Вот одно

CREATE FUNCTION Test
(@where int)
RETURNS varchar(100)
AS
BEGIN
declare @string varchar(100) set @string = ''
select @string = @string + s + ', ' from tmp where su = @where
RETURN @string
END
GO

create table tmp (su int, s varchar(10))
declare @s varchar(10)
insert into tmp select 1, 'a'
insert into tmp select 1, 'b'
insert into tmp select 2, 'b'
insert into tmp select 2, 'c'
insert into tmp select 2, 'd'
declare @tmp1 table (su1 int, su2 int, string varchar(100))
declare @string varchar(100) set @string = ''
insert into @tmp1 select su, (select sum(su) from tmp where su = t1.su), '' from tmp t1
update @tmp1 set string = dbo.Test(su) from tmp t where t.su = su1
select su2, string from @tmp1 group by su2, string
drop table tmp