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 | OrderId | ProductId | Quantity

What I'm trying to do is, calculate the subtotal price for each product (Quantity*Price) then SUM the TOTAL value for the entire order..

I'm trying something like this

SELECT Id, SUM(Quantity * (select Price from Product where Id = Id)) as qty
FROM OrderItem o
WHERE OrderId = @OrderId

But of course that doesn't work :)

Any help appreciated!

EDIT: I only want to show the grand total for the entire order, so basically the sum of Quantity*Price for every row in OrderItem. Here's some sample data.

Sample Data

TABLE Product

Id     Name            Price  
1      Tomatoes        20.09    
4      Cucumbers       27.72    
5      Oranges         21.13    
6      Lemons          20.05
7      Apples          12.05

Table OrderItem

Id         OrderId        ProductId        Quantity
151        883            1                22
152        883            4                11
153        883            5                8
154        883            6                62

M

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.

Answer 2

i think this - including null value = 0

 SELECT oi.id, 
         SUM(nvl(oi.quantity,0) * nvl(p.price,0)) AS total_qty 
    FROM ORDERITEM oi 
    JOIN PRODUCT p ON p.id = oi.productid 
   WHERE oi.orderid = @OrderId 
GROUP BY oi.id