Calculating the SUM of (Quantity*Price) from 2 different tables

Question

I have two tables as follows

PRODUCT table

Id | Name | Price

And an ORDERITEM table

Id | Orde         


        

Answer 1

Use:

  SELECT oi.orderid,
         SUM(oi.quantity * p.price) AS grand_total,
    FROM ORDERITEM oi
    JOIN PRODUCT p ON p.id = oi.productid
   WHERE oi.orderid = @OrderId
GROUP BY oi.orderid

Mind that if either oi.quantity or p.price is null, the SUM will return NULL.