mysql如何查询子节点

CREATE TABLE `menu` (
 `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '菜单id',
 `parent_id` int(11) DEFAULT NULL COMMENT '父节点id',
 `menu_name` varchar(128) DEFAULT NULL COMMENT '菜单名称',
 `menu_url` varchar(128) DEFAULT '' COMMENT '菜单路径',
 `status` tinyint(3) DEFAULT '1' COMMENT '菜单状态 1-有效；0-无效',
 PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=12212 DEFAULT CHARSET=utf8;

INSERT INTO `menu` VALUES ('0', null, '菜单0', ' ', '1');
INSERT INTO `menu` VALUES ('1', '0', '菜单1', '', '1');
INSERT INTO `menu` VALUES ('11', '1', '菜单11', '', '1');
INSERT INTO `menu` VALUES ('12', '1', '菜单12', '', '1');
INSERT INTO `menu` VALUES ('13', '1', '菜单13', '', '1');
INSERT INTO `menu` VALUES ('111', '11', '菜单111', '', '1');
INSERT INTO `menu` VALUES ('121', '12', '菜单121', '', '1');
INSERT INTO `menu` VALUES ('122', '12', '菜单122', '', '1');
INSERT INTO `menu` VALUES ('1221', '122', '菜单1221', '', '1');
INSERT INTO `menu` VALUES ('1222', '122', '菜单1222', '', '1');
INSERT INTO `menu` VALUES ('12211', '1222', '菜单12211', '', '1');

select id from (
 select t1.id,
 if(find_in_set(parent_id, @pids) > 0, @pids := concat(@pids, ',', id), 0) as ischild
 from (
 select id,parent_id from re_menu t where t.status = 1 order by parent_id, id
 ) t1,
 (select @pids := 要查询的菜单节点 id) t2
 ) t3 where ischild != 0