01. Select within Select

--01. Bigger than Russia. 
SELECT name FROM world
  WHERE population >
  (SELECT population FROM world
  WHERE name='Russia');
      
--02. Richer than UK
SELECT name FROM world
WHERE continent = 'Europe'
AND gdp / population > (SELECT gdp / population FROM world WHERE name = 'United Kingdom');

/*03. Neighbours of Argentina and Australia*/
SELECT name,continent 
FROM world
WHERE 
continent IN (SELECT continent FROM world
              WHERE name IN ('Australia','Argentina'))
              ORDER BY name;
              
/*04. Between Canada and Poland*/
SELECT name,population
FROM world
WHERE population >(
SELECT population FROM world WHERE name = 'Canada')
AND population <(
SELECT population FROM world WHERE name = 'Poland');

--05. Percentages of Germany
SELECT name,
CONCAT (ROUND(population /(SELECT population
FROM world WHERE name = 'Germany') * 100,0),'%') AS population
FROM world WHERE continent = 'Europe';

--06. Bigger than every country in Europe
SELECT name FROM world
WHERE gdp > ALL (SELECT gdp FROM world WHERE continent = 'Europe' AND gdp > 0);

--07. Largest in each continent
SELECT continent, name, area FROM world x
  WHERE area >= ALL
    (SELECT area FROM world y
        WHERE y.continent=x.continent);
        
--08. First country of each continent (alphabetically)
SELECT continent,name FROM world x
WHERE name <= ALL
(SELECT name FROM world y WHERE x.continent = y.continent)
ORDER BY name;