Try

SELECT SI.AccountCode, MAX(SI.InvoiceDate) AS Latest  
	FROM SalesInvoiceItem IV 
		INNER JOIN SalesInvoice SI ON IV.InvoiceNumber = SI.InvoiceNumber
	WHERE ProductCode='56.GTR003.001.01' AND MAX(SI.InvoiceDate) < DATEADD(MONTH, - 1,GETDATE())
GROUP BY SI.AccountCode

>Goal: To get all accounts that have purchased a specific product more then one month ago but have *not* purchased within the last month.
>
>I've tried the following but it's very slow where a lot of accounts have purchased the product in the last month:

WITH Temp_CTE (AccountCode,InvoiceDate)
>AS
>(
>SELECT SI.AccountCode,MAX(SI.InvoiceDate) FROM SalesInvoiceItem IV 
>INNER JOIN SalesInvoice SI ON IV.InvoiceNumber = SI.InvoiceNumber
>WHERE ProductCode='56.GTR003.001.01' AND SI.InvoiceDate >= DATEADD(MONTH, - 1,GETDATE())
>GROUP BY SI.AccountCode
>)
>SELECT SI.AccountCode,MAX(SI.InvoiceDate) AS Latest  FROM SalesInvoiceItem IV 
>INNER JOINSalesInvoice SI ON IV.InvoiceNumber = SI.InvoiceNumber
>LEFT JOIN Temp_CTE ON SI.AccountCode = Temp_CTE.AccountCode
>WHERE ProductCode='56.GTR003.001.01' AND SI.InvoiceDate < DATEADD(MONTH, - 1,GETDATE()) AND Temp_CTE.AccountCode IS NULL
>GROUP BY SI.AccountCode

>
>I'm useless at SQL - can anyone suggest a way of speeding this up?