Table of contents

I. Overview

Concept: Constraints are rules that act on fields in a table to limit the data stored in the table.
Purpose: To ensure the correctness, validity and integrity of the data in the database.
Classification:

constraint	describe	keywords
not-null constraint	Restrict the data of this field to not be null	NOT NULL
unique constraint	Ensure that all data in this field is unique and not repeated	UNIQUE
primary key constraint	The primary key is the unique identifier of a row of data, which requires non-null and unique	PRIMARY KEY
default constraint	When saving data, if the value of this field is not specified, the default value will be used	DEFAULT
check constraints (after version 8.0.16)	Ensure that the field value satisfies a certain condition	CHECK
foreign key constraints	It is used to establish a connection between the data of the two tables to ensure the consistency and integrity of the data	FOREIGN KEY

Note: Constraints are applied to the fields in the table, and constraints can be added when creating/modifying tables.

2. Constraint Demonstration

According to the requirements, complete the creation of the table structure:

field name field meaning Field Type Restrictions constraint keyword

id unique ID int Primary key, and auto-increment PRIMARY KEY，AUTO_INCREMENT

name Name varchar(10) is not empty and unique NOT NULL，UNIQUE

age age int Greater than 0 and less than or equal to 120 CHECK

status state char(7) If this value is not specified, it defaults to 1 DEFAULT

gender gender char(7) none

create table user(
    id int primary key auto_increment comment '主键',
    name varchar(10) not null unique comment '姓名',
    age int check ( age>= 0 && age <= 120 ) comment '年龄',
    status char(1) default '1' comment '状态',
    gender char(1) comment '年龄'
)comment '用户表';

-- 添加数据
insert into  user(id, name, age, status, gender)
values (1,'Tom',19,'1','男'),(2,'Rose',25,'0','女');

Error judgment:

Unique constraint (ID is unique):

Non-null constraint (Name cannot be empty):

Check constraints (age range):

3. Foreign key constraints

1. Concept

The foreign key is used to establish a connection between the data of the two tables, so as to ensure the consistency and integrity of the data.

Note: At present, the above two tables have not established foreign key associations at the database level, so the consistency and integrity of the data cannot be guaranteed.

2. Grammar

Add foreign key:

ALTER TABLE table name ADD CONSTRANT foreign key name FOREIGN KEY (foreign key field name) REFERENCES main table (main table column name);

-- 建表
create table emp(
    id int primary key auto_increment comment 'ID',
    name varchar(50) not null unique comment '姓名',
    age int comment '年龄',
    salary int  comment '薪资',
    entrydate date comment '入职时间',
    managerid int comment '直属领导id',
    dept_id int comment '部门id'
)comment '员工表';

create table dept(
    id int auto_increment comment 'ID' primary key ,
    name varchar(50) not null comment '部门名称'
)comment '部门表';

-- 添加数据
insert into dept(id, name)
values (1,'研发部'),(2,'市场部'),(3,'财务部'),(4,'销售部'),(5,'总经办');

insert into emp(id, name, age, salary, entrydate, managerid, dept_id) 
values (1,'金庸',66,'总裁',20000,'2000-01-01',null,5),
       (2,'张无忌',20,'项目经理',12500,'2005-12-05',1,1),
       (3,'杨逍',33,'开发',8400,'2000-11-03',2,1),
       (4,'韦一笑',48,'开发',11000,'2002-02-05',2,1),
       (5,'常同春',43,'开发',10500,'2004-09-07',3,1);

-- 添加外键
alter table emp add constraint fk_emp_dept_id 
      foreign key (dept_id) reference dept(id);

Delete foreign key: ALTER TABLE table name DROP FOREIGN KEY foreign key name;

-- 删除外键
alter table emp drop foreign key fk_emp_dept_id;

3. Delete/Update Behavior

Behavior	illustrate
NO ACTION	When deleting/updating the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, delete/update is not allowed. (consistent with RESTRICT)
RESTRICT	When deleting/updating the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, delete/update is not allowed. (consistent with NOACTION)
CASCADE	When deleting/updating the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, delete/update the record in the child table with the foreign key.
SET NULL	When deleting the corresponding record in the parent table, first check whether the record has a corresponding foreign key, and if so, set the foreign key value in the child table to null (this requires that the foreign key is allowed to take nul).
SET DEFAULT	When the parent table is changed, the child table sets the foreign key column to a default value (Innodb does not support)

Syntax: ALTER TABLE table name ADD CONSTRAINT foreign key name FOREIGN KEY (foreign key field) REERENCES main table name (main table field name) ON UPDATE CASOADE ON DELETE CASCAOE ;

-- 外键的删除和更新行为
-- cascade
alter table emp add constraint fk_emp_dept_id
    foreign key (dept_id) references dept(id)
        on update cascade on delete cascade ;
-- set null
alter table emp add constraint fk_emp_dept_id
    foreign key (dept_id) references dept(id)
        on update set null on delete set null ;

Database MySQL - Constraints