Which customer(s) (show the customer(s), have the highest order sum and which have the lowest order sum. I should also show the product in the order.

My code:

Select name, order_sum, product_name from customer c, belong b, order o, in_order i, product p WHERE order_sum=(select min(order_sum), max(order_sum) from order) AND p.banr=i.banr AND i.benr=o.benr AND o.benr=b.benr AND b.knr=c.knr;

The result of the code above:

operand should contain i column(s)

When I only use max() function i get the right result of max.
How should I use both max and min function in the code?

The question i will answer is: Which product sales best and how much?
For answer this question I need to use 2 rows; Product_name and order_amount.
I think the best way to solve the question is with sum() function and function for max().

I´d tried this code:
Select product_name, sum(order_amount) from Product p, IN_Order i WHERE order_amount = (select max(order_amount from in_order) AND i.banr=p.banr;

Results: Products sold most at one time. This is not the right answar.
What should I do to answer my question?

I´d tried this code with SUM() and Group by():
Select product_name, sum(order_amount) from Product p, IN_Order i WHERE i.banr=p.banr group by product_name;

With this code a get a list of all product with each order amount. I just want to get the product with the highest order amount