declare @A table(a int,  b varchar(2), c nchar(2))
insert into @A
select 1  ,'sf','t1' union all 
select 2  ,'s','t1' union all 
select 3  ,'dd','t2' union all 
select 4  ,'dd','t3' union all 
select 6  ,'dd','t6' union all 
select 7  ,'dd','t2' union all 
select 8  ,'cc','t1' union all 
select 9  ,'a','t2' union all 
select 10  ,'s','t2' 

select * from @A m where a in
  (select top 2 a from @A where c=m.c)
order by m.c 
a           b    c
----------- ---- ----
1           sf   t1
2           s    t1
3           dd   t2
7           dd   t2
4           dd   t3
6           dd   t6

(6 行受影响)

------解决方案--------------------

C# code
create table tb 
 ( 
 a int, 
 b nvarchar(50), 
 c nvarchar(50), 
 ) 
 insert tb select 1,'sf','t1' 
 insert tb select 2,'s','t1' 
 insert tb select 3,'dd','t2'  
 insert tb select 4,'dd','t3' 
 insert tb select 6,'dd','t6' 
  
  
 select * from tb t where a in 
 (select top 2 a from tb where c=t.c) 
  
 drop table tb 
  
  
 (1 個資料列受到影響) 
  
 (1 個資料列受到影響) 
  
 (1 個資料列受到影響) 
  
 (1 個資料列受到影響) 
  
 (1 個資料列受到影響) 
 a       b                          c 
 ----------- -------------------------------------------------- -------------------------------------------------- 
 1       sf                         t1 
 2       s                          t1 
 3       dd                         t2 
 4       dd                         t3 
 6       dd                         t6 
  
 (5 個資料列受到影響)