SQL: Find the teams containing a list users

Question

I have a game where I have teams containing a number of users. There are different amounts of users in each team but for anyone team the combination of users makes that team unique.

I have created three database tables for this.

 Team    User    TeamUsers
 t_id    u_id    t_id, u_id

If users 1, 4, 5 and 7 have been in a team together previously I would like to know the team ID of that team. What would the SQL look like for this?

I have experimented with something like this:

SELECT t_id FROM teamusers WHERE u_id IN (users[1], ..., users[i])
GROUP BY t_id HAVING COUNT(t_id) = users[].length;

users array is programatically filled with the values. Does this seem correct? Is there another way that's better?

Answer 1

This is a classical case of relational division. We have assembled a whole arsenal of queries under this closely related question:
How to filter SQL results in a has-many-through relation

One way to get all teams that have all given players would be what you already have. Should be among the most efficient queries.

For longer arrays, unnest() + JOIN performs better:

SELECT t_id
FROM  (SELECT unnest($users) AS u_id) u
JOIN   team_users USING (u_id)
GROUP  BY 1
HAVING count(*) = array_length($users, 1);

But for just a hand full of players, it hardly matters.
For repeated use, instead of building the query in your app, you can create a table function:

CREATE OR REPLACE FUNCTION f_get_teams(_users int[])
  RETURNS SETOF int AS
$func$

SELECT t_id
FROM  (SELECT unnest($1) AS u_id) u
JOIN   team_users USING (u_id)
GROUP  BY 1
HAVING count(*) = array_length($1, 1);

$func$ LANGUAGE SQL STRICT;

Call:

SELECT * FROM f_get_teams('{1,4,5,7}'::int[]);