create function [dbo].fn_inonerow(@p_empid int)
returns nvarchar(4000)
as
begin
 declare @orderids nvarchar(4000)
 select @orderids = coalesce(@orderids + ', ', '') +  rtrim(orderid)
 from                orders a
 inner join           employees b
 on                       b.employeeid=a.employeeid
 where                b.employeeid=@p_empid

 return @orderids
end
go

--Then call the function like this:
select [dbo].fn_inonerow(1) as orderids