--测试数据
declare @T table (id int identity,col varchar(5))
insert into @T
select 'AAA' union all
select 'AAA' union all
select 'BBB' union all
select 'CCC' union all
select 'CCC'

--SQL SERVER 2005/2008
select row_number() over (partition by col order by id) as num,col from @T
/*
num                  col
-------------------- -----
1                    AAA
2                    AAA
1                    BBB
1                    CCC
2                    CCC
*/

--SQL SERVER 2000
select (select count(1) from @T where col=t.col and id<=t.id) as num ,col from @T t
/*
num         col
----------- -----
1           AAA
2           AAA
1           BBB
1           CCC
2           CCC
*/