Mutual friends- SQL Algorithm

Question

I'm trying to fetch mutual friends.

I have a table structured like this:

 `friends` (   
    `inviter`  int(11) NOT NULL,   
    `accepter`  int(11) NOT NULL,   
    `time` datetime NOT NULL,)
ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_turkish_ci;

My load_my_friends(##MY ID##); query is like this:

SELECT  p.id , p.name,p.lastname,p.thumb
FROM  friends AS f
RIGHT JOIN  profiles AS p ON ( f.accepter = p.id OR f.inviter = p.id ) 
WHERE  f.inviter = ##MY ID##
OR  f.accepter = ##MY ID##
LIMIT 0 , 5000

Could you tell me how I can fetch the mutual friends data.

Note: I'm not using any social-network script like JCOW,Social Engine etc.

---

Edit for @2

In the database I have user1,user2,user3,user4,user5..userX

• user1's friends are: 2,3,6,7,8
• user2's friends are: 1,3,4,5,7,8

I want a query which can fetch these results:

     | id  | name |lastname| thumb
    1|  3  |raheel|shan    | /img01.jpg
    2|  7  | arqu |x       | /img02.jpg
    3|  8  |abcdef|hijklmno| /img06.jpg

Answer 1

A query that's probably easy to understand is the following:

SELECT p.* 
FROM 
    (
       -- all friends of User A
       SELECT accepter AS friend_id FROM friends WHERE inviter IN (##User_A_ID##)
       UNION
       SELECT inviter AS friend_id FROM friends WHERE accepter IN (##User_A_ID##)
    ) AS t1
    -- intersect with
    JOIN
    (
       -- all friends of User B
       SELECT accepter AS friend_id FROM friends WHERE inviter IN (##User_B_ID##)
       UNION
       SELECT inviter AS friend_id FROM friends WHERE accepter IN (##User_B_ID##)
    ) AS t2
    ON t1.friend_id = t2.friend_id

    JOIN profiles p on p.id=t1.friend_id

Answer 2

Perhaps someone can come up with a much simpler way to do this, but here's my go at it:

SELECT id,name,lastname FROM profiles, 
  (SELECT *
   FROM friends 
   WHERE  
       (inviter = 1 OR accepter = 1) 
       AND (accepter IN ( 
             SELECT inviter 
             FROM   friends 
             WHERE  accepter = 2 AND inviter != 1)
          OR accepter IN ( 
             SELECT accepter 
             FROM   friends 
             WHERE  inviter = 2 AND accepter != 1)
          OR inviter IN ( 
             SELECT inviter 
             FROM   friends 
             WHERE  accepter = 2 AND inviter != 1)
          OR inviter IN (
             SELECT accepter
             FROM   friends 
             WHERE  inviter = 2 AND accepter != 1)))
    f
    WHERE id != 1 AND (inviter = id OR accepter = id);

where user1's id = 1, and user2's id = 2

Have a look - http://sqlfiddle.com/#!2/15951/8

Answer 3

If you simply want a list of friends that are shared between two users - User_A and User_B - then this might be sufficient:

SELECT accepter 
FROM   friends 
WHERE  inviter = ##User_A_ID## 
       AND accepter IN ( 
          SELECT accepter 
          FROM   friends 
          WHERE  inviter = ##User_B_ID## )

Join with your profiles-table to get the other related data.

Answer 4

SELECT * 
FROM profiles p
WHERE p.id IN (
    SELECT f_id 
    FROM (
        SELECT accepter as f_id FROM friends where inviter IN (1,2)
        UNION
        SELECT inviter as f_id FROM friends where accepter IN (1,2)
    ) friends 
    GROUP BY f_id
    HAVING count(*) > 1
)

assuming that friendships are unique rows....