SQL ZOO练习——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

总结：

1. 注意理解WHERE，GROUP BY和HAVING的不同。