How to get a list of mutual friends

Hi IM trying to find out how i can get mutual friends

im currently having problems thinking this out.

I have a table called "users" and this is how it looks

id | name
-----------
1   Kenny
2   Jack
3   Jimmy
4   Chris
5   Meg
6   Jake
7   Micheal
8   Dude

I have a table called "friendship" and this is how it looks

user_a | user_b 
----------------
4      开发者_如何学Go   1        
7         5
8         1
2         4
2         1
5         2
1         6
1         7

user_a sends user_b a request to become friends and... BAM there are friends. Now if im user 2, and i go to user 1s friend list, i want to see what friends we have in common. What is the correct sql to do that?

Assuming that it is not possible for User 1 to be friends with User 7 twice, (meaning there can not be a row with user_a = 1, user_b=7 and another row user_a = 7, user_b = 1).

SELECT IF(user_a = 1 OR user_a = 2, user_b, user_a) friend
FROM friendship
WHERE (user_a = 1 OR user_a = 2) OR (user_b = 1 OR user_b = 2)
GROUP BY 1
HAVING COUNT(*) > 1