Why do you need foreign keys?

introduce

Think about it , When you are shopping in JD , Is there always such a time ： The receiving address or consignee is always different , You have a shipping address of your own home , There is also a shipping address of the school , There may even be other friends' shipping addresses . So how do we make jd.com fuliuqingfeng The user information and its multiple mailing addresses are saved in the database ?

The initial product

We may write the following code to realize this function at the beginning .

create table user_info(
  id char(36) primary key,
  user_name varchar(30) not null,
  password varchar(30) not null,
  real_name varchar(8),
  mobile char(11),
  address varchar(150)
);
insert into user_info (id,user_name,password,real_name,mobile,address) 
values ('51b28fe1-4ebf-41ac-a17b-d5e276861fd0','fuliuqingfeng','123456',' Zhang San ','18920120206',' Anyang, Henan ');
insert into user_info (id,user_name,password,real_name,mobile,address) 
values ('cc95772b-75a2-4702-bd99-4c3b0322d606','fuliuqingfeng','123456',' Li Si ','18617297545',' Beijing haidian ');
insert into user_info (id,user_name,password,real_name,mobile,address)
values ('c63028fd-cf8d-4dac-a278-b5cc8fd61e3c','fuliuqingfeng','123456',' Wang Wu ','17694976949',' Datong, Shanxi ');

But we found that , If you write like this , Too much code , data redundancy . Time consuming . that , How can we improve ？

Initial improvement

We think of ： We can split them into two tables , Put the same data on one sheet , Put different data on one sheet . Write the code as follows ：

create table user_info(
  id char(36) primary key,
  user_name varchar(30) not null,
  password varchar(30) not null
)
insert into user_info (id,user_name,password) values ('51b28fe1-4ebf-41ac-a17b-d5e276861fd0','fuliuqingfeng','123456');

create table address(
  id char(36) primary key,
  user_info_id char(36),
  real_name varchar(8) not null,
  mobile char(11) not null,
  address varchar(150) not null
)
insert into address (id,user_info_id,real_name,mobile,address) 
values ('bfb9472a-7911-4e6f-a479-3b719454ebab','51b28fe1-4ebf-41ac-a17b-d5e276861fd0',' Zhang San ','18920120206',' Anyang, Henan ');
insert into address (id,user_info_id,real_name,mobile,address) 
values ('5227c6b9-45a2-44aa-8ac0-1f63a38d3b65','51b28fe1-4ebf-41ac-a17b-d5e276861fd0',' Li Si ','18617297545',' Beijing haidian ');
insert into address (id,user_info_id,real_name,mobile,address) 
values ('30b8584b-aa6a-4516-a623-03f487058586','51b28fe1-4ebf-41ac-a17b-d5e276861fd0',' Wang Wu ','17694976949',' Datong, Shanxi ');

But in this case , We found out , If we were user_info perhaps address Add inside / Modifying data , Another table cannot synchronize its information .
for example ：
take user_info in id by 51b28fe1-4ebf-41ac-a17b-d5e276861fd0 Data deletion , But at this time, the data in the address table will no longer be complete
Can't find which user these addresses belong to ;
Another example is ：
Also you can ask the address Add a user_info_id Nonexistent address information
（ Such as ：insert into address (id,user_info_id,real_name,mobile,address) values (‘7da42cc6-36a6-4ad5-9998-71dbc30c8e17’,‘ddc376dd-f8b3-42a6-b42a-db22abed1941’,‘xiaowang’,‘18338970095’,‘ Dongcheng District, Beijing ’);）
—— Again , This data is not complete , Still can't find which user these addresses belong to .

therefore , We still need to improve again .

Second improvement

We thought of foreign keys , The solution is user_info_id Added foreign keys , Point to user_info Primary Key , This constraint protects data integrity ：
If you delete the user information id Already in address Used in table , Then the data cannot be deleted ; Unable to contact address Add users to the table id Nonexistent address information .

create table user_info(
  id char(36) primary key,
  user_name varchar(30) not null,
  password varchar(30) not null
)
insert into user_info (id,user_name,password) values ('51b28fe1-4ebf-41ac-a17b-d5e276861fd0','fuliuqingfeng','123456');
create table address(
  id char(36) primary key,
  user_info_id char(36),
  real_name varchar(8) not null,
  mobile char(11) not null,
  address varchar(150) not null,
  constraint address_user_info_id_fk foreign key(user_info_id) references user_info(id)
)
insert into address (id,user_info_id,real_name,mobile,address) 
values ('bfb9472a-7911-4e6f-a479-3b719454ebab','51b28fe1-4ebf-41ac-a17b-d5e276861fd0',' Zhang San ','18920120206',' Anyang, Henan ');
insert into address (id,user_info_id,real_name,mobile,address) 
values ('5227c6b9-45a2-44aa-8ac0-1f63a38d3b65','51b28fe1-4ebf-41ac-a17b-d5e276861fd0',' Li Si ','18617297545',' Beijing haidian ');
insert into address (id,user_info_id,real_name,mobile,address) 
values ('30b8584b-aa6a-4516-a623-03f487058586','51b28fe1-4ebf-41ac-a17b-d5e276861fd0',' Wang Wu ','17694976949',' Datong, Shanxi ');

If you want to change user_info The data in it , Need to put address Only when the data in the table is cleared , And when executing code , Must be user_info First create .

such , We need foreign keys .