Rewriting mysql subqueries with LEFT JOIN

I have a table with such structure:

• uid
• sid

all integers. This table simulates friendship between two user.

All I want to do is to find such records that exist only in one direction.

For example: I want to find number of records that have t1.u_id = 15, when there is no records that have t2.s_id = 15 and t2.u_id = t1.s_id.

So that relationship between two ids exists only once.

It seemed to me that I've managed to write the correct query with subqueries, but then I've开发者_运维技巧 started to rewrite the query using left join to increase speed and now...well it's not working.

My query right now is:

SELECT * FROM `friends` AS f1 LEFT JOIN `friends` AS f2 
ON f1.user_id = f2.friend_id 
WHERE f1.friend_id=15 AND f2.user_id IS NULL

but it's not working. Please help.

NB. I'm using mysql 5, so I can't use EXCEPT statement and all other good things.

How about this?

SELECT * 
FROM `friends` AS f1 
    LEFT JOIN `friends` AS f2 ON f1.user_id = f2.friend_id AND f1.friend_id = f2.user_id
WHERE f1.friend_id=15 
    AND f2.user_id IS NULL

And this one ?

SELECT * FROM friends f1 LEFT JOIN friends f2 ON f1.user_id = f2.friend_id WHERE f1.friend_id=15 AND f2.user_id IS NULL