1.List each country name where the population is larger than that of 'Russia'.
world(name, continent, area, population, gdp)
SELECT name FROM world
WHERE population >
(SELECT population FROM world
WHERE name='Romania')
2.Show the names of countries in Europe with a per capita GDP greater than 'United Kingdom'.
Per Capita GDP
select name
from world
where continent ='europe'and gdp/population > (select gdp/population from world
where name = 'United Kingdom') ;
3.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 ('South America','Oceania')
order by name;
4.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.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.
Decimal places
Percent symbol %
SELECT name,
CONCAT(
ROUND(100*population/(SELECT population FROM world WHERE name='Germany')), '%')
FROM world
WHERE continent='Europe';
6.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 >= ALL(SELECT gdp FROM world WHERE gdp>0
and continent='Europe');
7.Find the largest country (by area) in each continent, show the continent, the name and the area:
SELECT continent, name, area FROM world AS x
WHERE area >= ALL(SELECT area
FROM world AS y
WHERE y.continent=x.continent
AND area>0)
8.List each continent and the name of the country that comes first alphabetically.
select continent,name
from world as x
where x.name = (select y.name from world as y
where x.continent=y.continent
order by name
limit 1)
9.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 as x
where 25000000>all(select population from world as y
where x.continent=y.continent
and population>0)
10.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 AS x
WHERE population/3 > ALL(SELECT population
FROM world AS y
WHERE x.continent=y.continent
AND x.name <> y.name
AND population > 0)
补充
1.嵌套时先写字句（问题中的那个条件），再写主句。
2.题1：比较条件时，为同一个对象进行比较，例如人口比较人口，人均国内值比较人均国内值。
3.CONCAT允许您将两个或多个字符串粘在一起。此操作是串联的。
例如题5，contact(20,%),显示的结果为20%。
使用cantact和round函数，可以组成百分数 占比的形式。其中round（参数1，参数2）函数中，也可以只使用1个参数。
4.limit（第一个参数，第二个参数）第一个参数为输出的起始位置，第二个参数为偏移变量，偏移多少输出多少，例如limit（2,3）表示从位置2开始输出，输出到位置5的结果。
例如列字段：列名num：字段为12345，SELECT * FROM 表 LIMIT 2, 3 输出结果为3,4,5。
num
1
2
3
4
5
select num from 表
输出结果为
num
3
4
5
