Включете допълнителен брояч в MySQL резултатния набор

Опитайте това:

SET @counter = 0; 
Select sub.*
FROM
(
    select orderid, (@counter := @counter +1) as counter,
      round(sum(unitprice * quantity),2) as value
    from order_details
    group by orderid
) sub
order by 2 desc