--> 测试数据：[tb]
IF OBJECT_ID('[tb]') IS NOT NULL DROP TABLE [tb]
GO
CREATE TABLE [tb]([ID] INT,[billid] INT,[aid] int)
INSERT [tb]
SELECT 1,1000,NULL UNION ALL
SELECT 2,1000,NULL UNION ALL
SELECT 3,1000,NULL UNION ALL
SELECT 4,1001,NULL UNION ALL
SELECT 5,1001,NULL UNION ALL
SELECT 6,1001,NULL UNION ALL
SELECT 7,1001,NULL UNION ALL
SELECT 8,1001,NULL
GO

--> 测试语句：
;with t as
(
SELECT *,row_id=row_number() over(partition by billid order by id) FROM [tb]
)
update t set aid=row_id

select * from tb

/*
ID          billid      aid
----------- ----------- -----------
1           1000        1
2           1000        2
3           1000        3
4           1001        1
5           1001        2
6           1001        3
7           1001        4
8           1001        5

(8 行受影响)

*/

------解决方案--------------------
SQL code
--> 测试数据：[tb]
IF OBJECT_ID('[tb]') IS NOT NULL DROP TABLE [tb]
GO
CREATE TABLE [tb]([ID] INT,[billid] INT,[aid] int)
INSERT [tb]
SELECT 1,1000,NULL UNION ALL
SELECT 2,1000,NULL UNION ALL
SELECT 3,1000,NULL UNION ALL
SELECT 4,1001,NULL UNION ALL
SELECT 5,1001,NULL UNION ALL
SELECT 6,1001,NULL UNION ALL
SELECT 7,1001,NULL UNION ALL
SELECT 8,1001,NULL
GO

--> 2000：

update t set t.aid=(select count(*) from tb where billid=t.billid and id<=t.id)  from tb t

select * from tb

/*
ID          billid      aid
----------- ----------- -----------
1           1000        1
2           1000        2
3           1000        3
4           1001        1
5           1001        2
6           1001        3
7           1001        4
8           1001        5

(8 行受影响)

*/

------解决方案--------------------
用开窗函数
select ID billid,
row_number()over(partition by billid order by id) as aid
from table
--------------------------------------------
ID	billid	aid
1	1000	1
2	1000	2
3	1000	3
4	1001	1
5	1001	2
6	1001	3
7	1001	4
8	1001	5
这个很方便而且灵活，希望对你有所帮助：）