SELECT [September 2012 Billing File] = 
			CASE
				WHEN T2.[Application ID] IS NOT NULL 
					AND T2.[ServerName] IS NOT NULL 
					AND SUM([Billed This Month]) <> 0
				THEN 'Y'
				ELSE 'N'
			END
FROM		[AEG_DB].[dbo].[TBL_Work] T1
LEFT JOIN	[TBL_Data] T2
	ON (T1.[Application ID] = T2.[Application ID]
	AND T1.[ServerName] = T2.[ServerName])
WHERE		T2.[Data Period] = 'September 2012'
GROUP BY	T2.[Application ID],
			T2.[ServerName];