Calculate the total quantity of purchased product that has been prepared by employee number 257 and is in pending shipment status.
employee number 257 refers to EmployeeID = 257; pending shipment status refers to Status = 3
SELECT SUM(T2.OrderQty) FROM PurchaseOrderHeader AS T1 INNER JOIN PurchaseOrderDetail AS T2 ON T1.PurchaseOrderID = T2.PurchaseOrderID WHERE T1.Status = 1