lef join select

hvordan gør man nedenstående korrekt?

$sql = "SELECT forum.id, tmp.num
    FROM $DB.forum forum
    LEFT JOIN
    (
        SELECT COUNT(forum.id) AS num
        FROM $DB.forum forum
        LEFT JOIN $DB._att_forum att_forum ON att_forum.contr_id=forum.id
        WHERE forum.thread_id=76 && forum.id!=forum.thread_id && COALESCE(att_forum.att_remove, 0)=0
        GROUP BY forum.thread_id
    ) AS tmp";