SELECT  GemSQL.dbo.Invoice_Header.Customer,
        GemSQL.dbo.Invoice_Header.Document_Date,
        GemSQL.dbo.Invoice_Detail.Amount

FROM   GemSQL.dbo.Invoice_Header INNER JOIN
       GemSQL.dbo.Invoice_Detail ON
       GemSQL.dbo.Invoice_Header.[Document] = 
       GemSQL.dbo.Invoice_Detail.[Document]

ORDER BY GemSQL.dbo.Invoice_Header.Customer

SELECT  ih.Customer,
        ih.Document_Date,
        id.Amount
FROM   GemSQL.dbo.Invoice_Header ih INNER JOIN GemSQL.dbo.Invoice_Detail id ON ih.[Document] = id.[Document]
       INNER JOIN (SELECT Customer, MIN(DocumentDate) as EarliestInvoice FROM GemSQL.dbo.Invoice_Header GROUP BY Customer) AS Earliest on Earliest.Customer = ih.Customer
WHERE Earliest.EarliestInvoice > DATEADD(yy,-1,getdate())
ORDER BY GemSQL.dbo.Invoice_Header.Customer