SQL Server Memory used by a database

The query will let you know the database list of the instance and the memory in MB used by the databases.

SELECT      database_id AS DatabaseID,       DB_NAME(database_id) AS DatabaseName,       COUNT(file_id) * 8/1024.0 AS BufferSizeInMB   FROM      sys.dm_os_buffer_descriptors   GROUP BY DB_NAME(database_id),database_id   ORDER BY BufferSizeInMB DESC

The example picture shows the database name and its BufferSizeInMB, this information can helpful while investigating the database performance issue to know how much the memory is used by the database.