create table tab(id int ,cn int ,OPIS varchar(20))

insert into tab(id,cn,OPIS)
select 1,123,'AAA'
union all	
select 2, 000,'eee'
union all	

select 3,123,'BBB'
union all 

select 4,555,'ccc'




create function fn (@cn int)

returns varchar(max)as
begin 

declare @str  varchar(200)

select @str = coalesce(@str+',','')+ opis from tab where cn = @cn

return @str

end 

select distinct cn ,dbo.fn(cn) as  opis  from tab