How many drivers are older than 30?
select count(*) from driver where age > 30
How many are older than 40?
select count(*) from driver where age > 40
Show their home cities.
select home_city from driver where age > 40
Among these cities, which one has at least two drivers older than 40?
select home_city from driver where age > 40 group by home_city having count(*) >= 2