--> --> (Roy)生成測試數據
 
declare @T table([A] int,[B] int,[C] int)
Insert @T
select 1,2,3 union all
select 1,2,4 union all
select 2,1,1 union all
select 2,1,2 union all
select 2,1,3

;with a
as
(
Select *,ROW_NUMBER()over(order by (select 1)) as d from @T
)
select 
a.A,a.B,a.C,[d]=Case when b.A Is not null then 'Y' else '' end
from a
left join a as b on a.d=b.d+1 and a.a=b.a and a.b=b.b

/*
A    B    C    d
1    2    3    
1    2    4    Y
2    1    1    
2    1    2    Y
2    1    3    Y
*/