--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  
*/  

来源：http://blog.csdn.net/maco_wang/article/details/7236738