Hvordan specificerer man det overordnede forespørgselsfelt inde fra en underforespørgsel i MySQL?

Hvad med:

$query = "SELECT p1.id, 
                 (SELECT COUNT(1) 
                    FROM post_table p2 
                   WHERE p2.parent_id = p1.id) as num_children
            FROM post_table p1
           WHERE p1.parent_id = 0";

eller hvis du sætter et alias på p1.id, kan du sige:

$query = "SELECT p1.id as p1_id, 
                 (SELECT COUNT(1) 
                    FROM post_table p2 
                   WHERE p2.parent_id = p1.id) as num_children
            FROM post_table p1
           WHERE p1.parent_id = 0";