title: Need view to calculate difference even if there is no child record
body: <p>The following view is supposed to calc the <em>difference</em> between Total &amp; Amount. But if there is no corresponding payment entered in bill_pay_allocations, the view fails to calculate. How can I get a calc a difference even if there is no corresponding payment?</p>
  
  <pre><code>SELECT
  `alphabase`.`bill_ing_sheets`.`INVOICE_NO` AS `INVOICE_NO`,
  `alphabase`.`bill_ing_sheets`.`TOTAL` AS `TOTAL`,
  `alphabase`.`bill_pay_allocations`.`AMOUNT` AS `AMOUNT`,
  `alphabase`.`bill_pay_allocations`.`AMOUNT` - `alphabase`.`bill_ing_sheets`.`TOTAL` AS `Difference` 
  FROM
  `alphabase`.`bill_ing_sheets` left join `alphabase`.`bill_pay_allocations` 
  ON
  `alphabase`.`bill_pay_allocations`.`BILLING_ID` = `alphabase`.`bill_ing_sheets`.`BILLING_ID` 
  left join `alphabase`.`bill_payments` 
  ON
  `alphabase`.`bill_payments`.`PAYMENT_ID` = `alphabase`.`bill_pay_allocations`.`PAYMENT_ID`
  </code></pre>