大数据-子查询

cd /d D:\MyWork\MySQL\mysql-5.7.28-winx64\bin rem 目的是为了执行mysql.exe
mysql -h127.0.0.1 -P3306 -uroot -p123456 rem 作用是为了让客户端能够正确的连接服务器
mysql --host=127.0.0.1 --port=3306 --user=root --password=123456 world

-- 查看服务器中有哪些数据库
show databases;

-- 创建新的数据库
create database company;

-- 切换工作数据库
use company;

-- 查看当前数据库中的表
show tables;

-- 导入.sql文件中的数据
source d:/company.sql;

-- 查看表中的数据
select * from employees;

-- 练习 : 创建world数据库, 并切换, 再导入world.sql文件中的数据到world库.
create database world;

use world;

source d:/world.sql;

-- 丢弃数据库
drop database company;

drop database world;

-- 查看当前工作数据库
select database();

-- 查看当前工作数据库的版本
select version();

-- 查看服务器所有参数
show variables;

-- 和字符集设置相关的参数 : 
character_set_client                                     | gbk
character_set_connection                                 | gbk
character_set_database                                   | utf8
character_set_filesystem                                 | binary
character_set_results                                    | gbk
character_set_server                                     | utf8
character_set_system                                     | utf8

-- 如果client,connection和result的设置不是gbk, 必须执行以下语句

-- 修改客户端编码
set names gbk;

数据库结构
    mysqld 服务器
        数据库1(目录形式)
            表1
                数据(记录)1
                数据(记录)2
                数据(记录)3
                ....
            表2 
            表3
            ....
            
        数据库2
        ......
        
-- 查看表中数据
select * from 表名; 

-- 查看表结构
describe employees;
desc employees;

+----------------+--------------+------+-----+---------+----------------+
| Field          | Type         | Null | Key | Default | Extra          |
+----------------+--------------+------+-----+---------+----------------+
| employee_id    | int(6)       | NO   | PRI | NULL    | auto_increment |
| first_name     | varchar(20)  | YES  |     | NULL    |                |
| last_name      | varchar(25)  | YES  |     | NULL    |                |
| email          | varchar(25)  | YES  |     | NULL    |                |
| phone_number   | varchar(20)  | YES  |     | NULL    |                |
| job_id         | varchar(10)  | YES  | MUL | NULL    |                |
| salary         | double(10,2) | YES  |     | NULL    |                |
| commission_pct | double(4,2)  | YES  |     | NULL    |                |
| manager_id     | int(6)       | YES  |     | NULL    |                |
| department_id  | int(4)       | YES  | MUL | NULL    |                |
+----------------+--------------+------+-----+---------+----------------+

create table customer(
    id int,
    name varchar(20),
    age int,
    phone varchar(15),
    birthday date
);

insert into customer (
    id,
    name,
    age,
    phone,
    birthday
) values (
    1,
    '张三',
    30,
    '134234234',
    '1992-5-8'
);

insert into customer (
    id,
    name,
    age,
    phone,
    birthday
) values (
    2,
    '李四',
    40,
    '1599234234',
    '1991-5-8'
);

insert into customer (
    id,
    name,
    age,
    phone,
    birthday
) values (
    3,
    '王五',
    50,
    '135234234',
    '1995-11-8'
);

-- 更新记录(修改数据)
-- 如果更新时不加where过滤, 会导致所有数据被修改
update customer set 
    name = '某人',
    age = 10;
    
update customer set 
    name = '张三',
    age = 30
where 
    id = 1;
    
-- 删除记录
-- 如果没有where过滤, 会全部删除.
delete from customer;

delete from customer 
where id = 2;


DML 数据操纵语言.
insert C
select R 
update U
delete D

+----------------+---------------------------------------------------------------------------------------+------+-----+---------+-------+
| Field          | Type                                                                                  | Null | Key | Default | Extra |
+----------------+---------------------------------------------------------------------------------------+------+-----+---------+-------+
| Code           | char(3)   国家代码                                                                            | NO   | PRI |         |       |
| Name           | char(52)  国家名称                                                                    | NO   |     |         |       |
| Continent      | enum('Asia','Europe','North America','Africa','Oceania','Antarctica','South America') | NO   |     | Asia    |       |
| Region         | char(26)          地区                                                                    | NO   |     |         |       |
| SurfaceArea    | float(10,2)       国土面积m                                                                    | NO   |     | 0.00    |       |
| IndepYear      | smallint(6)       独立年                                                                   | YES  |     | NULL    |       |
| Population     | int(11)           国家人口                                                              | NO   |     | 0       |       |
| LifeExpectancy | float(3,1)        平均寿命                                                                    | YES  |     | NULL    |       |
| GNP            | float(10,2)       国民生产总值                                                                    | YES  |     | NULL    |       |
| GNPOld         | float(10,2)                                                                           | YES  |     | NULL    |       |
| LocalName      | char(45)                                                                              | NO   |     |         |       |
| GovernmentForm | char(45)          政府组织                                                                    | NO   |     |         |       |
| HeadOfState    | char(60)          领导人                                                                    | YES  |     | NULL    |       |
| Capital        | int(11)           首都                                                                    | YES  |     | NULL    |       |
| Code2          | char(2)                                                                                   | NO   |     |         |       |
+----------------+---------------------------------------------------------------------------------------+------+-----+---------+-------+
city
+-------------+----------+------+-----+---------+----------------+
| Field       | Type     | Null | Key | Default | Extra          |
+-------------+----------+------+-----+---------+----------------+
| ID          | int(11)  | NO   | PRI | NULL    | auto_increment |
| Name        | char(35) | NO   |     |         |                |
| CountryCode | char(3)  | NO   | MUL |         |                |
| District    | char(20) | NO   |     |         |                |
| Population  | int(11)  | NO   |     | 0       |                |
+-------------+----------+------+-----+---------+----------------+
SELECT *
FROM   departments;

SELECT department_id, location_id
FROM   departments;

select 
    code,
    name,
    continent
from 
    country;
    
select 
    continent,
    name,
    code
from 
    country;

SQL注意点 :     
    SQL 语言大小写不敏感。 
    SQL 可以写在一行或者多行
    关键字不能被缩写也不能分行
    各子句一般要分行写。
    使用缩进提高语句的可读性。
    
-- 查询国家的人口,名称和代码及首都.
select 
    population,
    name,
    code,
    capital
from 
    country;

-- AS用于给列起别名, AS关键字可以省略
SELECT last_name AS name, commission_pct comm
FROM   employees;

-- 别名可以使用""包围, 目的是让它能原样显示或包含特殊的符号
select 
    code 国家代码,
    name,
    continent as "国家 大洲"
from 
    country;

-- 过滤行 where 条件布尔 : 工作流程就是从基本表中测试每行数据, 都经过条件布尔测试一下, 如果结果为真留下, 为假则丢弃.     
SELECT employee_id, last_name, job_id, department_id
FROM   employees
WHERE  department_id = 90 ;

-- 查询所有亚洲国家
select 
    code,
    population,
    name
from 
    country 
where 
    continent = 'asia';
    
-- 查询所有亚洲国家 , 下面的SQL是错误的, where中不可以使用列的别名, 因为where先执行.
select 
    code,
    population,
    name,
    continent cont
from 
    country 
where 
    cont = 'asia';
    
执行顺序 : from => where => select 

--查询所有中国城市的人口和名称和id号和国家代码, 给国家代码起别名, 尝试在where中使用别名.
select 
    population,
    name,
    id,
    countrycode code 
from 
    city 
where 
    countrycode = 'chn';

-- between a and b --等效于salary >= a && salary <= 3500    
SELECT last_name, salary
FROM   employees
WHERE  salary BETWEEN 2500 AND 3500;

-- id in(a, b, c) -- 等效于id = a || id = b || id = c 
SELECT employee_id, last_name, salary, manager_id
FROM   employees
WHERE  manager_id IN (100, 101, 201);

select 
    code,
    continent,
    name,
    population
from 
    country 
where 
    name like 'china'; -- name = 'china' 如果模糊查询中的字符串没有通配符, 和=效果一样.
    
SELECT    first_name
FROM     employees
WHERE    first_name LIKE 'S%';

% 代表任意个任意字符
_ 代表一个任意字符
-- 查询国家名称中只要包含ch的都行

select 
    code,
    population,
    name,
    continent 
from 
    country 
where 
    name like '%ch%'
    
-- 名字中第3个和第4个是in的国家
select 
    code,
    population,
    name,
    continent 
from 
    country 
where 
    name like '__in%'
    
-- 查询城市表中, 名称的第2个和3个字母是or的城市.
select 
    id,
    name,
    countrycode 
from 
    city 
where 
    name like '_or%';
    
-- 查看哪些国家没有首都
-- null和任意的比较运算, 结果一定是false, 处理null, 必须使用特殊的is来判断
-- null值在进行统计时会被自动忽略.
select 
    code,
    name,
    continent,
    capital
from 
    country 
where 
    capital = null;
    
select 
    code,
    name,
    continent,
    capital
from 
    country 
where 
    capital is null;
    
select 
    code,
    name,
    continent,
    capital
from 
    country 
where 
    capital is not null;

-- 哪些国家尚未独立.
select 
    name,
    code,
    indepYear
from 
    country 
where 
    indepYear is null;


SELECT 
    employee_id, last_name, job_id, salary
FROM   employees
WHERE  
        salary >=10000
    AND    
        job_id LIKE '%MAN%';


SELECT 
    employee_id, last_name, job_id, salary
FROM   employees
WHERE  
        salary >= 10000
    OR     
        job_id LIKE '%MAN%';

-- 查询亚洲国家中人口大于5000万的国家.
select 
    code,
    name,
    continent,
    population
from 
    country 
where 
        continent = 'asia'
    and 
        population > 50000000;

-- 查询中国的城市人口小于20万的城市.
select 
    *
from 
    city 
where 
    countrycode = 'chn'
    and 
    population < 200000;
    
-- 去重 distinct, 后面的列最好是有重复数据的
select 
    distinct 
        continent,
        region
from 
    country;
    
-- 查看中国各有哪些不同的省.
select 
    distinct district
from 
    city 
where 
    countrycode = 'chn';

SELECT   last_name, job_id, department_id, salary
FROM     employees
ORDER BY salary asc;

SELECT   last_name, job_id, department_id, salary
FROM     employees
ORDER BY salary desc;

select 
    code,
    name,
    population pop 
from 
    country 
where 
    continent = 'asia'
order by 
    pop;

order by中可以使用列的别名, 原因是它最后执行

顺序 : from => where => select => order by 


SELECT last_name, department_id, salary
FROM   employees
ORDER BY department_id asc, salary DESC;

-- 查询亚洲人口最少的国家
select 
    code,
    name,
    continent,
    population pop 
from 
    country 
where 
    continent = 'asia'
order by 
    pop desc;

-- 查询山东省人口最多的城市
select 
    id,
    name,
    district,
    population
from 
    city 
where 
        countrycode = 'chn'
    and 
        district = 'shandong'
order by 
    population
    

    
-- 多表联接 : 会产生笛卡尔集
-- 为了学习, 创建简单表
create table city2
select * from city where name='london';
+------+--------+-------------+----------+------------+
| ID   | Name   | CountryCode | District | Population |
+------+--------+-------------+----------+------------+
|  456 | London | GBR         | England  |    7285000 |
| 1820 | London | CAN         | Ontario  |     339917 |
+------+--------+-------------+----------+------------+

create table country2
select * from country where code in('gbr', 'can');
+------+----------------+---------------+-----------------+-------------+-----------+------------+----------------+------------+------------+-------------
| Code | Name           | Continent     | Region          | SurfaceArea | IndepYear | Population | LifeExpectancy | GNP        | GNPOld     | LocalName
+------+----------------+---------------+-----------------+-------------+-----------+------------+----------------+------------+------------+-------------
| CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |           79.4 |  598862.00 |  625626.00 | Canada
| GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |           77.7 | 1378330.00 | 1296830.00 | United Kingd
+------+----------------+---------------+-----------------+-------------+-----------+------------+----------------+------------+------------+-------------

select * from city2, country2; 
结果集中的绝大多数都是垃圾. 必须要过滤掉
+------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+--
| ID   | Name   | CountryCode | District | Population | Code | Name           | Continent     | Region          | SurfaceArea | IndepYear | Population | L
+------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+--
|  456 | London | GBR         | England  |    7285000 | CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |
| 1820 | London | CAN         | Ontario  |     339917 | CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |
|  456 | London | GBR         | England  |    7285000 | GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |
| 1820 | London | CAN         | Ontario  |     339917 | GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |
+------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+--

select 
    * 
from 
    city2, 
    country2
where 
    countrycode = code;
    
+------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+-
| ID   | Name   | CountryCode | District | Population | Code | Name           | Continent     | Region          | SurfaceArea | IndepYear | Population |
+------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+-
| 1820 | London | CAN         | Ontario  |     339917 | CAN  | Canada         | North America | North America   |  9970610.00 |      1867 |   31147000 |
|  456 | London | GBR         | England  |    7285000 | GBR  | United Kingdom | Europe        | British Islands |   242900.00 |      1066 |   59623400 |
+------+--------+-------------+----------+------------+------+----------------+---------------+-----------------+-------------+-----------+------------+-

-- 下面的SQL是错误的, 因为列名模糊
select 
    name,
    population,
    name,
    continent,
    population
from 
    city2, 
    country2
where 
    countrycode = code;
    
select 
    city2.name as cityName,
    city2.population cityPop,
    country2.name countryName,
    country2.continent,
    country2.population countryPop
from 
    city2, 
    country2
where 
    city2.countrycode = country2.code;
    
-- 表名的使用也麻烦了. 给表起别名    
select 
    ci.name as cityName,
    ci.population cityPop,
    co.name countryName,
    co.continent,
    co.population countryPop
from 
    city2 as ci , 
    country2 co 
where 
    ci.countrycode = co.code;

-- 表名一旦指定了别名, 原名就不可以使用了.
select 
    city2.name as cityName,
    city2.population cityPop,
    country2.name countryName,
    country2.continent,
    country2.population countryPop
from 
    city2 as ci, 
    country2 as co 
where 
    city2.countrycode = country2.code;
    
-- 查询所有国家的名称和国家的首都的名称.
select 
    co.name country,
    ci.name captial,
    co.population countryPop,
    ci.population cityPop,
    co.continent 
from 
    country co,
    city ci 
where 
    co.capital = ci.id
    
-- where中有多个条件.
select 
    ci.name as cityName,
    ci.population cityPop,
    co.name countryName,
    co.continent,
    co.population countryPop
from 
    city2 as ci , 
    country2 co 
where 
        ci.countrycode = co.code -- 联接条件, 比普通过滤更重要.
    and 
        ci.population > 1000000
        
+----------+---------+----------------+-----------+------------+
| cityName | cityPop | countryName    | continent | countryPop |
+----------+---------+----------------+-----------+------------+
| London   | 7285000 | United Kingdom | Europe    |   59623400 |
+----------+---------+----------------+-----------+------------+

SQL99标准中 内联接不要用,号来写, 而是使用专门的关键字join.... on 联接条件
select 
    ci.name as cityName,
    ci.population cityPop,
    co.name countryName,
    co.continent,
    co.population countryPop
from 
    city2 as ci 
inner join 
    country2 co 
on 
    ci.countrycode = co.code 
where 
    ci.population > 1000000;
    
-- on 和 where 效果相同, 但是下面的写法不推荐
select 
    ci.name as cityName,
    ci.population cityPop,
    co.name countryName,
    co.continent,
    co.population countryPop
from 
    city2 as ci 
inner join 
    country2 co 
where 
        ci.countrycode = co.code 
    and 
        ci.population > 1000000;
    
-- on 和 where 效果相同, 但是下面的写法不推荐    
select 
    ci.name as cityName,
    ci.population cityPop,
    co.name countryName,
    co.continent,
    co.population countryPop
from 
    city2 as ci 
inner join 
    country2 co 
on 
        ci.countrycode = co.code 
    and 
        ci.population > 1000000;
    
-- 最好的写法是下面的.    inner关键字可以省略
select 
    ci.name as cityName,
    ci.population cityPop,
    co.name countryName,
    co.continent,
    co.population countryPop
from 
    city2 as ci 
join 
    country2 co 
on -- on后面只跟 联接条件 
    ci.countrycode = co.code 
where -- where 后面只跟  普通行过滤
    ci.population > 1000000;
    
-- 查询所有亚洲和欧洲国家的首都, 使用SQL99
select 
    co.name country,
    co.continent,
    ci.name capital
from 
    country co 
join 
    city ci 
on 
    co.capital = ci.id 
where 
    co.continent in('asia', 'europe');
    
-- 查看所有国家名称和首都及官方语言
select 
    co.name country,
    co.continent,
    ci.name capital,
    cl.language
from 
    country co 
join 
    city ci 
on 
    co.capital = ci.id 
join 
    countrylanguage cl 
on 
    cl.countrycode = co.code 
where 
    cl.isofficial = 't';
    
-- 查询简单表的国家及首都
-- 内联接的结果总是所有联接条件为真的记录. 为假的记录全部滤掉.
select 
    co.name country,
    ci.name capital,
    ci.district
from 
    country2 co 
join 
    city2 ci 
on 
    co.capital = ci.id;
    
    
-- 外联接可以保证某张表的数据完整, 即使联接条件为假, 也要保留数据
select 
    co.name country,
    ci.name capital,
    ci.district
from 
    country2 co 
left outer join 
    city2 ci 
on 
    co.capital = ci.id;
    
select 
    co.name country,
    ci.name capital,
    ci.district
from 
    country2 co 
right outer join 
    city2 ci 
on 
    co.capital = ci.id;
    
-- 查看所有国家名称和首都, 即使没有首都也要显示.
select 
    co.name country,
    co.continent,
    ci.name capital
from 
    country co 
left join 
    city ci 
on 
    co.capital = ci.id;

-- 查询哪些国家没有首都
select 
    co.name country,
    co.continent,
    ci.name capital
from 
    country co 
left join 
    city ci 
on 
    co.capital = ci.id
where 
    ci.name is null;
    
// 伪代码
// 内联接
Set leftTable;
Set rightTable;
Set resultSet = new Set();
for (int i = 0; i < leftTable.length; i++) {
    Row leftRow = leftTable[i];
    for (int j = 0; j < rightTable.length; j++) {
        Row rightRow = rightTable[j];
        if (联接条件(leftRow, rightRow)) {
            Row newRow = leftRow + rightRow;
            resultSet.add(newRow);
        }
    }
}

// 外联接 
Set leftTable;
Set rightTable;
Set resultSet = new Set();
for (int i = 0; i < leftTable.length; i++) {
    Row leftRow = leftTable[i];
    boolean flag = false;
    for (int j = 0; j < rightTable.length; j++) {
        Row rightRow = rightTable[j];
        if (联接条件(leftRow, rightRow)) {
            Row newRow = leftRow + rightRow;
            resultSet.add(newRow);
            flag = true; // 表明联接条件为真的记录保存起来
        }
    }
    
    if (!flag) {
        Row newRow = leftRow + 空行(右表);
        resultSet.add(newRow);
    }
}

-- 查询哪些国家没有官方语言
select 
    co.name country,
    cl.language,
    cl.isofficial
from 
    country co 
left join 
    countrylanguage cl 
on 
        co.code = cl.countrycode
    and 
        cl.isofficial = 'T'
where 
    cl.isofficial is null
    
-- 查询所有国家的首都和官方语言, 没有首都和官方语言的也要全部显示.

-- 单行函数, 查询结果中对每一行都单独执行的函数
select
    upper(name),
    lower(continent)
from 
    county;
    
select 
    concat(code, name),
    substr(name, 3, 2)
from 
    country;
    
-- 把国家的代码和名称及大洲用@符号连接. 如 : CHN@China@asia 
select
    concat(concat(concat(concat(code, '@'), name), '@'), continent)
from 
    country;
    
select
    concat(code, '@', name, '@', continent)
from 
    country;


-- 分组函数 : 作用于一组数据. 默认情况下虚表中的所有记录被当作一个组.
分组函数都是统计运算, 特点就是一组数据只能有一个结果. 结果也必须是所有数据处理完后才有的.

max() 最大值 
min() 最小值 
avg() 平均值 
sum() 求和
count() 计数

select 
    max(population),
    min(surfacearea)
from 
    country ;


select 
    -- name, 不可以再把普通的列放在这里
    max(name),
    max(population),
    min(surfacearea)
from 
    country ;

复习 :

C / S 架构
Server : mysqld 
Client : mysql 

客户端连接服务器必须要提供 : 主机地址, 端口号, 用户名, 密码, 默认数据库
mysql --host=127.0.0.1 --port=3306 --user=root --password=123456 world

-- 查看所有数据库
show databases;

-- 查看库中的表
show tables;

-- 查看其他库中的表
show tables from 库名;

-- 查看表结构 
desc 表名;

-- 查看表的建表语句
show create table 表名;

-- 通过建表语句可以看出表的存储引擎, 缺省字符集, 各种约束的名字....

myisam引擎不支持高级特性 : 事务, 外键, 行级锁. 速度极快
innodb引擎支持高级特性, 速度稍慢.

``的作用是专门用于包围数据库对象的名称. 
数据库 
表 
列
索引 
约束
函数
......

C : insert  
R : select 
U : update 
D : delete 
以上语句统称为DML -> 数据操纵语言 


create 
drop 
alter
以上语句统称DDL语句 -> 数据定义语言


跨库操作表 : 只需要在表名上加上库.限定
select * from company.employees;

select 
    *
from 
    country;
    
使用用户变量
set @var1 = 200;
set @var1 = 'abc';

-- 查询没有首都和官方语言的国家
select 
    100, -- 常量 
    'abc',
    20 + 30 / (50 - 3), -- 常量
    @var1, -- 用户变量
    @@datadir, -- 系统变量
    database(), -- 函数
    now(),
    co.*, -- 普通列
    ci.*,
    cl.*
from 
    country co 
left join 
    city ci 
on 
    co.capital = ci.id -- 联接条件
left join 
    countrylanguage cl 
on 
        co.code = cl.countrycode 
    and 
        cl.isofficial = 't'
where 
        cl.language is null
    and 
        ci.name is null
order by 
    co.name desc; 

    
-- 查询亚洲国家的平均人口
select 
    avg(population)
from 
    country 
where 
    continent = 'asia';

-- 查询全球平均寿命最高和最低
select 
    max(LifeExpectancy),
    min(LifeExpectancy)
from 
    country;
    
max() 最大值 
min() 最小值 
avg() 平均值 
sum() 求和
count() 计数

select 
    sum(population),
    count(capital)
from 
    country;
    
-- 查看表中的记录数
select 
    count(*)
from 
    country;
    
-- 解决的问题类似于, 查看 各大洲 的情况.
select 
    continent, -- 群体概念
    max(name),
    avg(surfacearea)
from 
    country 
group by 
    continent;
    
-- 查看各地区的最大人口数
select
    continent,
    region,
    max(population)
from 
    country 
group by 
    continent,
    region;
    
select
    max(continent),
    region,
    max(population)
from 
    country 
group by 
    region
order by 
    max(continent);


-- 查看各国的城市总人口数
select 
    countrycode,
    sum(population)
from 
    city 
group by 
    countrycode 

select
    name,
    population,
    sum(surfacearea)
from 
    country 
group by 
    name,
    population;
    
-- 对分组的结果进行过滤, 只能使用having    
select 
    countrycode,
    sum(population) sumPop
from 
    city 
group by 
    countrycode 
having
    sumPop > 10000000
order by 
    sumPop;

-- 查看亚洲国家各省城市总人口和城市个数中城市总人口数大于100万的.
select 
    co.name,
    ci.district,
    sum(ci.population) sumPop,
    count(ci.id) cities
from 
    country co 
join 
    city ci 
on 
    co.code = ci.countrycode 
where 
    co.continent = 'asia'
group by 
    co.name,
    ci.district
having 
    sumPop > 1000000
order by 
    cities desc;
    
解决SQL步骤 :     
1) 最基础的基表, from 
2) 考虑一张表的数据够不够, 如果不够进行连接 join 
3) 再考虑是外还是内, 如果是外还得考虑保证哪张表完整
4) 有join必须要有on 
5) 是否需要对当前的大基表进行基础的行过滤. where 
6) 是否需要分组, 分组依据的列是什么. group by 
7) 如果有分组,第一时间,把分组的列放在 select 后面, 并同时继续分析要选择哪些列或统计运算.
8) 是否要再对分组结果进行进一步过滤, 如果需要使用having
9) 是否需要排序, 如果需要使用order by .

练习 : 
1 列出所有在超过10个国家中使用的语言。
select
    language,
    count(countrycode) ct
from 
    countrylanguage
group by 
    language 
having 
    ct > 10;
    
2 查询每个大洲各有多少种政府组织形式和每个大洲最富有的收入
select 
    continent,
    count(distinct GovernmentForm),
    max(gnp)
from 
    country 
group by 
    continent;
    
    
3 列出不同的国家(country code)有居民超过7,000,000的城市， 它们有多少？
select 
    countrycode,
    count(*)
from 
    city 
where 
    population > 7000000 
group by 
    countrycode;
    
4 查询中国的每个省的总城市数量和总人口数
-- 能用where解决 不要用having解决.
select 
    district,
    count(*),
    sum(population)
from 
    city 
where 
    countrycode = 'chn'
group by 
    district
order by 
    sum(population) desc;
    

select
    countrycode,
    district,
    count(*),
    sum(population)
from 
    city 
group by 
    countrycode,
    district
having 
    countrycode = 'chn'
order by 
    sum(population) desc;
    
5 Sweden国家说的是什么语言？
select 
    co.name,
    cl.language,
    cl.isofficial
from 
    countrylanguage cl 
join 
    country co 
on 
    cl.countrycode = co.code 
where 
    co.name = 'sweden';
    
6 哪些国家没有列出任何使用语言？
select 
    co.name,
    cl.language
from 
    countrylanguage cl 
right join 
    country co 
on 
    cl.countrycode = co.code 
where 
    cl.language is null;
    
select 
    co.name,
    count(cl.language) ct 
from 
    countrylanguage cl 
right join 
    country co 
on 
    cl.countrycode = co.code 
group by 
    co.name 
having 
    ct = 0;


7 列出在城市表中80%人口居住在城市的国家
select 
    co.name,
    sum(ci.population) / co.population rate
from 
    country co 
join 
    city ci 
on 
    co.code = ci.countrycode 
group by
    co.population,
    co.name 
having 
    rate > 0.8;
    
    
    
    
-- 子查询 : 解决一次查询不能解决的问题.
select 
    name,
    surfacearea,
    continent
from 
    country 
where surfacearea >
                    (select 
                        avg(surfacearea)
                    from 
                        country) 
    
-- 子查询用在比较运算中时, 必须返回一行一列. 下面的SQL是错误的    
select 
    name,
    surfacearea,
    continent
from 
    country 
where surfacearea >
                    (select 
                        avg(surfacearea)
                    from 
                        country
                    group by continent) 
                    
-- 找出哪个城市的人口是最多的.
select 
    countrycode,
    name,
    population
from 
    city  
where     
    population = (select max(population) from city);
    
-- 找出平均寿命最高的和最低的国家.
select 
    continent,
    name,
    lifeExpectancy
from 
    country 
where 
    lifeExpectancy in ( (select max(lifeExpectancy) from country),
                        (select min(lifeExpectancy) from country))
                        
-- 查看哪些国家的人口是大于本大洲平均人口.
select 
    co1.name,
    co1.continent,
    co1.population,
    co2.avgPop 
from 
    country co1
join 
    (select 
        continent,
        avg(population) avgPop
    from 
        country 
    group by 
        continent) co2
on 
    co1.continent = co2.continent
where 
    co1.population > co2.avgPop
order by  
    continent;
    
-- 创建数据库
create database if not exists school charset gbk;

-- 修改数据库, 只能修改默认字符集
alter database school charset utf8;

-- 丢弃数据库
drop database if exists school;

-- 切换数据库
use school;


数据类型
int                4字节整数
bigint            8字节整数
char(长度)        定长字符串, 长度最大255, 在实际插入数据时, 如果长度不够, 会自动补齐.
varchar(长度)    变长字符串, 长度最大65535字节, 在实际插入数据时, 如果长度不够, 实际多少就占多少空间.
longtext        超长文本
double            8字节浮点数
decimal            多字节定点数
date            日期
time            时间
datetime        日期时间
timestamp        时间戳

-- 创建表, 必须至少要有一个列
-- 主键 : 数据必须是非空且唯一.
create table if not exists teacher(
    id int auto_increment, -- 将来这个列对应的数据可以自动生成. 必须作成主键
    name varchar(20), -- varchar数据类型必须要有长度.
    age int,
    salary double,
    gender enum('男', '女') default '男',
    primary key(id) -- 表级主键
) engine innodb charset utf8; -- 

create table if not exists classes(
    id int auto_increment,
    name varchar(20) not null, -- not null 就是非空
    room char(3),
    begindate date,
    master int, --班主任是一个整数, 将来是某个老师的id 
    primary key(id)
);

-- 创建学生表, id, name, age, gender, phone, class_id
create table if not exists student (
    id int auto_increment,
    name varchar(20),
    age int,
    gender enum('男', '女') default '男',
    phone varchar(15),
    class_id int,
    primary key(id)
);

insert into teacher(
    name,
    age,
    salary,
    gender
) values (
    '佟刚',
    40,
    2000,
    '男'
);

insert into teacher(
    id,
    name,
    age,
    salary,
    gender
) values (
    null,
    '芳芳',
    20,
    50000,
    '女'
);


-- 基于子查询建表, 把虚表变成实表
create table country2
as select * from world.country; -- 复制表, 只是机械的把虚表变实, 不能复制约束(主键)

-- 复制表结构建表, 没有数据
create table country3 
like world.country;


+----------+-----------------+------+-----+---------+----------------+
| Field    | Type            | Null | Key | Default | Extra          |
+----------+-----------------+------+-----+---------+----------------+
| id       | int(11)         | NO   | PRI | NULL    | auto_increment |
| name     | varchar(20)     | YES  |     | NULL    |                |
| age      | int(11)         | YES  |     | NULL    |                |
| gender   | enum('男','女') | YES  |     | 男      |                |
| phone    | varchar(15)     | YES  |     | NULL    |                |
| class_id | int(11)         | YES  |     | NULL    |                |
+----------+-----------------+------+-----+---------+----------------+

insert into student(
    name,
    age,
    phone,
    class_id,
    address 
) values(
    '小明',
    20,
    '2342342',
    1,
    '北京昌平'
);

-- 修改表
alter table 表名 
-- 支持若干子名

-- 添加新列
add column 新列名 数据类型(长度) 其他选项.

alter table student 
add column address varchar(200) not null; -- 新列追加在所有列的最后.

alter table student 
add column email varchar(50) after age; -- 在指定列后面添加新列

alter table student 
add column cardNo char(18) first; -- 在最前面添加新列

-- 修改列, 数据类型, 长度, 其他选项 
modify 列名 新数据类型(新长度) 新其他选项

alter table student 
modify name varchar(10) not null;

-- 重命名列
change 列名 新列名 新数据类型(新长度) 新其他选项;

alter table student 
change phone mobile char(11) unique; -- unique就是唯一.

-- 丢弃列
drop column 列名; -- 会导致删除的列所对应的所有数据全部丢失, 无法找回

alter table student
drop column cardNo;

-- 重新命名表
rename to 新表名

alter table student 
rename to students;

alter table teacher 
rename to teachers;

-- 修改约束
以上都是alter table 的子句, 用于修改表
---------------------------------------------------------------

-- 丢弃表 操作很危险, 如果没有数据备份, 就永远找不回来.
drop table 表名;

-- 丢弃表时可以批量丢弃
drop table country2, country3;

-- 清空表 DDL, 表中数据不能恢复. 效率高
truncate table 表名;

delete 是DML, 是可以后悔的. 效率低.


insert into teachers(
    name,
    age,
    salary,
    gender
) values (
    '红康',
    30,
    3000,
    '男'
);

insert into teachers(
    name,
    age,
    salary,
    gender
) value (
    '小柴',
    20,
    5000,
    '女'
);

insert into teachers(
    name,
    age,
    salary,
    gender
) values (
    '贺飞',
    35,
    4000,
    '男'
), (
    '老韩',
    40,
    3000,
    '男'
), (
    '安妮',
    32,
    6000,
    '女'
);

-- 不建议这样的写法. 因为它强烈依赖表的结构,如果表的结构发生变化, 下面的语句就容易失败!
insert into teachers values(
    null,
    '小邱',
    40,
    7000,
    '男'
);

+-----------+-------------+------+-----+---------+----------------+
| Field     | Type        | Null | Key | Default | Extra          |
+-----------+-------------+------+-----+---------+----------------+
| id        | int(11)     | NO   | PRI | NULL    | auto_increment |
| name      | varchar(20) | NO   |     | NULL    |                |
| room      | char(3)     | YES  |     | NULL    |                |
| begindate | date        | YES  |     | NULL    |                |
| master    | int(11)     | YES  |     | NULL    |                |
+-----------+-------------+------+-----+---------+----------------+

insert into classes (
    name,
    room,
    begindate,
    master 
) values (
    'Java0725',
    '305',
    now(), -- 值的部分可以使用函数调用
    2
);

-- 实现表的完全克隆!!!
create table if not exists country2 like world.country;

-- 基于子查询插入数据, 好处就是一次性插入多条记录.
insert into country2 
select * from world.country;


-- 事务 : 一组DML, 在执行时, 要么全部失败, 要么全部成功, 使得数据从一种状态转换为另一种状态. 数据没有丢失!!

A Atomic 原子性             所有操作不可分割.
C Consistence 一致性        数据在事务前和事务后数据是一致的.
I ISOLATION    独立性             不同事务之间具有隔离性
D Duration    持久性            事务一旦提交, 永久生效.

DCL : 数据控制语言 : commit, rollback, grant, revoke...

显式启动事务 : 
    set autocommit = false;
    set @@autocommit = off;
    
    多条DML语句;
    
    commit; 事务提交, 成功
    rollback; 事务回滚, 失败
    ddl语句执行 : 事务提交
    会话正常结束 提交
    异常结束 : 回滚
    
    
    还原设置
    set autocommit = true;
    set @@autocommit = on;