Using multiple JOINS. SUM() producing wrong value

Question

I am getting some basic invoice information in a SQL query and figuring the Order Total and Payment Totals in the same query. Here is what I have thus far:

S         


        

Answer 1

If you run the query without a group by, you'll see that some payments have multiple rows. That's because you're also joining on order items. The result set will contain a row for each combination of orderitem and payment.

One solution would be to change the sum to:

,        
,    (   select SUM(payments.amount) 
         from payments 
         where payments.orderID = orders.id
     ) AS totalPayments
,    

This would ensure the payments with multiple orderitems are not summed multiple times.