if not object_id('tb') is null
    drop table tb
Go
Create table tb([A] int,[B] nvarchar(1))
Insert tb
select 1,N'a' union all
select 2,N'b' union all
select 3,N'c'
Go
Select
     distinct  
     stuff((select ','+[B] from tb for xml path('')),1,1,'')
from tb t
/*
a,b,c

*/