SQLZOO练习三--SELECT within SELECT Tutorial

This tutorial looks at how we can use SELECT statements within SELECT statements to perform more complex queries.

name	continent	area	population	gdp
Afghanistan	Asia	652230	25500100	20343000000
Albania	Europe	28748	2831741	12960000000
Algeria	Africa	2381741	37100000	188681000000
Andorra	Europe	468	78115	3712000000
Angola	Africa	1246700	20609294	100990000000
...

1、Bigger than Russia

获取人口比俄罗斯多的国家名字

List each country name where the population is larger than that of 'Russia'.

select name
from world
where population>(select population from world where name='Russia');

2、Richer than UK

获取欧洲国家中，人均gdp高于United Kingdom的国家名称。

Show the countries in Europe with a per capita GDP greater than 'United Kingdom'.

select name from world
where continent='Europe' and gdp/population>(select gdp/population from world where name='United Kingdom');

3、Neighbours of Argentina and Australia

获取国家名称及所在大陆名称，条件是包含阿根廷或者澳大利亚的大陆，按照国家名称升序排序。

List the name and continent of countries in the continents containing either Argentina or Australia. Order by name of the country.

select name,continent
from world
where continent in(select continent from world where name in('Argentina','Australia'))
order by name;

4、Between Canada and Poland

获取人口大于加拿大，同时人口小于波兰的国家名称、人口。

Which country has a population that is more than Canada but less than Poland? Show the name and the population.

select name,population
from world
where population > (select population from world where name ='Canada') and population < (select population from world where name='Poland');

5、Percentages of Germany

获取欧洲各国家名称，以及各国家人口/德国人口的百分比。

Germany (population 80 million) has the largest population of the countries in Europe. Austria (population 8.5 million) has 11% of the population of Germany.

Show the name and the population of each country in Europe. Show the population as a percentage of the population of Germany.

select name,concat(round((population/(select population from world where name='Germany'))*100,0),'%') as percentage
from world
where continent='Europe';

解题思路：️获取德国人口的select语句；️用concat函数，转换成字符串后拼接百分号。

6、Bigger than every country in Europe

获取超过欧洲任何一个国家gdp的国家名字。

Which countries have a GDP greater than every country in Europe? [Give the name only.] (Some countries may have NULL gdp values)

select name
from world
where gdp>(select max(gdp) from world where continent='Europe');

解题思路：超过欧洲任何一个国家gdp，这里用到聚合函数max(gdp)，意思是取欧洲国家gdp的最大值。

7、Largest in each continent

获取每个大洲面积最大的国家，展示所在大陆、国家名称、面积。

Find the largest country (by area) in each continent, show the continent, the name and the area:

SELECT continent, name, area FROM world x
  WHERE area >= ALL
    (SELECT area FROM world y
        WHERE y.continent=x.continent
          AND area>0);

8、First country of each continent (alphabetically)

列出每个大洲以及按字母顺序排在第一位的国家/地区名称。

List each continent and the name of the country that comes first alphabetically. 

select continent,name
from world x
where name=(select min(name) from world y where x.continent=y.continent);

9、Difficult Questions That Utilize Techniques Not Covered In Prior Sections

Find the continents where all countries have a population <= 25000000. Then find the names of the countries associated with these continents. Show name, continent and population.

select name,continent,population
from world
where continent=(select continent from world
group by continent
having max(population)<=25000000);

解题思路：每个大洲全部国家人口都要小于2500万，用聚合函数max（）函数。注意，这时候，不能用where，必须用having，因为where不能和聚合函数一起使用。

10、获取同一大洲，该国家面积大于等于其他所有国家3倍以上的国家名称和大洲名称。

Some countries have populations more than three times that of any of their neighbours (in the same continent). Give the countries and continents.

select name,continent
from world x
where population>=ALL(select population*3 from world y
where x.continent=y.continent and x.name!=y.name and population>0);

解题思路：