what are the codes of all different cities where students are from?
select distinct city_code from student
how many students are living in city PHL?
select count(*) from student where city_code = 'PHL'
among them, how many are between ages 20 and 25?
select count(*) from student where city_code = 'PHL' and age between 20 and 25
show their first names.
select fname from student where city_code = 'PHL' and age between 20 and 25