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create table #pc (idno int,p varchar(8))

insert into #pc values (0,'AA1 BB1')

insert into #pc values (0,'AA1 BB1')

insert into #pc values (0,'AA1 BB1')

insert into #pc values (0,'AA2 BB2')

insert into #pc values (0,'AA2 BB2')

insert into #pc values (0,'AA2 BB2')

insert into #pc values (0,'AA2 BB2')

insert into #pc values (0,'AA2 BB2')

insert into #pc values (0,'AA3 BB3')



declare @idno int,@p varchar(8)

set @idno=0

set @p=''

update #pc set @idno=(case when p<>@p then 1 else @idno+1 end),idno=@idno,@p=p

select * from #pc



idno        p        

----------- -------- 

1           AA1 BB1

2           AA1 BB1

3           AA1 BB1

1           AA2 BB2

2           AA2 BB2

3           AA2 BB2

4           AA2 BB2

5           AA2 BB2

1           AA3 BB3