sqlzoo刷题笔记

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前言

sqlzoo：刷题笔记
在线题库：https://sqlzoo.net/wiki/SELECT_from_WORLD_Tutorial
答案参考：https://blog.csdn.net/zhangzhezhu/article/details/101850325
时间：0104-0110

1 SELECT basics

1. Introducing the world table of countries

The example uses a WHERE clause to show the population of ‘France’. Note that strings (pieces of text that are data) should be in ‘single quotes’;

Modify it to show the population of Germany

SELECT population FROM world 
WHERE name = 'Germany'

2. Scandinavia

Checking a list The word IN allows us to check if an item is in a list. The example shows the name and population for the countries ‘Brazil’, ‘Russia’, ‘India’ and ‘China’.

Show the name and the population for ‘Sweden’, ‘Norway’ and ‘Denmark’.

SELECT name, population FROM world
WHERE name IN ('Sweden', 'Norway', 'Denmark')

3. Just the right size

Which countries are not too small and not too big? BETWEEN allows range checking (range specified is inclusive of boundary values). The example below shows countries with an area of 250,000-300,000 sq. km. Modify it to show the country and the area for countries with an area between 200,000 and 250,000.

SELECT name, area FROM world
WHERE area BETWEEN 200000 AND 250000

2 SELECT from world

1. Introduction

Read the notes about this table. Observe the result of running this SQL command to show the name, continent and population of all countries.

SELECT name, continent, population FROM world

2. Large Countries

How to use WHERE to filter records. Show the name for the countries that have a population of at least 200 million. 200 million is 200000000, there are eight zeros.

SELECT name FROM world
WHERE population > 200000000

3. Per capita GDP

Give the name and the per capita GDP for those countries with a population of at least 200 million.

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

4. South America In millions

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.

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

5. France, Germany, Italy

Show the name and population for France, Germany, Italy

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

6. United

Show the countries which have a name that includes the word ‘United’

select name from world where name like '%United%'

7. Two ways to be big

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.

select name,population,area from world 
where area>3000000 or population>250000000

8. One or the other (but not both)

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 area> 3000000 xor population>250000000

9. Rounding

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.

select name,round(population/1000000,2),round(gdp/1000000000,2) from world 
where continent='South America'

10. Trillion dollar economies

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.

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

11. Name and capital have the same length

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.

You can use the LENGTH function to find the number of characters in a string

SELECT name,capital FROM world
WHERE LENGTH(name)=LENGTH(capital)

12. Matching name and 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.
You can use the function LEFT to isolate the first character.
You can use <> as the NOT EQUALS operator.

SELECT name, capital FROM world
where LEFT(name,1)=LEFT(capital,1) and name<>capital

12. All the vowels

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.

You can use the phrase name NOT LIKE ‘%a%’ to exclude characters from your results.

The query shown misses countries like Bahamas and Belarus because they contain at least one ‘a’

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 '% %'

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掌握SQL知识点总结：
一 、SQL基础教程：

1. <、>、=、<>、

2. and、or、xor、 not

3. between ~ and ~

二 、SQL高级教程：

1. SQL IN / NOT IN 语法

   SELECT column_name(s)
   FROM table_name
   WHERE column_name IN (value1,value2,…)

2. SQL LIKE 操作符语法

   SELECT column_name(s)
   FROM table_name
   WHERE column_name LIKE pattern

3. SQL 通配符

   在搜索数据库中的数据时，SQL 通配符可以替代一个或多个字符。
   SQL 通配符必须与 LIKE 运算符一起使用。 在 SQL中，可使用以下通配符：

三、SQL函数

1. ROUND() 函数

   ROUND 函数用于把数值字段舍入为指定的小数位数。
   ——若第二个参数为负数，表示四舍五入的精度位数
   

2. LENGTH() 函数

   // LENGTH函数返回文本字段中值的长度。
   // SQL LENGTH() 语法
   SELECT LENGTH(column_name) FROM table_name

3. LEFT 和 RIGHT

   // LEFT、RIGHT函数返回ARG最左边、右边的LENGTH个字符串，
   // ARG可以是 CHAR或BINARY STRING
   LEFT(ARG,LENGTH)、RIGHT(ARG,LENGTH)

3 SELECT from nobel

1. Winners from 1950

Change the query shown so that it displays Nobel prizes for 1950.

SELECT yr, subject, winner FROM nobel 
WHERE yr = 1950

2. 1962 Literature

Show who won the 1962 prize for Literature.

SELECT winner FROM nobel 
WHERE yr = 1962 AND subject = 'Literature'

3. Albert Einstein

Show the year and subject that won ‘Albert Einstein’ his prize.

select yr,subject from nobel 
where winner='Albert Einstein'

4. Recent Peace Prizes

Give the name of the ‘Peace’ winners since the year 2000, including 2000.

select winner from nobel 
where subject='Peace' and yr>=2000

5. Literature in the 1980’s

Show all details (yr, subject, winner) of the Literature prize winners for 1980 to 1989 inclusive.

select*from nobel 
where subject='Literature' and yr between 1980 and 1989

6. List item

Only Presidents

Show all details of the presidential winners:

Theodore Roosevelt
Woodrow Wilson
Jimmy Carter
Barack Obama

SELECT * FROM nobel 
WHERE winner IN ('Theodore Roosevelt',
                  'Woodrow Wilson',
                  'Jimmy Carter',
                  'Barack Obama')

7. John

Show the winners with first name John

select winner from nobel 
where winner like 'John %'

8. Chemistry and Physics from different years

Show the year, subject, and name of Physics winners for 1980 together with the Chemistry winners for 1984.

select*from nobel 
where (yr=1980 and subject='Physics') 
or (yr=1984 and subject='Chemistry')

9. Exclude Chemists and Medics

Show the year, subject, and name of winners for 1980 excluding Chemistry and Medicine

select*from nobel where yr=1980 
and subject not in ('Chemistry','Medicine')

10. Early Medicine, Late Literature

Show year, subject, and name of people who won a ‘Medicine’ prize in an early year (before 1910, not including 1910) together with winners of a ‘Literature’ prize in a later year (after 2004, including 2004)

select*from nobel 
where (subject='Medicine' and yr<1910) 
or (subject='Literature' and yr>=2004)

11. Umlaut

Find all details of the prize won by PETER GRÜNBERG

select*from nobel 
where winner='PETER GRÜNBERG'

12. Apostrophe

Find all details of the prize won by EUGENE O’NEILL

select*from nobel 
where winner='EUGENE O\'NEILL'

13. Knights of the realm

Knights in order

List the winners, year and subject where the winner starts with Sir. Show the the most recent first, then by name order.

select winner,yr,subject from nobel 
where winner like 'Sir%' order by yr desc,winner

14. Chemistry and Physics last

The expression subject IN (‘Chemistry’,‘Physics’) can be used as a value - it will be 0 or 1.

Show the 1984 winners and subject ordered by subject and winner name; but list Chemistry and Physics last.

SELECT winner, subject FROM nobel 
WHERE yr=1984 
ORDER BY subject IN ('Physics','Chemistry'),subject,winner

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本章总结：
题目14 Chemistry and Physics last

The expression subject IN (‘Chemistry’,‘Physics’) can be used as a value - it will be 0 or 1.
Show the 1984 winners and subject ordered by subject and winner name; but list Chemistry and Physics last.

分析：因为需要将化学和物理排在最后，根据提示“subject IN(‘Chemistry’,‘Physics’)”的结果位0或1，所以可以先利用“subject IN(‘Chemistry’,‘Physics’)”的结果进行升序排序，这样结果位1的化学和物理就会显示在最后。

我的答案：

SELECT winner, subject
  FROM nobel
 WHERE yr=1984
 ORDER BY (subject IN ('Chemistry','Physics')),subject,winner;

掌握SQL知识点总结：

1. ORDER BY 语句

   // ORDER BY 语句用于根据指定的列对结果集进行排序。默认按照升（ASC），降序 DESC 关键字。

   用法：
   SELECT columns FROM talbe
   ORDER BY column [ ASC/DWSC] , …

2. subject in (‘c’,‘p’)会返回0或1，可以作为排序的依据。

4 SELECT in SELECT

1. Bigger than Russia

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

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

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

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

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*100/(select population from world where name='Germany')),'%') from world 
where continent='Europe'

6. Bigger than every country in Europe

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. Largest in each continent

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)

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

SELECT continent, MIN(name) AS name
FROM world
GROUP BY 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 x 
where 25000000>=all(select population from world y where population>0 
and x.continent=y.continent)

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 x 
where population/3>=all(select population from world y where y.continent=x.continent 
and x.name!=y.name)

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本章知识点总结：
（——主要是select嵌套）
(1) concat(数据，‘%’）的使用——给数据加上单位(第5题)
(2) all的使用（使用>all判断时，如果有数据为空null，加个判断 is not null）

5 SUM and COUNT

1. Total world population

Show the total population of the world.

SELECT SUM(population) FROM world

2. List of continents

List all the continents - just once each.

select distinct continent from world

3. GDP of Africa

Give the total GDP of Africa

select sum(gdp) from world 
where continent='Africa'

4. Count the big countries

How many countries have an area of at least 1000000

select count(name) from world 
where area>1000000

5. Baltic states population

What is the total population of (‘Estonia’, ‘Latvia’, ‘Lithuania’)

select sum(population) from world 
where name in ('Estonia', 'Latvia', 'Lithuania')

6. Counting the countries of each continent

For each continent show the continent and number of countries.

select continent,count(name) from world group by continent

7. Counting big countries in each continent

For each continent show the continent and number of countries with populations of at least 10 million.

select continent,count(name) from world 
where population>10000000 group by continent

8. Counting big continents

List the continents that have a total population of at least 100 million.

select continent from world group by continent 
having sum(population)>100000000

6 JOIN

13. List every match with the goals scored by each team as shown. This will use “CASE WHEN” which has not been explained in any previous exercises.

Notice in the query given every goal is listed. If it was a team1 goal then a 1 appears in score1, otherwise there is a 0. You could SUM this column to get a count of the goals scored by team1. Sort your result by mdate, matchid, team1 and team2.

SELECT game.mdate,team1,
sum(CASE WHEN teamid = team1 THEN 1 ELSE 0 END) AS score1,team2,
sum(CASE WHEN teamid = team2 THEN 1 ELSE 0 END) AS score2 
FROM game LEFT JOIN goal  ON(game.id = matchid) 
GROUP BY mdate, matchid, team1, team2

之所以用做连接是因为goal表记录的是进球的记录，因此如果双方都没有进球，那么则不会记录，所以需要左连接game来显示所有的比赛记录而不仅仅是进球记录.

总结

SQL中默认是跳过null？或者说当列筛选跳过null；联合查询时因为其它列得原因，有时候要补null形成一行。

<、>、=、<>、！=

and、or、xor、 in、 not in、like、not like

between a and b
(边界是否包括得看SQL的引擎)

round(column_name,decimals)
函数用于把数值字段舍入为指定的小数位数。
decimals正数表示指定保留位数，为负数表示四舍五入精度

length(column_name)
LENGTH函数返回文本字段中值的长度。

LEFT(ARG,LENGTH)、RIGHT(ARG,LENGTH)
// LEFT、RIGHT函数返回ARG最左边、右边的LENGTH个字符串，
// ARG可以是 CHAR或BINARY STRING

// 强调group by得使用，有时候时双重排序索引——注意其分组得逻辑
ORDER BY column [ ASC/DWSC] , …
ORDER BY 语句用于根据指定的列对结果集进行排序。默认按照升序（ASC），降序 DESC 关键字。

concat(a,b）
将a变为字符串后，添加字符b
eg：concat(10,’%’)——>10%

all的使用
使用>all判断时，如果有数据为空null，加个判断 is not null

sum(column)、count(column)、distinct(column)

注意以下两者的差别：（强调+1）
select * from table
where 条件 --------- （条件限制后，再进行分组）（分组的下面）
group by coulmn

select * from table
group by coulum
having 条件 --------- （分组后，对分的组中进行筛选）（分组这一层）

join得使用
inner join（默认） 内连接（两者的主键意义对应的上的部分）
left join 左连接（以左表为主键为主，作为主表，对不上右表就补null）
right join 右连接（以右表主键为主，作为主表，对不上左表就补null）
full join 完全连接（上面两个主键的综合，每一行对应的列，补不上就null）

CASE WHEN a THEN b ELSE c END
// 使用a的条件进行判断，是就b，不是就c
例如：6-13，例子如下：
“
select teacher.name,
case when dept in (1,2) then ‘Sci’
when dept = 3 then ‘Art’
else ‘None’
end
from teacher
”

null
只能使用is null/ not null —— 为什么不能使用 =，类似java中的数值类型的比较机制！

//这个函数主要用来进行空值处理
COALESCE ( expression,value1,value2……,valuen) 函数
COALESCE()函数的第一个参数expression为待检测的表达式，而其后的参数个数不定。
COALESCE()函数将会返回包括expression在内的所有参数中的第一个非空表达式。
如果expression不为空值则返回expression；否则判断value1是否是空值，
如果value1不为空值则返回value1；否则判断value2是否是空值，
如果value2不为空值则返回value2；……以此类推，
如果所有的表达式都为空值，则返回NULL。

我们将使用COALESCE()函数完成下面的功能，返回人员的“重要日期”：

如果出生日期不为空则将出生日期做为“重要日期”，如果出生日期为空则判断注册日期是否为空，如果注册日期不为空则将注册日期做为“重要日期”，如果注册日期也为空则将“2008年8月8日”做为“重要日期”。实现此功能的SQL语句如下：

SELECT FName,FBirthDay,FRegDay,
COALESCE(FBirthDay,FRegDay,‘2008-08-08’) AS ImportDay
FROM T_Person

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临时表的使用

limit的使用
limit a offset b 从b+1开始获取前a个数据