text: -- 1.
  -- Show the total population of the world.
  -- world(name, continent, area, population, gdp)
  
  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
  