SELECT INInvoiceHeaders.ordno
FROM INInvoiceHeaders
WHERE (MONTH(INInvoiceHeaders.invdate)=11
  AND YEAR(INInvoiceHeaders.invdate)=2016)
UNION DISTINCT
SELECT INArchivedInvoiceHeaders.ordno
FROM INArchivedInvoiceHeaders
WHERE (MONTH(INArchivedInvoiceHeaders.invdate)=11
  AND YEAR(INArchivedInvoiceHeaders.invdate)=2016)
GROUP BY ordno