Question

I am trying to create a query that will take information out of four tables for a billing system that I am creating. I have the following tables:

Table Invoice

InvoiceID (PK)
ClientID
Date
Status
...

Table Client

ClientID (PK)
ClientName
...

Table InvoiceItem

ItemID (PK)
InvoiceID
Amount
...

Table Payments

PaymentID (PK)
InvoiceID
Amount
...

I need to create a query where I can access information from the Invoice table along with the client name, and the sum of all invoice items and payments associated with the invoice.

I have tried the following:

SELECT 
    Invoice.InvoiceID, 
    Invoice.`Date`, 
    Invoice.Terms, 
    Invoice.DateDue, 
    Invoice.Status, 
    Client.ClinicName, 
    SUM(InvoiceItem.Amount), 
    SUM(Payment.PaymentAmount)
FROM Invoice
JOIN (Client, InvoiceItem, Payment) ON
    (Client.ClientID=Invoice.ClientID AND
     InvoiceItem.InvoiceID=Invoice.InvoiceID AND 
     Payment.InvoiceID=Invoice.InvoiceID)

And while this kind-of works, it is multiplying the SUM() by the number of records used to get the sum (i.e. if there are two payments – 800,400 – It gives me (800+400)*2 — 2400). I am guessing that there is something with how I am using the join, and I have honestly never had to use join for more than one table, and I would always use GROUP BY, but I can’t seem to get that to work correctly.

To make matters worse, I have been lost to the world of vb.net/MSSQL client-side programming for the past several years, so my MySQL is rather rough.

Answer 1

Your problem is that you can’t aggregate over two independent tables at once in a single query. However you can do it using subqueries.

SELECT Invoice.InvoiceID, Invoice.`Date`, Invoice.Terms, Invoice.DateDue, Invoice.Status, Client.ClinicName, InvoiceItemSum.SumOfAmount, PaymentSum.SumOfPaymentAmount
  FROM Invoice
  INNER JOIN Client ON Client.ClientID = Invoice.ClientID
  INNER JOIN (
    SELECT InvoiceID, SUM(Amount) AS SumOfAmount
      FROM InvoiceItem
      GROUP BY InvoiceID
  ) InvoiceItemSum ON InvoiceItemSum.InvoiceID = Invoice.InvoiceID
  INNER JOIN (
    SELECT InvoiceID, SUM(PaymentAmount) AS SumOfPaymentAmount
    FROM Payment
    GROUP BY InvoiceID
  ) PaymentSum ON PaymentSum.InvoiceID = Invoice.InvoiceID