mysql 显示行号，以及分组排序

CREATE TABLE `my_tb` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`parent_code` varchar(255) DEFAULT NULL,
`code` varchar(255) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=8 DEFAULT CHARSET=utf8;

INSERT INTO `my_tb` (  `parent_code`, `code`) VALUES ('01', '001');
INSERT INTO `my_tb` (  `parent_code`, `code`) VALUES ('01', '002');
INSERT INTO `my_tb` (  `parent_code`, `code`) VALUES ('02', '001');
INSERT INTO `my_tb` (  `parent_code`, `code`) VALUES ('01', '003');
INSERT INTO `my_tb` (  `parent_code`, `code`) VALUES ('02', '002');
INSERT INTO `my_tb` (  `parent_code`, `code`) VALUES ('03', '001');
INSERT INTO `my_tb` (  `parent_code`, `code`) VALUES ('04', '001');

-- 生成 行号
select @r:=@r+1 as row_num , a.* from  my_tb a ,(select @r:=0) b

-- 生成 分组排序号

select
@group_row:=CASE when @parent_code=a.parent_code then  @group_row+1 else 1 end as groupRow,
@parent_code:=a.parent_code as parent_code,
a.code

from  my_tb a ,( select @group_row:=1, @parent_code:='') as b
ORDER BY   a.parent_code , a.code