SQLZOO(SUM and COUNT GROUP BY)答案

Show the total population of the world.

SELECT SUM(population)

FROM world

List all the continents - just once each.

很明显的一个去重，用DISTINCT

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 continent,COUNT(name) FROM world GROUP BY continent

For each continent show the continent and number of countries with populations of at least 10 million.

只要限定条件就好了

SELECT continent,COUNT(name) FROM world WHERE population >= 10000000 GROUP BY continent

List the continents that have a total population of at least 100 million.

SELECT continent

FROM world

GROUP BY continent HAVING SUM(population)>=100000000