Question

I have coded up an example of exactly what the MySQL query needs to do, I had tried to combine the two queries but I didn’t have any success with doing so. Basically, what this does is selects all users that $session is not friends with already.

I don’t know how much I need to explain as far as my table structure goes, as you can see what I need to do using my query below, but basically my friends table only has one row for each friendship. (a friendship is only valid if the state is 1 and the CASE statement is necessary to get the friends ID from the friendship, since my table does only have one row for a friendship.

$getUsers = mysql_query("SELECT id FROM users WHERE id!=$session");
$getFriends = mysql_query("SELECT CASE WHEN userID=$session THEN userID2 ELSE userID END AS friendID 
                                    FROM friends 
                                    WHERE (userID=$session OR userID2=$session) 
                                    AND state='1'");

        $usersArray = array();
        $friendsArray = array();

        while($usersC = mysql_fetch_array($getUsers))
        {
            $usersID = $usersC['id'];
            array_push($usersArray, $usersID);
        }
        while($usersF = mysql_fetch_array($getFriends))
        {
            $friendID = $usersF['friendID'];
            array_push($friendsArray, $friendID);   
        }

        print_r(array_merge(array_diff($usersArray, $friendsArray), array_diff($friendsArray, $usersArray)));

Answer 1

You want an anti-join, which you can effect through an outer join and a filter for records where the joined table is NULL:

SELECT users.id
FROM   users LEFT JOIN friends f
    ON (f.userID = $session AND f.userID2 = users.id AND f.state='1')
    OR (f.userID = users.id AND f.userID2 = $session AND f.state='1')
WHERE   f.userID IS NULL    AND f.userID2 IS NULL
   AND users.id <> $session

See it on sqlfiddle.