MySQL database - constraints (overview, constraint demonstration, foreign key constraints, delete/update behavior)

Table of contents

Overview

Constraint demonstration

Table creation

verify

Graphical interface constraints

foreign key constraints

concept

grammar

Foreign key delete/update behavior

Summarize

Overview

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

constraint	describe	Keywords
non-null constraint	The data in this field cannot be NULL	NOT NULL
unique constraint	Ensure that all data in this field are unique and non-duplicate	UNIQUE
primary key constraints	The primary key is the unique identifier of a row of data and must be non-empty and unique.	PRIMARY KEY
Default constraints	When saving data, if the value of this field is not specified, the default value is used	DEFAULT
Check constraints (after version 8.0.16)	Save field values that meet a certain condition	CHECK
foreign key constraints	Used to establish a connection between the data of the two tables to ensure the consistency and integrity of the data.	FOREIGN KEY

Constraint demonstration

Table creation

Complete the creation of the form as required:

Field name	Field meaning	Field Type	Restrictions	constraint keyword
id	ID unique identification	int	Primary key, and automatically grows	PRIMARY KEY,AUTO_INCREMENT
name	Name	varchar(10)	Not empty and unique	NOT NULL,UNIQUE
age	age	int	Greater than 0 and less than or equal to 120	CHECK
status	state	char(1)	If this value is not specified, it defaults to 1	DEFAULT
gender	gender	char(1)	none

Automatic growth: the id increases automatically with the number of rows in the table, the keyword is auto_increment

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 and age <= 120) comment '年龄',
    status char(1) default '1' comment '状态',
    gender char(1) comment '性别'
) comment '用户表';

verify

1. Primary key and automatic growth

When we insert data, we do not enter the ID

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 and age <= 120) comment '年龄',
    status char(1) default '1' comment '状态',
    gender char(1) comment '性别'
) comment '用户表';

At this time, we found that id is used as the primary key and has automatic growth constraints. It will automatically increase as the number of rows increases, and we do not need to enter it.

2. The name is not empty and unique

When our name input is empty, an error will be reported; and when the name is repeated, after applying for the primary key for the inserted value, it will not be actually inserted, so the id value will jump by 1. (For example: 1, 2,3,5)

3. Age must be greater than 0 and less than or equal to 120

4. When the status is not specified, the default is 1

insert into user (name,age,gender)
            values ('Jack6',18,'男');

Graphical interface constraints

foreign key constraints

concept

Foreign keys are used to establish a connection between data in two tables to ensure data consistency and integrity.

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

In other words, if the R&D department in the department table is deleted, the employees of the R&D department in the employee table will not be deleted, causing data integrity problems.

grammar

Add foreign key

CREATE TABLE 表名(
    字段名 数据类型,
    ...
    [CONSTRAINT] [外键名称] FOREIGN KEY (外键字段名) REFERENCES 主表 (主列表名)
);

-- 或
ALTER TABLE 表名 
    ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段名) REFERENCE 主表 (主列表名);

Delete foreign key

ALTER TABLE 表名 DROP FOREIGN KEY 外键名称;

Example

alter table emp 
    add constraint fk_emp_dept_id foreign key (dept_id) references dept(id);

alter table emp drop foreign key fk_emp_dept_id;

Foreign key 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, deletion/update is not allowed. (Same as 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, deletion/update is not allowed. (Consistent with NO ACTION)
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 with the foreign key in the child table.
SET NULL	When deleting the corresponding record in the parent table, first check whether the record has a corresponding foreign key. If so, set the foreign key value in the child table to null (requires that the foreign key allows null)
SET DEFAULT	When the parent table changes, the child table sets the foreign key column to a default value (Innodb does not support this)

ALTER TABLE 表名 
    ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段名) REFERENCE 主表 (主列表名)
        ON UPDATE CASCADE ON DELETE CASCADE;

Demo

alter table emp
    add constraint fk_emp_dept_id foreign key (dept_id) references dept(id)
        on update cascade  on delete  cascade ;

At this time, change the id of the main table R&D department to 6:

The subtable is also updated:

Summarize

Non-null constraint: NOT NULL
Unique constraint: UNIQUE
Primary key constraint: PRIMARY KEY (increment: AUTO_INCREMENT)
Default constraint: DEFAULT
Check constraints: CHECK
Foreign key: FOREIGN KEY

end

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