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id, id1, cid
1 null 3
2 null 3
3 null 3
4 null 4 
5 null 8
6 null 8
7 null 6 
8 null 6 

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---

update tb set id1=id from tb where cid='3'
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---

declare @i int
set @I = 0 
update id1= @i,@i = @I +1
where cid = 3

---

SQL code:
--> Éú³É²âÊÔÊý¾Ý±í:tb

IF NOT OBJECT_ID('[tb]') IS NULL
DROP TABLE [tb]
GO
CREATE TABLE [tb]([id] INT,[id1] NVARCHAR(10),[cid] INT)
INSERT [tb]
SELECT 1,NULL,3 UNION ALL
SELECT 2,NULL,3 UNION ALL
SELECT 3,NULL,3 UNION ALL
SELECT 4,NULL,4 UNION ALL
SELECT 5,NULL,8 UNION ALL
SELECT 6,NULL,3 UNION ALL
SELECT 7,NULL,3 UNION ALL
SELECT 8,NULL,6
GO
--SELECT * from [tb]

-->SQL²éѯÈçÏÂ:
update A set
a.id1=(select COUNT(1) from tb where cid=3 and id<=a.id)
from tb a
where cid=3


select * from tb
/*
id id1 cid
----------- ---------- -----------
1 1 3
2 2 3
3 3 3
4 NULL 4
5 NULL 8
6 4 3
7 5 3
8 NULL 6

(8 ÐÐÊÜÓ°Ïì)
*/

{{--