MySQL Query to find friends and number of mutual friends

Question

I have looked through the questions but I cant find anything that does exactly what I need and I can\'t figure out how to do it myself.

I have 2 tables, a user table

Answer 1

If A is friend of B, then B is also a friend of A? Wouldn't it be better to use just a link instead of two links (and instead of two rows in friends_links)? Then you have to use two status columns, status1 and status2, and A is friend of B only if status1 = status2 = "a".

There are many ways to show mutual friends, e.g.:

SELECT friend_id
FROM friend_links
WHERE friend_links.user_id = $user1 or friend_links.user_id = $user2
  AND NOT (friend_links.friend_id = $user1 or friend_links.friend_id = $user2)
GROUP BY friend_id
HAVING Count(*)>1

And this query shows for each user and anyone who's not his/her friend:

SELECT
  users.user_id,
  users.first_name,
  users_1.user_id,
  users_1.first_name
FROM
  users INNER JOIN users users_1 ON users.user_id <> users_1.user_id
WHERE
  NOT EXISTS (SELECT *
              FROM friend_links
              WHERE
                friend_links.user_id = users.user_id
                AND friend_links.friend_id = users_1.user_id)

(The only think I didn't check is the friendship status, but it's easy to add that check).

I'm still working on it, but it's not easy to combine nicely these two queries togheter. So this isn't exactly an answer, I'm just showing some ideas that i've tried.

But what do you need exactly? A query that returns every user with anyone who's not his/her friend and the number of friends in common, or is the user_id already given?

With some code it's not a problem to answer your question... but there has to be a nice way just by using SQL! :)

EDIT:

I'm still wondering if there's a better solution to this, in particular the next query could be extremely slow, but it looks like this might work:

SELECT
  users_1.user_id,
  users_2.user_id,
  Sum(IF(users_1.user_id = friend_links.user_id AND users_2.user_id = friend_links_1.friend_id, 1, 0)) As CommonFriend
FROM
  users users_1 INNER JOIN users users_2
    ON users_1.user_id <> users_2.user_id,
  (friend_links INNER JOIN friend_links friend_links_1
    ON friend_links.friend_id = friend_links_1.user_id)
GROUP BY
  users_1.user_id,
  users_2.user_id
HAVING
  Sum(IF(users_1.user_id = friend_links.user_id AND users_2.user_id = friend_links.friend_id, 1, 0))=0

(as before, i didn't check friendship status)

If user is given, you could put WHERE users_1.user_id=$user1 but it's better to just leave one user table, and filter the next INNER JOIN whith that user.