7.Find the largest country (by area) in each continent, show the continent, the name and the area:

The above example is known as a correlated or synchronized sub-query.

SELECT 
    c1.continent,
    c1.name,
    c1.area
FROM 
    world c1
JOIN (
    SELECT 
        continent, 
        MAX(area) AS max_area
    FROM 
        world
    WHERE 
        area IS NOT NULL
    GROUP BY 
        continent
) c2
ON 
    c1.continent = c2.continent 
    AND c1.area = c2.max_area;

8.List each continent and the name of the country that comes first alphabetically.

SELECT 
    c1.continent,c1.name
FROM 
    world c1
JOIN (
    SELECT 
        name
    FROM 
        world
    GROUP BY 
        continent

) c2
ON 
    c1.name = c2.name

9.Find the continents where all countries have a population <= 25000000. Then find the names of the countries associated with these continents. Show name, continent and population.

SELECT a.name,a.continent,a.population
  FROM world a 
join  (SELECT continent,max(population) p
                           FROM world
                          WHERE population> 0 group by continent having p<=25000000) b
 on a.continent=b.continent

10.Some countries have populations more than three times that of all of their neighbours (in the same continent). Give the countries and continents.

SELECT c1.name, c1.continent
FROM world c1
WHERE c1.population >= 3 * (
    SELECT max(c2.population)
    FROM world c2
    WHERE c2.continent = c1.continent AND c2.name <> c1.name
);