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

Mutual friends can be found by joining the friend_links table to itself on the friend_id field like so:

SELECT *
FROM friend_links f1 INNER JOIN friend_links f2
  ON f1.friend_id = f2.friend_id
WHERE f1.user_id = $person1
  AND f2.user_id = $person2

But bear in mind that this, in its worst case, is essentially squaring the number of rows in the friend_links table and can pretty easily jack up your server once you have a non-trivial number of rows. A better option would be to use 2 sub-queries for each user and then join the results of those.

SELECT *
FROM (
  SELECT *
  FROM friend_links
  WHERE user_id = $person1
) p1 INNER JOIN (
  SELECT *
  FROM friend_links
  WHERE user_id = $person1
) p2
  ON p1.friend_id = p2.friend_id

Also, you can simplify your friend_links table by removing the surrogate key link_id and just making (user_id,friend_id) the primary key since they must be unique anyway.

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Edit:

How would this be applied to the original query of searching for users that aren't already friends, I would like to do both in a single query if possible?

SELECT f2.user_id, COUNT(*) 'friends_in_common'
FROM friend_links f1 LEFT JOIN friend_links f2
  ON f1.friend_id = f2.friend_id
WHERE f1.user_id = $person
GROUP BY f2.user_id
ORDER BY friends_in_common DESC
LIMIT $number

I am also thinking that the user_id constraints can be moved from the WHERE clause into the JOIN conditions to reduce the size of the data set created by the self-join and preclude the use of subqueries like in my second example.