>>>Actually, further to last I think all it needs is :

SELECT SI.AccountCode,MAX(SI.InvoiceDate) as LastInvoiceDate FROM LV1_Planglow.LV1_DB.SalesInvoiceItem IV 
>>>INNER JOIN LV1_Planglow.LV1_DB.SalesInvoice SI ON IV.InvoiceNumber = SI.InvoiceNumber
>>>WHERE ProductCode='56.GTR003.001.01' 
>>>GROUP BY SI.AccountCode
>>>HAVING MAX(SI.InvoiceDate) < DATEADD(MONTH, - 1,GETDATE())

>>
>>Yes, exactly. But don't forget to add alias to the ProductCode. Even when the column comes from a particular table, as long as you're using more than 1 table in a query give every single column an alias, so maintenance will become easier later.
>
>Thx.
>BTW I tried using TIMESTAMP instead of GETDATE() and it was actually slower ......

How exactly did you measure the performance?