MySQL database foreign key operation, cascade, changes to the table

1. Foreign key: used to establish the relationship between the two tables
- many
- many to many
- one

Study the relationship between tables and table:
　　1. Define a department employee table
　　　　id, name, gender, dep_name, dep_desc

　　- All data is stored in a table of drawbacks:
　　　　1. The structure is not clear ---> nonlethal
　　　　2. waste of space ---> lethal
　　　　3. Scalability poor ---> disadvantages can not be ignored
　　　　　　- like All py python code stored in a file, a strong coupling together ----> ---- decoupled> split table

　　　　- Split the table to solve the above problems.

　　　　- between two tables need to establish a strong relationship between the use of "foreign keys"

- How !!!!! ****** confirm the relationship between the table and the table is ( one to many, many to many, one to one )
　　- Note: To establish a relationship between two tables, must stand to think in two locations:
　　　　- location of the employees table: multiple employees can correspond to a department? can!!!

　　　　　　- Employees and departments: Many

　　　　　　　　- Many-sector employees to form table

　　　　- standing position of the department table: multiple departments can correspond to an employee? Can not !!!

　　　　Summary: Any way many-to-table relationships, called many of the foreign key relationships.

　　- Foreign key: Syntax: Foreign Key (current relationship table foreign key field) references are associated with the table name (id)

- many

# Create two tables
1. must first establish the association table, then association table

# 被关联表:
dep:
　　create table dep(
　　　　id int primary key auto_increment,
　　　　dep_name varchar(16),
　　　　dep_desc varchar(255)
　　);

# 关联表:
emp:
　　create table emp(
　　　　id int primary key auto_increment,
　　　　name varchar(16),
　　　　age int,
　　　　gender enum('male', 'female', 'others') default 'male',
　　　　dep_id int not null,
　　　　foreign key(dep_id) references dep(id)
　　);

Note: KEY representative of a foreign key in MUL

# Insert data:
- 1 must be inserted in the data relation table (DEP), and then inserted into association table (EMP) data.

# DEP: 
            INSERT INTO DEP (dep_name, dep_desc) values ( ' Nb_ Ministry of Foreign Affairs ' , ' international ambassador sector ' ), 
            ( ' Sb_ Teaching Department ' , ' making programmers department !!!! ' ), 
            ( ' technology ' , ' technology limited sector ' ); 

        # EMP: 
            INSERT INTO EMP (name, Age, Gender, the dep_id) 
            values ( ' Tank ' ,. 17, ' MALE ' ,. 1 ), 
            ( 'jason', 70, 'male', 2),
            ('sean', 50, 'male', 2),
            ('egon', 88, 'male', 2),
            ('owen', 95, 'female', 3);

            # 报错,
            insert into emp(name, age, gender, dep_id) values('大饼', 100, 'others', 999);

- Cascade update and cascade delete (to change along with associated data)

　　- on update cascade
　　- on delete cascade

- Create a table

#被关联表:
    dep2:
        create table dep2(
            id int primary key auto_increment,
            dep_name varchar(16),
            dep_desc varchar(255)
        );
#关联表:
    emp2:
        create table emp2(
            id int primary key auto_increment,
            name varchar(16),
            age int,
            gender enum('male','female','others') default 'male',
            dep_id int not null,
            foreign key(dep_id) references dep2(id)
            on update cascade
            on delete cascade
        );

- insert data

# DEP: 
    INSERT INTO dep2 (dep_name, dep_desc) values ( 'Nb_ Ministry of Foreign Affairs ' , ' international ambassador sector ' ), 
( ' Sb_ Teaching Department ' , ' making programmers department !!!! ' ), ( ' technology ' , ' technology limited sector ' );
 # EMP: 
    INSERT INTO EMP2 (name, Age, Gender, the dep_id) 
    values ( ' Tank ' ,. 17, ' MALE ' ,. 1 ), 
    ( ' Jason ' , 70, ' MALE ',2),
    ('sean',50,'male',2),
    ('egon',88,'male',2),
    ('owen',95,'female',3);

#报错
insert into emp(name,age,gender,dep_id)values('大饼',100,'others',999);

- updating data or deleting data

- update records 
Update dep2 the SET the above mentioned id = 200 the WHERE the above mentioned id = 1 ;

 - delete record 
the Delete from dep2 the WHERE the above mentioned id = 200;

Because cascade, data associated with the change of 200

Note: mysql is not a many, many only

- many to many

We must also stand to think about the position of two tables;

- Demonstration error:

#- 创建book表
    create table book(
        id int primary key auto_increment,
        title varchar(20),
        price int,
        book_content varchar(255),
        author_id int,
        foreign key(author_id) references author(id)
        on update cascade
        on delete cascade
        );

#- 创建author表
    create table author(
        id int primary key auto_increment,
        name varchar(16),
        age int,
        book_id int,
        foreign key(book_id) references book(id)
        on update cascade
        on delete cascade
        );

- 问题: 无法知道哪张表是被关联表

- 利用第三张表，为两张表建立“多对多外键关系”。

#-book:
create table book(
    id int primary key auto_increment,
    title varchar(20),
    price int,
    book_content varchar(255)
);
#-auther:
create table author(
    id int primary key auto_increment,
    name varchar(16),
    age int
);
#-book2author:
create table book2author(
    id int primary key auto_increment,
    book_id int,
    author_id int,
    foreign key(book_id) references book(id)
    on update cascade
    on delete cascade,
    foreign key(author_id) references author(id)
    on update cascade
    on delete cascade    
);

- 插入数据

#- book
insert into book(title, price, book_content) values
('金瓶mei', 199, '讲述朦胧时光的小故事'),
('python从入门到断气', 2000, '学习如何一夜秃头'),
('三体', 200, '跟着大佬进入宇宙奇幻世界')
;

 - author
insert into author(name, age) values
('egon', 68),
('jason', 88);

- book2author：
insert into book2author(book_id, author_id) values
(1, 1),
(1, 2),
(2, 2),
(3, 1);

# 报错, 插入的数据，book_id, author_id必须存在
insert into book2author(book_id,author_id) values(4, 4);

# 更新或删除
#更新
update book set price =666 where id =1;
update book set id=4 where id=1;
#删除
delete from book where id=4;  (会删除关联表内容)

- 一对一:

- user_info:
　　id, name, age, gender, hobby, id_card

- user:
　　id , name, age, detail_id（外键）

- detail:
　　id, gender, hobby, id_card

user与detail表建立了一对一的外键关系。
foreign key 应该建在使用频率较高的一方。

-创建表

#被关联表
create table customer(
    id int primary key auto_increment,
    name varchar(16),
    media varchar(32)
);
#关联表
create table student(
    id int primary key auto_increment,
    addr varchar(255),
    phone_char(11),
    id_card char(18),

    # 外键必须设置成唯一的
    customer_id int unique,
    forign key(customer_id) references customer(id)
    on update cascade
    on delete cascade
);

- 插入数据

insert into customer(name,media) values
('hcy','facebook'),
('zsb1','ig'),
('zsb2','vk'),
('hb','探探');

insert into student(addr,phone,id_card,customer_id) values
 ('上海', '15214546711', '440888888888888888', 1),
 ('北京', '18888888888', '440777777777777777', 2);

  # 报错，一对一，关系必须 一一对应
insert into student(addr, phone, id_card, customer_id) values ('上海', '15214546711', '440888888888888888', 1);

- 插入数据:
insert into

修改表的操作

- 语法：注意: mysql 关键字不区分大小写
1. 修改表名
ALTER TABLE 表名 RENAME 新表名;

2. 增加字段
ALTER TABLE 表名 ADD 字段名数据类型 [完整性约束条件…],ADD 字段名数据类型 [完整性约束条件…]; # 添加到最后一列

ALTER TABLE 表名 ADD 字段名数据类型 [完整性约束条件…] FIRST; # 添加到第一列

ALTER TABLE 表名 ADD 字段名数据类型 [完整性约束条件…] AFTER 字段名; # 添加到某一列之后

3. 删除字段
ALTER TABLE 表名 DROP 字段名;

4. 修改字段
ALTER TABLE 表名 MODIFY 字段名数据类型 [完整性约束条件…]; # 修改数据类型

ALTER TABLE 表名 CHANGE 旧字段名新字段名旧数据类型 [完整性约束条件…]; # 修改字段名，保留字段类型

ALTER TABLE 表名 CHANGE 旧字段名新字段名新数据类型 [完整性约束条件…]; # 修改字段名与字段类型

- 复制表的操作:
复制表结构＋记录（key不会复制: 主键、外键和索引）
mysql> create table new_service select * from service;

只复制表结构
# 将select * from service where 1=2; ---> 不要真实数据，需要表结构
mysql> create table new_customer select * from customer where 1=2;

MySQL database foreign key operation, cascade, changes to the table

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