Setting a clause for cross join (sql) - mysql

Question

I am looking to show a mutual friends function but my user friends list is stored on another table.
I want to know if it's possible to use the data of each row as a clause for join B
SELECT * FROM ( SELECT * FROM `users`) a
CROSS JOIN (SELECT COUNT(*) AS MUTUAL_FRIENDS
FROM `user_friends` WHERE `friend_to` = `a`.`userid`) b
ORDER BY `b`.`MUTUAL_FRIENDS` ASC
What I am looking to do is use the userid from users table as a clause for the join with:
`friend_to` = `a`.`userid`
Expected results:
+--------=+---------+-----------+--------------+
| userid | Username |Photo_URL |MUTUAL_FRIENDS|
+---------+---------+-----------+--------------+
1 Somename1 /image1.png 3
4 Somename4 /image4.png 2
2 Somename2 /image2.png 1
3 Somename3 /image3.png 0
5 Somename5 /image5.png 0
Friends table:
+--------=+---------+-----------+
| friend_id | userid |friend_to |
+---------+---------+-----------+
1 1 2
2 2 1
3 1 3
4 3 1
5 1 4
6 4 1
Users Table
+--------=+---------+-----------+
| userid | Username |Photo_URL |
+---------+---------+-----------+
1 Somename1 /image1.png
2 Somename2 /image2.png
3 Somename3 /image3.png
4 Somename4 /image4.png
5 Somename5 /image5.png
Or am I doing this totaly wrong?

Answer 1

Here is one approach.
With CROSS JOIN
SELECT u.userid,u.Username,u.Photo_URL,IFNULL(Num,0) MUTUAL_FRIENDS
FROM Users u
LEFT JOIN (SELECT userid, COUNT(*) Num
FROM (SELECT a.userid
FROM Friends a
CROSS JOIN Friends b
WHERE a.userid = b.friend_to and a.friend_to = b.userid) r
GROUP BY userid) k ON u.userid = k.userid
ORDER BY MUTUAL_FRIENDS DESC, u.userid
Without CROSS JOIN
SELECT u.userid,u.Username,u.Photo_URL,IFNULL(Num,0) MUTUAL_FRIENDS
FROM Users u
LEFT JOIN (SELECT userid, COUNT(*) Num
FROM (SELECT a.userid
FROM Friends a, Friends b
WHERE a.friend_to = b.userid and b.friend_to = a.userid) r
GROUP BY userid) k ON u.userid = k.userid
ORDER BY MUTUAL_FRIENDS DESC, u.userid
I will try to explain little by little and please correct me if there is error.
The condition a.friend_to = b.userid and b.friend_to = a.userid is
used to select only mutual friend. Then if a user has this kind of
"mutual connection," the corresponding userid will appear once.
Get the count along with the userid.
Left join the userid and count table.
Result
userid Username Photo_URL MUTUAL_FRIENDS
1 Somename1 /image1.png 3
2 Somename2 /image2.png 1
3 Somename3 /image3.png 1
4 Somename4 /image4.png 1
5 Somename5 /image5.png 0
Given the data, user1 is mutual friend of user2, user3 and user4. User2 is only mutual friend of user 1 and so as user 3 and user4.
With CROSS JOIN SQL Fiddle
Without CROSS JOIN SQL Fiddle