SUM and COUNT
注意：where语句中对表示条件的需要用单引号， 下面的译文使用的是有道翻译如有不正确，请直接投诉有道
01.Show the total population of the world.
译文：展示世界总人口。
SELECT SUM(population) FROM world
02.List all the continents - just once each.
译文：列出所有的大陆——每个大陆只列出一次。
SELECT DISTINCT continent FROM world
03.Give the total GDP of Africa
译文：看看非洲的GDP总量
SELECT SUM(gdp) FROM world WHERE continent = 'Africa'
04.How many countries have an area of at least 1000000
译文：有多少国家的面积至少有100万。
SELECT COUNT(name) FROM world WHERE area >= 1000000
05.What is the total population of ('Estonia', 'Latvia', 'Lithuania')
译文：爱沙尼亚、拉脱维亚、立陶宛的总人口是多少?
SELECT SUM(population) FROM world WHERE name IN ('Estonia', 'Latvia', 'Lithuania')
06.For each continent show the continent and number of countries.
译文：表示每个洲的洲数和国家数。
SELECT continent, COUNT(name) FROM world GROUP BY continent
07.For each continent show the continent and number of countries with populations of at least 10 million.
译文：每一洲显示人口至少为1 000万的洲和国家数目。
SELECT continent, COUNT(name) FROM world WHERE population > 10000000 GROUP BY continent
08.List the continents that have a total population of at least 100 million.
译文：列出总人口至少为1亿的大洲。
SELECT continent FROM world GROUP BY continent HAVING SUM(population) >= 100000000
练习网址：https://sqlzoo.net/wiki/SUM_and_COUNT
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