CREATE Procedure CustomerAlsoBought
(
    @ProductID int
)
As

/* We want to take the top 5 products contained in
    the orders where someone has purchased the given Product */
SELECT  TOP 5 
    OrderDetails.ProductID,
    Products.ModelName,
    SUM(OrderDetails.Quantity) as TotalNum

FROM    
    OrderDetails
  INNER JOIN Products ON OrderDetails.ProductID = Products.ProductID

WHERE   OrderID IN 
(
    /* This inner query should retrieve all orders that have contained the productID */
    SELECT DISTINCT OrderID 
    FROM OrderDetails
    WHERE ProductID = @ProductID
)
AND OrderDetails.ProductID != @ProductID 

GROUP BY OrderDetails.ProductID, Products.ModelName 

ORDER BY TotalNum DESC