Stored Function Problem

Hi,

when i run this code, function does not get through; should i use procedure or can this be done within function


set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
go


CREATE function [dbo].[fn_user_product_groups]
(@userid as int) returns varchar (1000)
as
begin
declare @product_groups as varchar(900);
set @product_groups = '';

select @product_groups =

userid
,group1
,group2
,group3
from
(select
userid
,SUM(CASE WHEN groups.id = 6 THEN 1 ELSE 0 END) AS group1
,SUM(CASE WHEN groups.id = 8 THEN 1 ELSE 0 END) AS group2
,SUM(CASE WHEN groups.id = 9 THEN 1 ELSE 0 END) AS group3

from
groups_products as groups
join users as u
on u.userid = groups.userid

where
u.userid = @userid
and condition1 = 1
and condition2 = 2

group by userid) as a

group by

userid
,group1
,group2
,group3

return @product_groups;
end


thank you for help :)