What are the districts out there?
select district_name from district
Tell me the city area of these districts.
select district_name, city_area from district
Of all these districts, which ones have a below average city area?
select district_name from district where city_area < (select avg(city_area) from district)
Which of these districts has the least city area?
select district_name from district where city_area = (select min(city_area) from district)