show the names of all different amenities.
select amenity_name from dorm_amenity
find the total number of each amenity in all of the dorms.
select count(*), t2.amenid from dorm_amenity as t1 join has_amenity as t2 on t1.amenid = t2.amenid group by t2.amenid
which one is the most common amenity used in all dorms? List its name.
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