Question

I have a bridging table that looks like this

clients_user_groups
id = int
client_id = int
group_id = int

I need to find all client_id’s of of clients that belong to the same group as client_id 46l

I can achieve it doing a query as below which produces the correct results

SELECT client_id FROM clients_user_groups WHERE group_id = (SELECT group_id FROM clients_user_groups WHERE client_id = 46);

Basically what I need to find out is if there’s a way achieving the same results without using 2 queries or a faster way, or is the method above the best solution

Answer 1

You’re using a WHERE-clause subquery which, in MySQL, ends up being reevaluated for every single row in your table. Use a JOIN instead:

SELECT a.client_id
FROM   clients_user_groups a
JOIN   clients_user_groups b ON b.client_id = 46
                            AND a.group_id  = b.group_id

Since you plan on facilitating clients having more than one group in the future, you might want to add DISTINCT to the SELECT so that multiple of the same client_ids aren’t returned when you do switch (as a result of the client being in more than one of client_id 46’s groups).

If you haven’t done so already, create the following composite index on:

(client_id, group_id)

With client_id at the first position in the index since it most likely offers the best initial selectivity. Also, if you’ve got a substantial amount of rows in your table, ensure that the index is being utilized with EXPLAIN.