create table test(
  id int identity(1,1) not null
    primary key
, class tinyint not null
, name nvarchar(120) not null )



insert into test values 
(1, N'a'),
(1, N'c'),
(1, N'c'),
(1, N'd'),
(1, N'd'),
(2, N'd'),
(3, N'd'),
(3, N'e'),
(3, N'f')

--For each class, no letter will be repeated
select dbo.ConcatDistinct(name + ' '),class
from test
group by class

drop table test