Get total buffer usage by database

-- Get total buffer usage by database
SELECT DB_NAME(database_id) AS [Database Name] ,
COUNT(*) * 8 / 1024.0 AS [Cached Size (MB)]
FROM sys.dm_os_buffer_descriptors
WHERE database_id > 4 -- exclude system databases
AND database_id <> 32767 -- exclude ResourceDB
GROUP BY DB_NAME(database_id)
ORDER BY [Cached Size (MB)] DESC ;