How many people are older than 23?
select count(*) from people where age > 23
How about the number of people younger than 20?
select count(*) from people where age < 20
Please show their hometowns.
select hometown from people where age < 20
How about the hometowns shared by people older than 23 and younger than 20?
select hometown from people where age > 23 intersect select hometown from people where age < 20