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

select orderID, sum(subtotal) as order_total from
(
    select orderID, productID, price, qty, price * qty as subtotal
    from product p inner join orderitem o on p.id = o.productID
    where o.orderID = @orderID
) t
group by orderID