MySQL练习题--sqlzoo刷题
首先查看world表的字段：
| name | continent | area | population | gdp | capital | tld | flag |
|---|
SELECT * FROM world；
2、显示人口至少为2亿的国家/地区的名称。2亿=200million
SELECT name FROM world WHERE population >= 200000000;
3、给出人口至少2亿的国家的名称和人均国内生产总值。
select name,(gdp/population) as per_capita_gdp from world where population>=200000000;
4、显示continent ='South America'的国家的名称和人口。 将人口除以100万，以获得数百万人口,也就是population的单位为百万。
select name,(population/1000000) as population from world where continent='South America' ;
5、显示法国，德国，意大利的名称和人口（France、Germany、Italy）
select name,population from world where name in ('France','Germany','Italy')； #注意 #France、Germany等是以字符串形式出现的，加引号，否则会出错
6、显示名称中包含“United”字样的国家/地区
select name from world where name like '%united%';
7、如果一个国家面积超过300万平方公里，或者人口超过2.5亿，那么这个国家就很大。
按人口显示面积大或面积大的国家。 显示名称，人口和面积。
select name,population,area from world where population>250000000 or area>3000000;
8、
Exclusive OR (XOR). Show the countries that are big by area or big by population but not both. Show name, population and area.
- Australia has a big area but a small population, it should be included.
- Indonesia has a big population but a small area, it should be included.
- China has a big population and big area, it should be excluded.
- United Kingdom has a small population and a small area, it should be excluded.
select name,population,area from world where (population<=250000000 and area>3000000) or (population>250000000 and area<3000000);
9、Show the name and population in millions and the GDP in billions for the countries of the continent 'South America'. Use the ROUND function to show the values to two decimal places.
select name,round(population/1000000,2) as population,round(gdp/1000000000,2) as gdp from world where continent='South America';
10、显示GDP至少为1万亿（100亿，即12个零）的国家的名称和人均GDP。 将此值舍入到最接近的1000。
将万亿美元国家的人均GDP显示为最接近的1000美元。
select name,round(gdp/population,-3) as per_capita_gdp from world where gdp>1000000000000 ;
11、Greece has capital Athens.
Each of the strings 'Greece', and 'Athens' has 6 characters.
Show the name and capital where the name and the capital have the same number of characters.
select name,capital from world where length(name)=length(capital);
12、The capital of Sweden is Stockholm. Both words start with the letter 'S'.
select name,capital from world where left(name,1)=left(capital,1) and name!=capital;
13、Equatorial Guinea and Dominican Republic have all of the vowels (a e i o u) in the name. They don't count because they have more than one word in the name.
Find the country that has all the vowels and no spaces in its name.
SELECT name FROM world where name like '%a%' and name like '%e%' and name like '%i%' and name like '%o%' and name like '%u%' and name not like '% %';
虽然运行正确，但是感觉并不对，后面会找更好的答案解决此问题
总结：
1、round函数的使用
链接：http://www.w3school.com.cn/sql/sql_func_round.asp
2、like操作符
3、XOR操作符（第8题）
4、length函数
用法：
length（str）：一个汉字是算三个字符，一个数字或字母算一个字符
char_length: 不管汉字还是数字或者是字母都算是一个字符
5、left
用法：left(str, length)，即：left(被截取字符串， 截取长度)
#用法： left(str, length) #即：left(被截取字符串， 截取长度)
#从右侧截取： right(str, length) #即：right(被截取字符串， 截取长度)
#截取特定长度字符串 substring(str, pos, length) #substring(被截取字符串，从第几位开始截取，截取长度)
