UPDATE NOMOR ROWNUMBER()

declare @coba table (nomer int,id_unik int,id_group int, jual money)

insert into @coba
values (null,2,3,100),(null, 3,3,100),(null,4,3,100), (null,5,5,200),(null,6,5,200)
-------------------------
select row_number() over ( partition by id_group order by id_group),* from @coba

 (No column name)    nomer    id_unik    id_group    jual
1    NULL    2    3    100.0000
2    NULL    3    3    100.0000
3    NULL    4    3    100.0000
1    NULL    5    5    200.0000
2    NULL    6    5    200.0000
-----------------------------
UPDATE T
SET T.nomer = TT.ROW_ID
--select *
FROM @coba AS T
INNER JOIN (SELECT ROW_NUMBER() OVER ( partition by id_group order by id_group ) AS ROW_ID, id_unik
            FROM @coba ) AS TT
    ON T.id_unik = TT.id_unik

select row_number() over ( partition by id_group order by id_group),* from @coba



(No column name)    nomer    id_unik    id_group    jual
1    1    2    3    100.0000
2    2    3    3    100.0000
3    3    4    3    100.0000
1    1    5    5    200.0000
2    2    6    5    200.0000
-------------------------------------
update @coba set jual=0 where nomer>1
select row_number() over ( partition by id_unik order by id_unik),* from @coba
1    1    2    3    100.0000
1    2    3    3    0.0000
1    3    4    3    0.0000
1    1    5    5    200.0000
1    2    6    5    0.0000