text: -- SUM and COUNT
  
  -- 1) Show the total population of the world.
  SELECT SUM(population) FROM world;
  
  -- 2) List all the continents - just once each.
  SELECT DISTINCT continent FROM world;
  
  -- 3) Give the total GDP of Africa
  SELECT SUM(gdp) FROM world 
    WHERE continent = 'Africa';
  
  -- 4) How many countries have an area of at least 1000000
  SELECT count(name) FROM world 
    WHERE area >=  1000000;
  
  -- 5) What is the total population of ('Estonia', 'Latvia', 'Lithuania')
  SELECT SUM(population) FROM world 
    WHERE name IN ('Estonia', 'Latvia', 'Lithuania')
  
  -- 6) For each continent show the continent and number of countries.
  SELECT continent, COUNT(name) FROM world GROUP BY continent
  
  -- 7) For each continent show the continent and number of countries with populations of at least 10 million.
  SELECT continent, COUNT(name) FROM world 
    WHERE population >= 10000000 GROUP BY continent;
  
  -- 8) List the continents that have a total population of at least 100 million.
  SELECT continent FROM world GROUP BY continent HAVING SUM(population) >= 100000000;
  