/* 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(*)
  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(*)
  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(*)
  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;
