using result of SELECT in another SELECT - sql

Question

I'm trying to do question 9 here:
https://sqlzoo.net/wiki/SELECT_within_SELECT_Tutorial
I currently have the code:
SELECT continent, SUM(y.population) as Population
FROM world AS y
GROUP BY(y.continent)
HAVING SUM(y.population) < 250000000;
This returns the continents with a sum of their respective populations less than 250000000. I know I need to encase this in another select to make use of the continent returned, but don't know how to do this?
I tried something like this:
SELECT A.continent from world A
INNER JOIN(
SELECT B.continent, SUM(B.population) as Population
FROM world B
GROUP BY(B.continent)
HAVING SUM(B.population) < 250000000
) ON A.continent = B.continent;
^This was to try and get a single list of the continents which i could then encase in another select to iterate through and print the country names, although I feel there must be a way to directly iterate through the continent column from the first example?
This is likely something pretty trivial, but regardless any help would be great

Answer 1

there are multiple ways to solve this - i used a count of all countries on the continent = count of countries with population<25000000
your main mistake is in logic - SUM - it should be EACH country, not the summary
select name,w1.continent,population
from world w1
join
(
SELECT distinct continent, count(name) cnt
FROM world x
WHERE population<=25000000
group by continent
)w2
on w1.continent=w2.continent
where cnt=(select count(name) from world where w1.continent=continent)