Question

My MySQL tables structure is like this.

USER
uid

FRIENDS
fuid,fuid2,fapproved

For each friend relationship I insert 2 records in FRIENDS. If user 1 is friend of user 2 then the next rows are inserted into FRIENDS

1,2,1

2,1,1

1,3,1

3,1,1

2,3,1

3,2,1

User id 3 is friend of user id 1 and user id 2

How to get user id 3 in one sql query?

Answer 1

Given two users @friend1 and @friend2 find all the users who are mutual friends of them:

SELECT user.uid
FROM user
WHERE EXISTS(
    SELECT TOP 1 1 
    FROM Friends 
    WHERE Friends.fuid = @friend1 AND Friends.fapproved = 1 
      AND Friends.fuid2 = User.uid
  )
  AND EXISTS(
    SELECT TOP 1 1 
    FROM Friends 
    WHERE Friends.fuid = @friend2 AND Friends.fapproved = 1 
      AND Friends.fuid2 = User.uid
  )