'friends of friends' SQL query - mysql

Question

This question is related to previous one. You can check my first post here
I'm trying to pull data from a user table and I need 'friends of friends', those who are two steps away from the chosen user but not directly connected to the chosen user
I tried with this query:
select
u.*
from user u
inner join friend f
on u.user_id = f.friend_id
inner join friend ff
on f.user_id = ff.friend_id
where ff.user_id = {$user_id} AND u.user_id <> {$user_id};
I didn't know how to pull users who are not directly connected to the chosen user. I get all friends of friends of the current user, but I also get direct friends of the current user.

Answer 1

You just need to exclude the ones who are direct friends as well as being friends-of-friends. I've rearranged the table aliases so it's a bit clearer (to me, anyway) what's being retrieved:
SELECT
u.*
FROM
user u
INNER JOIN friend ff ON u.user_id = ff.friend_id
INNER JOIN friend f ON ff.user_id = f.friend_id
WHERE
f.user_id = {$user_id}
AND ff.friend_id NOT IN
(SELECT friend_id FROM friend WHERE user_id = {$user_id})
It also removes the need to exclude the user ID being queried.

Answer 2

You are on the right track. You need to add a where condition that excludes those who are directly related:
select u.*
from user u
inner join friend f on u.user_id = f.friend_id
inner join friend ff on f.user_id = ff.friend_id
where ff.user_id = {$user_id} AND u.user_id <> {$user_id};
AND not exists
(select f2.friend_id
from friend f2
where f2.friend_id = ff.friend_id
and u.user_id = f2.user_id)
The extra not exists clause that I added to your query verifies that the second degree friend is also not a first degree friend.

Answer 3

I'd use a non-correlated subquery, which retrieves the ids of the direct friendships of {$user_id}, and exclude these users from the final result:
select u.*
from user u
inner join friend f on u.user_id = f.friend_id
inner join friend ff on f.user_id = ff.friend_id
where ff.user_id = {$user_id} AND u.user_id <> {$user_id}
AND u.user_id not in (
select directFriend.friend_id
from friend directFriend
where directFriend.user_id = {$user_id})