Sum(IIf([Date] Between #01/08/2012# And #31/08/2012#,[Value]))
Sum(IIf([Date] Between #01/09/2012# And #30/09/2012#,[Value]))

SELECT

		CASE
		WHEN InvoiceDate between '2012-01-01' and '2012-01-31' 
		THEN SUM (InvoiceValue)
		WHEN InvoiceDate between '2012-02-01' and '2012-02-29' 
		THEN SUM (InvoiceValue)		
		END AS Jan_2012_Value

FROM
tbSalesHistory

GROUP BY
InvoiceValue, InvoiceDate

SELECT SUM(CASE WHEN InvoiceDate BETWEEN '2012-01-01' AND '2012-01-31' THEN InvoiceValue ELSE 0 END),
       SUM(CASE WHEN InvoiceDate BETWEEN '2012-02-01' AND '2012-02-29' THEN InvoiceValue ELSE 0 END)
FROM   tbSalesHistory

-- instead of
BETWEEN '2012-01-01' AND '2012-01-31'
-- use
InvoiceDate >= '20120101' AND InvoiceDate <  '20120201'