What is the name of the most common amenity in all dorms?
SELECT T1.amenity_name FROM dorm_amenity AS T1 JOIN has_amenity AS T2 ON T1.amenid   =   T2.amenid GROUP BY T2.amenid ORDER BY count ( * )  DESC LIMIT 1
most common amenity = MAX(COUNT(amenity_name)); in all dorms refers to grouping by dorm and counting amenities;