show the codes of all different cities where students are living.
select distinct city_code from student
find the number of students who are not living in the city with code HKG.
select count(*) from student where city_code != 'HKG'
what are their first and last names?
select fname, lname from student where city_code != 'HKG'
sort the result by their ages.
select fname, lname from student where city_code != 'HKG' order by age