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