SQL JOIN, GROUP BY on three tables to get totals

Question

I\'ve inherited the following DB design. Tables are:

customers
---------
customerid  
customernumber

invoices
--------
invoiceid  
amount

invoicepayments
         


        

Answer 1

I am not sure I got you but this might be what you are looking for:

SELECT i.invoiceid, sum(case when i.amount is not null then i.amount else 0 end), sum(case when i.amount is not null then i.amount else 0 end) - sum(case when p.amount is not null then p.amount else 0 end) AS amountdue
FROM invoices i
LEFT JOIN invoicepayments ip ON i.invoiceid = ip.invoiceid
LEFT JOIN payments p ON ip.paymentid = p.paymentid
LEFT JOIN customers c ON p.customerid = c.customerid
WHERE c.customernumber = '100'
GROUP BY i.invoiceid

This would get you the amounts sums in case there are multiple payment rows for each invoice