SQL combined SELECT statement

SELECT name, continent, population 
FROM world w
WHERE NOT EXISTS (                  -- there are no countries
   SELECT *
   FROM world nx
   WHERE nx.continent = w.continent -- on the same continent
   AND nx.population > 25000000     -- with more than 25M population 
   );

SELECT name, continent, population 
FROM world x 
WHERE 25000000 > ALL(SELECT population 
                     FROM world y 
                     WHERE y.continent = x.continent 
                     )

The 'ALL' part compares the population of all the countries in a continent with 25000000 and if it less than 25000000, it prints names, population of all countries in it.

The following code worked for me:

SELECT name, continent, population FROM world x
  WHERE 25000000>=ALL (SELECT population FROM world y
                         WHERE x.continent=y.continent
                         AND population>0)