which rooms have reservations ?
select distinct room from reservations
show me the average number of beds of the rooms with reservations.
select avg ( beds ) from rooms where roomid in ( select distinct room from reservations )
how about the rooms that do not have any reservations ?
select avg ( beds ) from rooms where roomid not in ( select distinct room from reservations )
what are the number of rooms that do not have any reservations ?
select count ( * ) from rooms where roomid not in ( select distinct room from reservations )