create table dbo.testmult (id int, value numeric( 8 , 2 ))
go
create function dbo.mult  (@id int)
returns numeric( 8 , 2 )
as
begin
   declare @mult numeric( 8 , 2 )
   if exists( select id from testmult where id = @id ) begin
        set @mult =  1 . 0 
        declare @value numeric( 8 , 2 )
        declare mult_cursor cursor for
            ( select value from testmult where id = @id )
        open mult_cursor
        fetch next from mult_cursor into @value
        while @@FETCH_STATUS =  0  begin
            set @mult = @mult * @value
            fetch next from mult_cursor into @value
        end
        close mult_cursor
        deallocate mult_cursor
    end
    else
        set @mult =  0 . 0 

    return(@mult)
end
go
insert testmult (id, value)
    values(  1 ,  2 . 5  )
insert testmult (id, value)
    values( 1  ,  1 . 5  )
insert testmult (id, value)
    values( 2  ,  3 . 5  )
insert testmult (id, value)
    values( 3  ,  1 . 7  )
insert testmult (id, value)
    values( 3  ,  1 . 8  )
insert testmult (id, value)
    values( 3  ,  1 . 3  )
go
select distinct id, dbo.mult(id) as mult
    from testmult 
    order by  1 
go

 --- results ---
 
id          mult       
 ----------- ---------- 
 
 1 . 00          3 . 75 
 2 . 00          3 . 50 
 3 . 00          3 . 98 

( 3  row(s) affected)