sql sub-queries

Elad, your first answer is almost correct, just missing one very key component:

SELECT x.name, x.continent
  FROM world x
  WHERE x.population >ALL(SELECT population*3
                            FROM world y
                            WHERE y.continent = x.continent
                            AND x.name<>y.name)

You see when you do the sub-query that checks x.population>3*(all of y.populations for same continent) YOU MUST SPECIFY NOT TO CHECK AGAINST THE SAME COUNTRY; otherwise you are stating to check that x>3x which is mathematically impossible.

SELECT name, region 
FROM bbc x 
WHERE population/3 >= ALL
    (SELECT population
     FROM bbc y
     WHERE y.region=x.region
     AND x.name != y.name)