在LEFT JOIN中获得共同朋友的数量

Question

I've two tables:

users :

id | user | fname | sname | photo
1  | Igor | Igor  | Souza | profile.jpg
2  | John | John  | Jhow  | pic.png
3  | Lucas| Lucas | Coll  | photo.jpg

friends :

id | friend1 | friend2 | status
1  | 1       | 3       | 2
2  | 1       | 2       | 2
3  | 3       | 2       | 2

In friends table, status = 2 it means that they are friends and the friend1 is the user that send the friend request.

To select the friends of user logged, I did ( $userID corresponds to the user ID profile page accessed):

SELECT u.user, u.fname, u.sname, u.photo
FROM friends f
INNER JOIN users u
ON (u.id = f.friend1 AND f.friend1 <> '$userID') OR (u.id = f.friend1 AND f.friend2 <> '$userID')
WHERE (f.friend1 = '$userID' OR f.friend2 = '$userID') AND f.status = 2

I want to do a count of mutual friends between the user logged ( $userIDLog ) and the user of profile page accessed ( $userID ). But I have no idea of how to do this, use a count in a subquery on LEFT JOIN ? If yes, how to do the subquery? Obs.: I need users information too ( users )

Answer 1

First let's assume $userId = 1 and $userIDLog = 2, now we can find out the logged in user's friends with.

SELECT friend2 as f FROM friends where friend1 = 1 
UNION SELECT friend1 as f FROM friends WHERE friend2 =1

Similiarly the other users friends are given by

SELECT friend2 as f FROM friends where friend1 = 2 
UNION SELECT friend1 as f FROM friends where friend2 =2 

Now if you join them on f, you have your answer.

SELECT COUNT(*) FROM 
  (SELECT friend2 as f FROM friends where friend1 = 1 
   UNION SELECT friend1 as f FROM friends WHERE friend2 =1) AS a
  INNER JOIN
  (SELECT friend2 as f FROM friends where friend1 = 2 
   UNION SELECT friend1 as f FROM friends where friend2 =2 ) AS b
  ON a.f = b.f

The only friend they both have in common is 3 and you get the answer count = 1 for this query.