Can some redundancy in this nested SELECT COUNT query be eliminated?
At least the inner countings are not necessary. You can easily use an EXISTS query, in negative:

SELECT name, continent, population
FROM world x
WHERE NOT EXISTS (
  SELECT 1
  FROM world y
  WHERE y.population &gt; 25000000
  AND x.continent = y.continent
)
ORDER BY continent, population DESC


This is more readable also since it literally says "give me the continents, except if there is a country where population is larger than the limit" and the person reading it doesn't have to spend time wondering what the counts are. Also the DB engine doesn't have to do any counting.

Another way would be to ask for the continents which have smaller countries and exclude those:

SELECT name, continent, population
FROM world x
WHERE continent NOT IN (
  SELECT DISTINCT continent
  FROM world y
  WHERE y.population &gt; 25000000
)
ORDER BY continent, population DESC


This may have better performance since it needs to only once find the continents that don't match and then run the main query as opposed to the previous one checking continents separately.