Question

I have two tables

users table:

id|name

user_relationships

id | user_id | friend_id

and want to get names of mutual friends of 2 users. i.e:

user_relationships
1 | 1 | 3
2 | 2 | 3  

users
3| sammy

users 1 and 2 have mutual friend 3. I want to get his name ‘sammy’ in one query.

How do I do that?

Answer 1

SELECT id, name
FROM users
WHERE id IN (
  SELECT friend_id
  FROM user_relationships
  WHERE user_id IN ( 1, 2 )
  GROUP BY friend_id
  HAVING COUNT(friend_id) >= 2
)

or with one join:

SELECT friend_id, name
FROM user_relationships r
  INNER JOIN users u ON r.friend_id = u.id
WHERE user_id IN ( 1, 2 )
GROUP BY friend_id
HAVING COUNT(friend_id) >= 2