alias 정렬하기
select name, surfacearea, population po, round(surfacearea/population,3) as "1인당 점유면적"
from country
where population >= 100000000
order by `1인당 점유면적`, po;
select name, surfacearea, population, round(surfacearea/population,3) as "1인당 점유면적"
from country
where population >= 100000000
order by 3;