Question

I have tables below:

user
+-----------------------------------------------+
|  user_id    |  username   | Password | ...    |
+-----------------------------------------------+
|     1       |     a       |  ***     | ...    |
+-----------------------------------------------+
|     2       |     b       |  ***     | ...    |
+-----------------------------------------------+
|     3       |     c       |  ***     | ...    |
+-----------------------------------------------+
|     4       |     d       |  ***     | ...    |
+-----------------------------------------------+
|     5       |     e       |  ***     | ...    |
+-----------------------------------------------+

friends
+-----------------------------------------------+
|  f_id    |  user_id   | friend_id | ...       |
+-----------------------------------------------+
|     1    |     4      |  2        | ...       |
+-----------------------------------------------+
|     2    |     4      |  1        | ...       |
+-----------------------------------------------+
|     3    |     4      |  5        | ...       |
+-----------------------------------------------+
|     4    |     4      |  3        | ...       |
+-----------------------------------------------+

I want to get all users that are available to be added as friends (in this case, the user_id of 1 will have 3 more friends to be added (2, 3, 5). However, by using the following SQL statement below I only get 1 user (4) available to be added:

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

But this works great when I logged in as user 4 which there are no users available to be added. This is a bit tricky to me. Any idea would be very much appreciated.

Thanks

Answer 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