MySQL constraints

MySql constraint

brief introduction , Concept

constraint

Constraints are actually constraints on the data in the table

effect

The purpose of adding constraints in the design of the table is to ensure the integrity and effectiveness of the records in the table , For example, the value of some columns in the user table cannot be empty , The values of some columns cannot be repeated .

classification

• Primary key constraint (primary key) PK
• Self growth constraints (auto_increment)
• Non empty constraint (not null)
• Uniqueness constraint (unique)
• Zero fill constraint (zerofill)
• Foreign key constraints (foreign key) FK

Primary key constraint primary key

Concept

• MySql A primary key constraint is a column or a combination of columns , Its value can uniquely identify each row in the table , Convenient in RDBMS Find a line as soon as possible .

• A primary key constraint is equivalent to a unique constraint + A combination of nonnull constraints , Primary key constraint columns are not allowed to be duplicate , Null values are not allowed .

• Only one primary key is allowed per table .

• The key of the primary key constraint is ：primary key

• When creating a primary key constraint , By default, the system will establish a unique index on the column and column combination .

Operation primary key

• Add single column primary key
• Add multi column joint primary key
• Delete primary key

constraint Named primary key

Single column primary key

There are two ways to create a single column primary key , One is to specify the primary key while defining the field , One is to specify the primary key after defining the field .

The way 1 grammar ：

--  stay create table In the sentence , adopt  PRIMARY KEY  Keyword to specify the primary key 
--  Specify the primary key while defining the field , The syntax is as follows ：
crate table  Table name (
	...
	< Field name > < data type > primary key
)

The way 1 Implementation example ：

crate table demo(
	sid int primay key,
	name VARCHAR(20)
);

The way 2 grammar ：

-- Define the primary key after defining the field 
create table  Table name (
	...
	[constraint < Constraint name >] primary key [ Field name ]
);

The way 2 Realization ：

create table demo2(
	id int,
	name varchar(20),
	constraint pk1 primary key (id)
);

Combined the primary key

A primary key consists of multiple fields in a table .

Be careful ：

• When the primary key is composed of multiple fields , You cannot declare a primary key constraint directly after a field name
• A table can only have one primary key , A federated primary key is also a primary key

grammar ：

create table  Table name (
	...
	primary key ( Field 1, Field 2, Field 3)
);

Example ：

create table demo3 (
	name varchar(20),
	depId int,
	num int,
	constraint nam1 primary key (name,depId)
);

No field can be empty , Two primary keys cannot be duplicate values at the same time .

Modify table structure and add primary key

grammar ：

create table  Table name ( ... );
--  adopt alter table modify 
ALTER TABLE < Table name > add primary key ( Field list );

Example ：

--  Add single column primary key 
create table demo4(
	id int,
	name varchar(20),
	depId int
);
alter table demo4 add primary key (id);

Add multi column primary key

alter table demo4 add primary key(id,name);

Delete primary key

Format ：

alter table < Data table name > drop primary key;

Realization ：

--  Delete single column primary key 
alter table demo1 drop primary key;
--  Delete multi column primary key 
alter table demo1 drop primary key;

Self growth constraints auto_increment

Concept

When the primary key definition grows , The value of this primary key There is no need for users to enter data 了 , The database system automatically assigns values according to the definition . Every record added , The primary key will automatically grow in the same step size .

By adding auto_increment Property to realize the primary key self growth .

grammar ：

 Field name   data type  auto_increment	

operation

create table user (
	id int primary key auto_increment,
	name varchar(20)
);

characteristic

• By default ,auto_increment The initial value of the yes 1, Every new record , field value Add 1.
• Only one field can be used in a table auto_increment constraint , The field must have a unique index , To avoid repetition of serial numbers （ It is the primary key or part of the primary key ）
• auto_increment The constrained field must have NOT NULL attribute
• auto_increment The field of constraint can only be integer type (TINYINT, SMALLINT, INT, BIGINT) etc. .
• auto_increment The maximum value of a constraint field is constrained by the data type of the field , If it reaches the upper limit auto_increment It will fail. .

Specify the initial value of the auto increment field

If the first record sets the initial value of this field , Then the newly added record will increase from this initial value . If the number inserted in the table is 1 Bar record id value yes 5, Then the subsequent records will start from 5 Begin to increase .

-- The way 1, When creating a table, specify 
create table emp1 (
	id int primary key auto_increment,
	name varchar(20)
) auto_increment=100;

If there is data in the table , Want to increase the primary key from 1 Start , Then you need to delete this field , Then add it again

-- Delete ID Field 
alter table student drop sid;
--  add to sid Field 
alter table student add id int(8) not null primary key AUTO_INCREMENT first;

delete and truncate Change of auto increment column after deletion

• delete After that, from the self increase to the last ID Start
• truncate from 1 Start

Non empty constraint

not null

This field cannot be null , It must be worth .

Add non empty constraints ：

 The way 1：
< Field name > < data type > not null;

 The way 2：
alter table  Table name  modify  Field   type  not null

alter table student modify name varchar(20) not null;

Delete non empty constraints

--  Use it directly modify Keyword modification , No addition not null
alter table student modify name varchar(20);

Unique constraint unique

The record in the field column cannot have duplicate values .

grammar ：

-- The way 1
< Field name > < data type > unique
-- The way 2
alter table  Table name  add constraint  Constraint name  unique( Column );

Example ：

-- Specify... When creating a table 
create table student (
	id int,
	name varchar(20),
	phone number varchar(20) unique -- Specify a unique constraint 
);

-- Modify table structure 
ALTER TABLE student add constraint  Constraint name  unique( Column )

-- Delete unique constraint 
ALTER TABLE student drop index < Unique constraint name >;

Default constraint

MySQL The default value constraint is used to specify the default value of a column .

grammar

-- The way 1：
< Field name > < data type > default < The default value is >
-- The way 2：
alter table  Table name  modify  list   type  default  The default value is ;

Example

-- The way 1
create table student (
	id int,
	name varchar(20),
	address varchar(20) default ' Beijing ' -- Specify the default constraint 
);
--  The way 2
alter table student modify age int(20) default 20;

Zero fill constraint

Concept

• When inserting data , When the value of this field When the length of is less than the defined length , This value will be Put the corresponding... In front of 0
• zerofill The default is int(10)
• When using zerofill when , By default, it will automatically add unsigned( Unsigned ) attribute , Use unsigned After attribute , The value range is the original value Of 2 times , for example ： The sign is -128~+127, No sign is 0-256

--  Use in a similar way , The attribute is  zerofill

create table demo1 (
	id int zerofill, --  Zero fill constraint 
	name varchar(20)
);

--  Delete 
alter table demo1 modify id int; --  Directly reset the properties of the field column , Remove zerofill Key words can be used 

summary

• Classification of constraints
• The role of constraints
• Use of constraints