My solution used LEFT JOIN with COUNTRY as the left table and COALESCE to handle NULLs, so it actually included all continents as the problem stated. The accepted INNER JOIN solution only shows 5 out of 7 continents, which doesn’t seem like 'all' continents to me.

SELECT CO.CONTINENT, COALESCE(ROUND(AVG(C.POPULATION), 0), 0)
FROM COUNTRY AS CO
LEFT JOIN CITY AS C
    ON C.COUNTRYCODE = CO.CODE
GROUP BY CO.CONTINENT;

SELECT B.CONTINENT,FLOOR(AVG(A.POPULATION)) AS PROMEDIO FROM CITY A INNER JOIN COUNTRY B ON A.COUNTRYCODE = B.CODE GROUP BY B.CONTINENT ORDER BY PROMEDIO DESC;

MS Sql -

select c.continent , round(avg(ci.population),2) from country c inner join city ci on c.code = ci.countrycode group by c.continent;

For MySQL Platform

The given answer is wrong. Actually we need to use the ROUND function since it's mentioned to round down to nearest integer. I used FLOOR function just to match the answer

SELECT country.continent, FLOOR(AVG(city.population)) FROM country
JOIN city ON country.code = city.countrycode
GROUP BY 1;

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