Give this a shot.

; with Year2010 AS (

Select

AREA,SRC

,case

when DATE between '1/1/2010' AND getdate() THEN 'YTD 2010'

when DATE between '1/1/2009' AND dateadd(yy,-1,getdate()) THEN 'YTD 2009'

else 'Not Required'end AS PERIOD

,COUNT(distinct ACCOUNT_NO) as AC_Count

fromdbo.Dim1 a

where FILTER_CODE not in ('4','9','unknown')

and DATE between '1/1/2010' AND getdate()

group by

AREA,SRC

,case

when DATE between '1/1/2010' AND getdate() THEN 'YTD 2010'

when DATE between '1/1/2009' AND dateadd(yy,-1,getdate()) THEN 'YTD 2009'

else 'Not Required'end

)

, Year2009 AS (

Select

AREA,SRC

,case

when DATE between '1/1/2010' AND getdate() THEN 'YTD 2010'

when DATE between '1/1/2009' AND dateadd(yy,-1,getdate()) THEN 'YTD 2009'

else 'Not Required'end AS PERIOD

,COUNT(distinct ACCOUNT_NO) as AC_Count

fromdbo.Dim1 a

where FILTER_CODE not in ('4','9','unknown')

and DATE between '1/1/2009' AND dateadd(yy,-1,getdate())

group by

AREA,SRC

,case

when DATE between '1/1/2010' AND getdate() THEN 'YTD 2010'

when DATE between '1/1/2009' AND dateadd(yy,-1,getdate()) THEN 'YTD 2009'

else 'Not Required'end

)

SELECT

Year2010.

,

Year2010.[SRC],

Year2010.[AC_COUNT][AC_COUNT YTD2010],

Year2009.[AC_COUNT][AC_COUNT YTD2009],

Year2010.[AC_COUNT] - Year2009.[AC_COUNT] AS [CountDifference]

FROM Year2010

INNER JOIN Year2009

ON Year2010.AREA = Year2009.AREA

AND Year2010.SRC =Year2009.SRC

--AND Year2010.PERIOD=Year2009.PERIOD