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

select 	[September 2012 Billing File] = CASE 	WHEN 	EXISTS
							(
								SELECT	*
								FROM	[TBL_Data] T2
								WHERE	T2.[Application ID]	= T1.[Application ID]
								AND	T2.[ServerName]		= T1.ServerName
							)
                                             	AND 	T1.[Billed This Month] <> 0 
						THEN 	'Y' 
						ELSE 	'N' 
						END
FROM 	[AEG_DB].[dbo].[TBL_Work] T1