How many apartments are there?
select count ( * ) from apartments
How many different facilities are there?
select count ( * ) from apartment_facilities
Can you list the different facilities?
select * from apartment_facilities
How many apartments have facilities?
select count ( * ) from apartments where apt_id in ( select apt_id from apartment_facilities )
How many apartments have no facilities?
select count ( * ) from apartments where apt_id not in ( select apt_id from apartment_facilities )