MySQL - NOT IN produce unwanted result

Question 1

Edit:

How about this: (in else clause use an invalid id or the same id as the logged in user)

select * from users where id not in
(
   select (
            case
              when uid = 1 then id
              when fid = 1 then uid
             else 0
            end
          ) from friends where uid = 1 or fid = 1
) and id != 1 order by rand() limit 5;

http://sqlfiddle.com/#!2/12de1/62

One more way to solve this (but might not be an optimal solution):

Union query below might not be a costly query compared to complete join between two tables if number of friends an user has is less than total number of users in system.

Also don't forget to add index on uid and fid column of friends table.

select * from users where id not in 
(
     select id from friends where uid = 1
     union
     select uid from friends where fid = 1
) and id != 1 order by rand() limit 5;

http://sqlfiddle.com/#!2/12de1/40

Question 2

How about this? The null shows where user 4 shouldn't be adding user 4 as a friend.. Infact you can filter it out by And FID is not null. If you want to select to be added friends a particular user as per your session id then you can spcify that in another condition as well :)

Updated adding another join to make sure the names are shown for friends, not for ther user. As well as there's no longer 4 exists.

SQLFIDDLE DEMO

Query:

select x.id, a.name, x.fid
from users a
join (
select u.id, u.name, f.fid
from users u
left join friends f
on u.id <> f.fid
and u.id <> f.uid) x
on x.fid = a.id
;

Results:

| ID | NAME | FID |
-------------------
|  1 |    b |   2 |
|  1 |    e |   5 |
|  1 |    c |   3 |
|  2 |    a |   1 |
|  2 |    e |   5 |
|  2 |    c |   3 |
|  3 |    b |   2 |
|  3 |    a |   1 |
|  3 |    e |   5 |
|  5 |    b |   2 |
|  5 |    a |   1 |
|  5 |    c |   3 |

For a specifi user e.g. 1

SQLFIDDLE DEMO USER 1

select x.id, x.fid, a.name
from users a
join 
(select u.id, f.fid
from users u
inner join friends f
on u.id <> f.fid
and u.id <> f.uid
and u.id = 1)x
on x.fid = a.id
;

| ID | FID | NAME |
-------------------
|  1 |   2 |    b |
|  1 |   3 |    c |
|  1 |   5 |    e |

Question 3

What your SQL query is doing is: Get all users where id is not equal to ANY friend_id and not the current user (I assume).

What you want, is to check that no records exist for the pair of friend_id and user_id in the friend table, so something like:$sql =

"SELECT * FROM user WHERE user.user_id NOT IN 
(SELECT friends.friend_id FROM friends WHERE friend.user_id = user.user_id) AND 
user.user_id <> $_SESSION['id']." ORDER BY RAND() LIMIT 5";

Question 4

Instead of using NOT IN with a subquery, you can try to JOIN the friends table.

I JOINed it twice. Once where the 1 is the user_id and once where 1 is the friend_id. Then I got all users that aren't friends with 1 or are 1.

SELECT user.user_id,user.username

FROM user

LEFT JOIN friends AS fA ON fA.user_id = 1
LEFT JOIN friends AS fB ON fB.friend_id = 1

WHERE user.user_id != 1
AND NOT (user.user_id <=> fB.user_id)
AND NOT (user.user_id <=> fA.friend_id)

DEMO: http://sqlfiddle.com/#!2/1a7ae/30

Question 5

You're missing the where clause in your sub query to limit it to the current users friends that you're excluding.

$sql = "select * from user u 
    where u.user_id not in 
    (select f.friend_id from friends f where f.user_id = ".$_SESSION['id'].")
    and u.user_id <> ".$_SESSION['id']." ORDER BY RAND() LIMIT 5";

I'd also like to recommend not using string concatenation to do this. PDO would be safer, you would have no risk of SQL injection.

http://php.net/manual/en/ref.pdo-mysql.php

Question 6

SELECT u1.id u1_id
     , u2.id u2_id
  FROM users u1
  JOIN users u2
    ON u2.id <> u1.id
  LEFT
  JOIN friends f
    ON (f.fid = u1.id AND f.uid = u2.id)
    OR (f.fid = u2.id AND f.uid = u1.id)
 WHERE f.id IS NULL;