Есть несколько способов получить его.
I.Примените статистическую функцию к каждому столбцу, который вы хотите иметь в своем результате
SELECT Orders.OrderID, MAX(Orders.ShipDate) As ShipDate, MAX(Orders.OrderStatus) As OrderStatus, SUM(Trackingnumbers.Shipment_Cost) AS Shipping_Cost
FROM Orders
INNER JOIN Trackingnumbers
ON Orders.OrderID = TrackingNumbers.OrderID
WHERE Orders.OrderStatus = 'Shipped' AND Orders.ShipDate > (GETDATE()-6) AND Orders.PaymentAmount = Orders.Total_Payment_Received
Group By Orders.OrderID
II.Поместите все поля в предложение GROUP BY
SELECT Orders.OrderID, Orders.ShipDate, Orders.OrderStatus, SUM(Trackingnumbers.Shipment_Cost) AS Shipping_Cost
FROM Orders
INNER JOIN Trackingnumbers
ON Orders.OrderID = TrackingNumbers.OrderID
WHERE Orders.OrderStatus = 'Shipped' AND Orders.ShipDate > (GETDATE()-6) AND Orders.PaymentAmount = Orders.Total_Payment_Received
III.Используйте подзапрос
SELECT *, (SELECT SUM(Trackingnumbers.Shipment_Cost)
FROM Trackingnumbers
WHERE TrackingNumbers.OrderID=Orders.OrderID) AS Shipping_Cost
WHERE Orders.OrderStatus = 'Shipped' AND Orders.ShipDate > (GETDATE()-6) AND Orders.PaymentAmount = Orders.Total_Payment_Received FROM Orders