
| 2       | 4          | coach  |
| 2       | 1          | player |

The scenario is that Jill is building a team, and the restriction is that you cannot be a player on more than one team. So I’m trying to build a query that pulls up those eligible to join Jill’s team.

My first attempt was:

SELECT `users`.`profile_id`
FROM `users` LEFT JOIN `user_role_to_team_lookup` AS `utr` USING(`profile_id`)
WHERE `utr`.`role` != 'player' OR `utr`.`role` IS NULL

The problem is that because Joe is a coach, he matches the criteria~ even though he is also already a player.

What would be the proper way to exclude users that are already players from the result set?

Answer 1

I would write this without the subquery that most people use:

SELECT u.profile_id
FROM users AS u 
LEFT OUTER JOIN user_role_to_team_lookup AS utr 
  ON u.profile_id = utr.profile_id AND utr.role = 'player'
WHERE utr.profile_id IS NULL

In other words, look for a user who is already a player. Those who aren’t a player will match no rows in the outer join, and therefore any column of utr will be NULL.

But it’s best to put the condition in the ON clause of the join.