第2关， https://sqlzoo.net/wiki/SELECT_from_WORLD_Tutorial



-- Observe the result of running this SQL command to show the name, continent and population of all countries.

SELECT name, continent, population FROM world


-- Show the name for the countries that have a population of at least 200 million. 200 million is 200000000, there are eight zeros.
-- 练习where

SELECT name
  FROM world
 WHERE population > 200000000

-- Give the name and the per capita GDP for those countries with a population of at least 200 million.
-- 练习 select 的计算

select name, gdp/population 
from world
where population > 200000000

-- Show the name and population in millions for the countries of the continent 'South America'. Divide the population by 1000000 to get population in millions.
-- 练习 select 计算

select name ,population/1000000
from world 
where continent = 'South America' 


-- Show the name and population for France, Germany, Italy
-- 练习 where in 多行筛选。

select name,population 
from world 
where name in ('France','Germany','Italy')

-- Show the countries which have a name that includes the word 'United'
-- 练习 like 
select name 
from world 
where name like '%United%'

/*Two ways to be big: A country is big if it has an area of more than 3 million sq km or it has a population of more than 250 million. Show the countries that are big by area or big by population. Show name, population and area.*/
-- 练习 where or 
select name
,population
,area
from world 
where area> 3000000 or population > 250000000 

-- Exclusive OR (XOR). Show the countries that are big by area (more than 3 million) or big by population (more than 250 million) but not both. Show name, population and area.
-- 练习 xor
select name
,population
,area
from world
where area> 3000000 xor population > 250000000 


/*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. For South America show population in millions and GDP in billions both to 2 decimal places.*/
-- 练习 round 
select name
,round(population/1000000,2)
,round(gdp/1000000000,2)
from world 
where continent='South America'


/*Show the name and per-capita GDP for those countries with a GDP of at least one trillion (1000000000000; that is 12 zeros). Round this value to the nearest 1000. Show per-capita GDP for the trillion dollar countries to the nearest $1000.*/
-- 练习 round里的数值为负数。

select name
, round(gdp/population,-3)
from world 
where gdp > 1000000000000


-- Show the name and capital where the name and the capital have the same number of characters.
-- 练习 length
select name
,capital 
from world 
where length(name)=length(capital)

/* The capital of Sweden is Stockholm. Both words start with the letter 'S'. Show the name and the capital where the first letters of each match. Don't include countries where the name and the capital are the same word. */
-- 练习 left <>
select name 
,capital 
from world 
where left(name,1) = left(capital,1) and name <> capital

/*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.*/
-- 练习 not like 

select name
from world 
where name not like '% %'
and name like '%a%'
and name like '%e%'
and name like '%i%'
and name like '%o%'
and name like '%u%'