MySQL根据父节点id查询所有子节点

<!--根据id查询子节点-->
    <select id="sonId" parameterType="java.lang.String" resultType="com.tl.visual.model.SystemRelate">
        SELECT class_id FROM (
              SELECT t1.class_id,
              IF(FIND_IN_SET(parent_id, @pids) > 0, @pids := CONCAT(@pids, ',', class_id), 0) AS ischild
              FROM (
                   SELECT class_id,parent_id FROM 表名 t WHERE t.state = '01' ORDER BY parent_id, class_id
                  ) t1,
                  (SELECT @pids := #{要查询的id}) t2
             ) t3 WHERE ischild != 0
    </select>