SQL Zoo 5.SUM and COUNT

以下题目均来自sql zoo 

1.Show the total population of the world.（显示世界总人口）

select sum(population) from world

2.List all the continents - just once each.(列出所有的大洲——每个只列出一个)

select distinct(continent) from world

解：distinct用来查询不重复记录的条数

3.Give the total GDP of Africa.(非洲的GDP总量)

select sum(gdp) from world where continent = 'africa'

4.How many countries have an area of at least 1000000.(列出面积至少有100万的国家)

select count(name) from world where area >=1000000

解：COUNT() 函数返回匹配指定条件的行数

5.What is the total population of ('Estonia', 'Latvia', 'Lithuania')（求出爱沙尼亚、拉脱维亚、立陶宛的总人口）

select sum(population) from world where name in 
('Estonia', 'Latvia', 'Lithuania')

6.For each continent show the continent and number of countries.(对于每个洲，显示洲和国家数量)

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.(对于每个洲，请显示人口至少为1000万的洲和国家数目)

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.(列出人口总数超过1亿的大洲)

select continent from world group by continent 
having sum(population) >=100000000