Which DMV do I have to join with sys.dm_os_waiting_tasks to get the database ID?
You can try something as follows:
SELECT TOP 100 w.session_id, e.database_id, D.name
FROM sys.dm_os_waiting_tasks w
LEFT JOIN sys.dm_exec_sessions e ON w.session_id = e.session_id
LEFT JOIN sys.databases d ON e.database_id = d.database_id