Show me all the districts.
select district_name from district
What is the average city area of these districts?
select avg(city_area) from district
Which districts have a higher than average city area?
select district_name from district where city_area > (select avg(city_area) from district)
What is the total population of these districts?
select sum(city_population) from district where city_area > (select avg(city_area) from district)