如何构造一个SQL查询以获取用户之间的共同朋友数？

Question

I am busy trying to create a query that returns the number of mutual friends between users in a userlist. I have tried a few but they haven't been working as I had hoped, here is one of the queries I have tried

query("SELECT * FROM userlist WHERE userid = '$userid' AND status ='friend' AND friendid != '$userid' ");

the sql table below renders the relationships between users.

userid | friendid | type | status |
-----------------------------------
1      |2         | buddy | friend |
-----------------------------------
1      |3         | buddy | friend |
-----------------------------------
2      |3         | buddy | friend |
-----------------------------------
3      |2         | buddy | friend |
-----------------------------------
4      |2         | buddy | friend |
-----------------------------------
3      |1         | buddy | friend |
-----------------------------------
2      |4         | buddy | friend |
------------------------------------

How would I structure a query to find the number of mutual friends between userid 4 and userid 1 .

Thanks.

Answer 1

I think this will help, Please note that, here i do not have used condition of "status", you can modify this query as per your requirement.

SELECT count(*) FROM userlist as a 
INNER JOIN  userlist as b 
ON(b.friendid=a.friendid and  b.userid=4 and b.friendid!=1)
WHERE a.userid=1 and b.friendid!=4

This will return number of mutual friends.

As per your data this will give result 1 as for user ID 1 and 4 there is only one mutual friend 2.

Answer 2

Select friendid
FROM userlist as F
WHERE userid = 1

Select friendid
FROM userlist as S
WHERE userid = 4

Select Distinct *
From F Join S
ON F.friendid = S.friendid

Would give you only the mutual friends of 1 and 4.

Answer 3

SELECT Distinct a.friendid as mutual_friend
FROM user_list AS a
JOIN user_list b ON a.friendid = b.friendid
WHERE (a.userid =1 or a.friendid=1)
AND (b.userid =2 or b.friendid=2) 

It would give you the Mutual friends id

Answer 4

I made a test Table upon your query and made up a query which will show ids of two friends and no of mutual friends between them.

    SELECT
    a.ID as Person1, b.ID as Person2, COUNT(a.FriendID) as NoOfMutualFriends 
    FROM
    Friends a INNER JOIN Friends b 
    ON a.FriendID = b.FriendID 
    WHERE a.ID <> b.ID     //other filters can be used here
    GROUP BY a.ID, b.ID 
    ORDER BY a.ID 

Ask if any Doubts ;)