show all cities that have an airport.
select city from airports
which of them are in the United States?
select city from airports where country = 'United States'
just show cities that have more than 2 airports.
select city from airports where country = 'United States' group by city having count(*) > 2
how about more than 3?
select city from airports where country = 'United States' group by city having count(*) > 3