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join에 능숙해지는 게 아주 관건일 것 같다는 생각이 든다...

문제

Given the CITY and COUNTRY tables, query the names of all the continents (COUNTRY.Continent) and their respective average city populations (CITY.Population) rounded down to the nearest integer.
Note: CITY.CountryCode and COUNTRY.Code are matching key columns.
Input Format
The CITY and COUNTRY tables are described as follows:

풀이

SELECT COUNTRY.Continent, FLOOR(AVG(CITY.Population)) AS AverageCityPopulation
FROM CITY
JOIN COUNTRY ON CITY.CountryCode = COUNTRY.Code
GROUP BY COUNTRY.Continent;