Question

I have table in MySQL database with two integer columns for example:

userid | groupid
10 | 300
11 | 300
11 | 301
12 | 302

Given two groupids, I am looking for the best and quickest way to find userids which are in both groups. My table contains 23M rows and I need to that for each distinct pair of groupids. Currently both columns are indexed however it takes so long to get the result even for a single pair of groups and I have 1000 distinct groupids. The query I am running now is:

select count(t2.userid) 
from usergroup t1, usergroup t2 
where t1.groupid = 27 and t2.groupid = 714 and t1.userid = t2.userid

Is there a way to do it fast?

Answer 1

Why the join?

select 
  u.userid
from
  usergroup u
where
  u.groupid in (27, 714)
group by
  u.userid
having 
  count(u.userid) > 1

Assuming a combination of userid and groupid is unique, which I figure it should be in a table like this.