如何在MySQL的子查询中指定父查询字段？

怎么样：

$query = "SELECt p1.id,       (SELECT COUNT(1)          FROM post_table p2         WHERe p2.parent_id = p1.id) as num_children FROM post_table p1WHERe p1.parent_id = 0";

或者，如果您在p1.id上放置一个别名，您可能会说：

$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 p1WHERe p1.parent_id = 0";