如何构建一个SQL查询来获取用户之间的共同朋友数量？

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.