Вы используете старое соединение, которое вы можете попробовать
SELECT
OrderItems.orderID,
Items.itemID,
Items.itemDescription,
Items.itemPrice,
OrderItems.quantity,
OrderItems.discountPercentage,
(OrderItems.quantity*Items.itemPrice)*(OrderItems.discountPercentage/100) AS discountPerItem,
(OrderItems.quantity*Items.itemPrice)-((OrderItems.quantity*Items.itemPrice)*(OrderItems.discountPercentage/100)) AS totalOrder
FROM Items, OrderItems
Where Items.ItemID = OrderItems.ItemID;
Но я бы предпочел, чтобы вы делали так.
SELECT
OrderItems.orderID,
Items.itemID,
Items.itemDescription,
Items.itemPrice,
OrderItems.quantity,
OrderItems.discountPercentage,
(OrderItems.quantity*Items.itemPrice)*(OrderItems.discountPercentage/100) AS discountPerItem,
(OrderItems.quantity*Items.itemPrice)-((OrderItems.quantity*Items.itemPrice)*(OrderItems.discountPercentage/100)) AS totalOrder
FROM Items
Inner Join OrderItems on Items.ItemID = OrderItems.ItemsID;