Mysql database foundation: constraint and identification columns

constraint

1、 Definition

A limitation , Used to limit the data in the table , In order to ensure the accuracy and reliability of the data in the table .

2、 classification

• NOT NULL： Non empty , To ensure that the value of this field cannot be empty
• DEFAULT： Default , Used to ensure that the field has a default value
• PRIMARY KEY： Primary key , Used to ensure that the value of this field is unique , And not empty
• UNIQUE： only , Used to ensure that the value of this field is unique , Can be null .
• CHECK： Check constraint （mysql China does not support it. ）
• FOREIGN KEY： Foreign key constraints , Limit the relationship between two tables , To ensure that the value of this field must come from the value of the associated column of the main table . Add foreign key constraint from table , Used to reference the value of a column in the main table .

3、 Add constraints when creating tables

3.1 Column level constraints

Column level constraints are usually written directly after the field type , as follows ：

create table  Table name (
	 Field name   Field type   Column level constraints ,
	 Field name   Field type   Column level constraints ,
);

• for example

#  Student list 
create table student(
	id int primary key, #  Primary key 
	stuName varchar(20) not null, #  Non empty 
	gender char(1) check(gender=' male ' or gender=' Woman '), #  Check 
	seat int unique, #  Unique constraint 
	age int default 18, #  Default constraint 
	majorId int foreign key references major(id) #  Foreign key constraints 
)

#  The curriculum 
create table major(
	id int primary key,
	majorName varchar(20)
);

Be careful ： The six constraints are syntactically supported , No mistake. , But foreign key constraints and check constraints have no effect

3.2 Table level constraints

Table level constraints are usually at the bottom of all fields , The grammar is as follows ：

create table  Table name (
	 Field name   Field type ,
	 Field name   Field type ,
	#  Table level constraints 
	【constraint  Alias 】  Constraint type ( Field name ),
	...
);

• for example

#  Student list 
create table student(
	id int,
	stuName varchar(20),
	gender char(1),
	seat int,
	age int,
	majorId int,
	#  Add table level constraints 
	constraint pk primary key(id), #  to id Add primary key constraint 
	constraint uq unique(seat),
	constraint ck check(gender=' male ' or gender=' Woman '), # Check constraint 
	constraint fk_student_major foreign key(majorId) references major(id) #  Foreign key constraints 
)

#  The curriculum 
create table major(
	id int primary key,
	majorName varchar(20)
);

In table level constraints , Non null and default constraints are not supported

Compare primary key with unique key
（1） Both primary keys and unique keys must be unique
（2） The primary key cannot be empty , The only key can be empty
（3） At most one primary key in a table , But you can have multiple unique keys

4、 Add constraints when modifying tables

grammar ：

#  Column level constraints 
alter table  Table name  modify column  Name   type   Constraint type 

#  Token constraint 
alter table  Table name  add 【constraint  Constraint name 】  Constraint type ( Field name ) 【 Foreign key references 】

for example ：

#  Add non empty constraints 
alter table student modify column stuName varchar(20) not null;

#  Add default constraint 
alter table student modify column age int default 18;

#　 Add primary key 
# (1)  Column level constraints 
alter table student modify column id int primary key;
# (2)  Table level constraints 
alter table student add primary key(id);

#　 Add unique key 
# (1)  Column level constraints 
alter table student modify column  seat int unique;
# (2)  Table level constraints 
alter table student add unique(seat);

#　 Add foreign keys 
alter table student add foreign key(majorid) references major(id);

Column level constraints do not support aliasing , Table level constraints can be aliased

4、 Delete constraints when modifying tables

#  Delete non empty constraints 
alter table student modify column stuName varchar(20) null;

#  Delete primary key 
alter table student drop primary key;

#  Delete default constraint 
alter table student modify column age int;

#  Delete unique 
alter table student drop index seat;

#  Delete foreign key 
alter table student drop foreign key majorid;

Identity column

Definition

Also known as self growing column , You don't have to insert values manually , The system provides the default sequence value

Use

• Set identity column when creating table

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

#  insert data 
insert into tab_identity values(null,'john');
insert into tab_identity values(null,'john');
insert into tab_identity values(null,'john');
insert into tab_identity values(null,'john');

select * from tab_identity;

Through the following statement, you can view the step size and starting value of the identification column

show variables like '%auto_increment%';

The step size of the identification column can be modified by the following statement ,mysql Modifying the starting value... Is not supported in .

set auto_increment_increment=3;

characteristic
（1） The identification column needs to be matched with a key （ Primary key , Unique key, etc ）
（2） A table can have at most one identity column
（3） The type of the identity column can only be numeric

• Set the identity column when modifying the table

alter table tab_identity modify column id int primary key auto_increment;

• Delete identity column when modifying table

alter table tab_identity modify column id int ;