9. Write a SELECT statement that uses an aggregate window function to get the total
amount of each order. Return these columns:
The order_id column from the Order_Items table
The total amount for each order item in the Order_Items table (Hint: You can
calculate the total amount by subtracting the discount amount from the item
price and then multiplying it by the quantity)
The total amount for each order
Sort the result set in ascending sequence by the order_id column.

10. Modify the solution to exercise 9 so the column that contains the total amount for
each order contains a cumulative total by item amount.
Add another column to the SELECT statement that uses an aggregate window
function to get the average item amount for each order.
Modify the SELECT statement so it uses a named window for the two aggregate
functions.