MySQL select语句从其他两个表中排除字段

Question

I am trying to execute a query to find all users that are not 'friends' of the current user. For example, if the user is logged in as 'alex', I want to display all users in the users table that are not friends of alex (ie david).

I'm using PHP Codeigniter if that helps. This is the query I came up with (but it returns nothing):

public function getUser($username) {
    $this->db->query("SELECT username FROM users WHERE username NOT IN 
    (SELECT user1 FROM friendships WHERE user2 = '$username' 
    UNION SELECT user2 FROM friendships WHERE user1 = '$username')");
    return $query->result_array();
}

MySQL Tables

Users

+----+----------+
| id | username |
+----+----------+
| 1  | alex     |
+----+----------+
| 2  | james    |
+----+----------+
| 3  | david    |
+----+----------+

Friendships

+----+-------+-------+
| id | user1 | user2 |
+----+-------+-------+
| 1  | alex  | james |
+----+-------+-------+
| 2  | james | david |
+----+-------+-------+

Answer 1

You should've stored the user id in friendship table instead of names.

Try this:

select username
from users
where username <> '$username'
    and username not in (
        select case when user1 = '$username' then user2 else user1 end
        from friendships
        where '$username' in (user1, user2)
        )