sqlzoo练习9-sum-count-quiz

本文中主要是对sum and count进行了一个小测，熟悉SQL中聚合函数的使用

Sum and Count Quiz

练习

1. Select the statement that shows the sum of population of all countries in ‘Europe’

欧洲所有国家的总人口

select sum(population)
from bbc
where region='Europe';

1. Select the statement that shows the number of countries with population smaller than 150000

统计人口小于150000的国家总数

select count(name)
from bbc
where population < 150000;

1. Select the list of core SQL aggregate functions

列出SQL中aggregate函数，返回的是单一结果的函数

AVG(), COUNT(), CONCAT(), FIRST(), LAST(), MAX(), MIN(), SUM()

1. Select the result that would be obtained from the following code，根据代码选择结果

select region, sum(area)
from bbc
where sum(area) > 15000000  -- 错误的写法
group by region;

• area总数大于15000000；（写法错误）
• 根据地区region进行分组

原因：where无法对区域总和进行分组，需要使用having来过滤行

正确写法：

select region, sum(area)
from bbc
group by region
having sum(area) > 15000000;  --使用having进行过滤分组

1. Select the statement that shows the average population of ‘Poland’, ‘Germany’ and ‘Denmark’

求解3个国家的平均人口数

select avg(population)
from bbc
where name in ('Poland', 'Germany', 'Denmark')

1. Select the statement that shows the medium population density of each region

显示每个region的平均人口密度

select region, sum(population)/sum(area) as density
from bbc
group by region;

1. Select the statement that shows the name and population density of the country with the largest population

显示人口最多国家的人口密度

select name, population/area as density
from bbc
where population = (select max(population)
                    from bbc);  -- 子查询中现将最大的人口数的国家选出来

1. Pick the result that would be obtained from the following code

select region, sum(area)
from bbc
group by region
having sum(area) <= 20000000;

• 先求出每个region的人口总数
• 再把人口总数小于等于2000000的过滤掉