Try this :
select sum(data) as 'Data Cache Size(MB)' from
(
SELECT count(*)*8/1024 AS 'data',
CASE database_id WHEN 32767 THEN 'RESOURCEDB' ELSE db_name(database_id) END AS 'DatabaseName' FROM sys.dm_os_buffer_descriptors GROUP BY db_name(database_id),database_id ORDER BY 'Data Cache Size(MB)' DESC
) as t
You can add your group by columns to the above.