A solution that can be easily understood for question 9

[satishjasthi]

select y.name, y.continent, y.population
from world as y
Join
(select continent,max(population)
from world
group by continent
having max(population) <= 25000000) as x
on y.continent = x.continent

[AdjWang]

Try this one ;-)

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