Simplify MySQL query - mysql

Question

I have a MySQL query that I used to use to return mutual friends between two users, but now that I am recoding my website, I am trying to simplify this code, or at least make it better.
So here's my code below to check mutual friends:
SELECT a.friendID
FROM
(SELECT CASE WHEN userID = $id
THEN userID2
ELSE userID
END AS friendID
FROM friends
WHERE (userID = $id OR userID2 = $id)
AND state='1'
) a
JOIN
(SELECT CASE WHEN userID = $session
THEN userID2
ELSE userID
END AS friendID
FROM friends
WHERE (userID = $session OR userID2 = $session)
AND state='1'
) b
ON b.friendID = a.friendID
My table is set up like this:
userID -- userID2 -- state
1 ------- 2 ------- 1
2 ------- 3 ------- 1
1 ------- 3 ------- 0
(sorry, I don't know how to do the pretty database structure design, so if someone could edit that for me...)
but for the above, when user 1 is on user 3's profile, since user 1 and user 2 are friends, and user 2 and user 3 are friends, but user 1 and user 3 are not, it should return user 2 as a mutual friend. (state 1 means friendship accepted, state 0 means friendship pending, so only if state is 1 should it be counted as a friend)
Also note that userID and userID2 can be in any order, it depends on who requests who as a friend, so like the above query does, I need to also have the "friendID" returned, as the above query does right.

Answer 1

SELECT t.f FROM friends JOIN (
SELECT IF(userID = $id, userID2, userID) f
FROM friends
WHERE state AND $id IN (userID, userID2)
) t ON t.f IN (userID, userID2)
WHERE state AND $session IN (userID, userID2)
See it on sqlfiddle.

Answer 2

Not necessarily the fastest but simple to read:
Create View ActiveFriends As
Select
UserId, UserID2
From
friends
Where
State = '1'
Union All
Select
UserId2, UserID
From
friends
Where
State = '1'
Select
f1.userID2 as MutualFriendId
From
ActiveFriends f1
Inner Join
ActiveFriends f2
On f1.UserID2 = f2.UserID
Where
f1.UserID = $session And
f2.UserID2 = $id
http://sqlfiddle.com/#!2/5748f/1/0

Answer 3

You could do :
SELECT userID2
FROM friends_table
WHERE userID IN ( $id, $session )
AND state = 1
GROUP BY userID2
HAVING COUNT(userID2) >= 2
friends_table :
user_id | friend_id | state
-----------------------------
1 | 7 | 1
2 | 3 | 0
7 | 1 | 1
User 1 and 7 are friends, user 2 wants to be friends with user 3, but user 3 hasn't responded yet.

Answer 4

at the first glance I would say that using intersect is a good solution
so this query should do the trick for you
SELECT userID2
FROM friends
WHERE userID = $fromID
INTERSECT
SELECT userID2
FROM friends
WHERE userID = $theOtherID
EDIT:
another less clear solution would be
SELECT userID2
FROM friends INNER JOIN friends f2
USING (userID2)
WHERE friends.UserID = $fromID AND f2.userID = $theOtherID