mysql中如何按分组添加序号

select  (@i := @i + 1) as rownum,b.*  from    table b, (SELECT  @i := 0) as a    order  by   region_id desc

set @i=0;
select @i:=@i+1 as rownum, b.* from table b order  by   region_id desc

select 
   -- rownum  判断 @now_region_id是否和当前的region_id一样,true:让 @i+=1   false:重置@i
   (@i := case  when  @now_region_id=region_id then @i + 1 else 1 end )  rownum,  
   --  设置 @now_region_id等于region_id
(@now_region_id:=region_id),  
 b.*,
fromtable b, 
(SELECT  @i := 0, @now_region_id:='') as a  
order  by   region_id desc

UPDATE tb_desc SET sort = 0;
-- @C这里想从几开始 就从几开始. @A表示上一行的名称.
SET @C = 0;SET @A = '';
UPDATE tb_desc d
SET d.`sort` = 
(
	SELECT 
		CASE WHEN @A = d.`name`
		THEN @D := @D + 1
	ELSE
		-- 这里(@A := d.`name`) and 0 )的and,只是为了和0进行与运算,不影响和@D相加最后的结果
	     ((@A := d.`name`) and 0 )+@D:= @C
	END
)
ORDER BY d.`name`;

SET @rank:=0;
SELECT * FROM 
(SELECT a.*,
	IF(@tmp=deptno,@rank:=@rank + 1,@rank:=1) AS group_id,
	@tmp:=deptno AS tmp
	FROM employee a  ORDER BY deptno,sal DESC) b
WHERE b.group_id<=5