What is the average annual amount of shipped sales from 1997 to 1998?
from 1997 to 1998 refers to ShippedDate > '1996-1-1' and ShippedDate < '1998-12-31'; average annual amount refers to SUM(MULTIPLY(UnitPrice, Quantity, SUBTRACT(1, Discount)))
SELECT SUM(T2.UnitPrice * T2.Quantity * (1 - T2.Discount)) / 3 FROM Orders AS T1 INNER JOIN `Order Details` AS T2 ON T1.OrderID = T2.OrderID WHERE T1.ShippedDate BETWEEN '1996-01-01 00:00:00' AND '1998-12-31 23:59:59'