SQL learning notes (03) -- data constraint relationship

One 、 Entity integrity

1. Primary key constraint （primary key）

• only , No repetition , Can't be empty , There is only one primary key constraint in a table
• Fields in the primary key can be set Automatic growth (auto_increment)

1-1. Create a primary key constraint while creating a table

Format 1 ：

create table  Table name (
	 Name 1  data type  primary key,
	 Name 2  data type 
);

create table stu(sno int(10) primary key auto_increment);

Format two ： The writing of the primary key constraint name ：PK_ Name

create table  Table name (
	 Name 1  data type ,
	 Name 2  data type ,
	constraint  The name of the primary key constraint 
);

create table stu(
sno int(10),
sname varchar(10),
constraint PK_name primary key(sname)
);

Format three ：

create table  Table name (
	 Name 1  data type ,
	 Name 2  data type ,
	primary key ( Name 1)
);

create table stu(sno int(10),primary key(sno));

1-2. For existing tables , Add primary key constraint

Format 1 ：

alter table  Table name  modity  Name   data type  primary key;

Format two ：

alter table  Table name  add primary key( Name );

Format three ：

alter table  Table name  add constraint  The name of the primary key constraint  primary key( Name );

2. Unique constraint （unique）

• 1. It is not allowed to have repeat Value , Guarantee the data Uniqueness .
• 2. There can be Null value
• 3. In a table , There can be Multiple Unique constraint
• 4. By default , The names of unique constraints and column names remain — Cause
• 5. Add columns with unique constraints , The system will also add a... To this column by default unique index
• Indexes ： A catalogue equivalent to a book , In the future, it can speed up the query of data .

2-1. Create a table while creating a table — constraint （UN_ Name ）

create table Table name (
	 Name 1  data type  unique,
	 Name 2  data type  unique,
	 Name 3  data type 
);

create table stu(sno int(10) unique,sname varchar(20) unique,ssex varchar(10));

2-2. For existing tables , Add unique constraints

alter table  Table name  add unique( Name 1[, Name 2]);

2-3. Delete unique constraint

alter table  Table name  drop index  The name of the unique constraint ;

 

Two 、 Domain integrity

1. Default constraint （default）

• Ensure that... Is not entered in the table Invalid The data of
• When a column is decorated with default constraints , Even if the modified column does not write data, it will default to a value

1-1. Create default constraints while creating tables

create table  Table name (
	 Name 1  data type  default ' The default value of string type or date type ',
	 Name 2  data type  default  The number ,
	 Name 3  data type 
);

create table student(ssex varchar(10) default ' male ',record int(10) default '70');

1-2. For existing constraints , Add default constraint

alter table  Table name  modify  Name   data type  default ' The number ';

alter table student modify record int(10) default '80';

1-3. Delete default constraint

alter table  Table name  drop  Name   data type ;

alter table student drop ssex varchar(10);

2. Non empty constraint (not null) The current column must have a value

2-1. Create a non empty constraint while creating the table

create table  Table name (
	 Name 1  data type  not null,
	 Name 2  data type 
);

create table student(sno int(10),sname varchar(20) not null);

2-2. For existing constraints , Add non empty constraints

alter table  Table name  modify  Name   data type  not null;

alter table student modify ssex varchar(10) not null;

2-3. Delete non empty constraints

alter table  Table name  drop  Name   data type ;

alter table student drop ssex varchar(10);

3、 ... and 、 Referential integrity

Foreign key constraints （foreign key）

• Foreign key constraints are based on From the table ;
• When the column of the secondary table refers to the column of the primary table , Column names can be different , But in the column The data type and content should be consistent ;
• Reference the columns of the main table from the table , The columns of the main table must have Primary key constraint or unique constraint ;
• When the data of the main table , Referenced from table , The data in the main table cannot be deleted ;
• When you want to operate from the data in the table , First ask the main table for advice , The main table has , Allow operations from tables , The main table doesn't have , Reject operation from table .

Create foreign key constraints while creating tables

create table  Table name (
	 Name 1  data type ,
	 Name 2  data type ,
	constraint  The name of the foreign key constraint  foreign key( From the column name of the table 1) references  Main table name ( The column name of the main table 1)
);

For existing tables , Add a foreign key constraint

alter table  From table name  add constraint  The name of the foreign key constraint  foreign key( Column name from table 1) references  Main table name ( Name of main table );

Delete foreign key constraint

alter table  Table name  drop foreign key  The name of the foreign key constraint ;