Constraint is a kind of restriction , It limits the data of a table's rows or columns , To ensure the data integrity of the table 、 Uniqueness .

MYSQL in , Several common constraints :

Constraint type : Primary key The default value is only Foreign keys Non empty
keyword : PRIMARY KEY DEFAULT UNIQUE FOREIGN KEY

NOT NULL

===================================================

Primary key (PRIMARY KEY) Is used to constrain a row in a table , As the identifier for this line , In a table, you can accurately locate a row through the primary key , So the primary key is very important . The primary key requires that the data in this row cannot be duplicated and cannot be empty .

There is also a special primary key —— Composite primary key . A primary key can be more than just a column in a table , It can also be identified by two or more columns in the table

===================================================

Default constraint (DEFAULT) Regulations , When there is DEFAULT Bound columns , What to do when the insert data is empty .

DEFAULT Constraints will only be used INSERT sentence ( The last experiment introduced ) When it comes to ,INSERT In the sentence , If you are DEFAULT The position of the constraint has no value , So this position will be DEFAULT The value of

===================================================

Unique constraint (UNIQUE) Relatively simple , It specifies that the values of a column specified in a table must not have duplicate values , That is, each value in this column is unique .

When INSERT Statement when the newly inserted data and the existing data are repeated , If there is UNIQUE constraint , be INSERT Failure .

===================================================

Foreign keys (FOREIGN KEY) To ensure data integrity , Can also show the relationship between tables .

A table can have multiple foreign keys , Every foreign key must REFERENCES( Reference resources ) The primary key of another table , Columns constrained by foreign keys , The value must have a corresponding value in the column it references .

stay INSERT when , If the value constrained by the foreign key does not correspond in the reference column , For example, the following command , Reference column (department Tabular dpt_name) There is no dpt3, be INSERT Failure

===================================================

Non empty constraint (NOT NULL), Listen to the name to understand , Columns constrained by non empty constraints , Must be non empty when inserting value .

stay MySQL Violation of nonempty constraint in , No mistake. , There will only be warnings .

attach : SQL sentence

 CREATE DATABASE mysql_shiyan;

 use mysql_shiyan;

 CREATE TABLE department
(
dpt_name CHAR(20) NOT NULL,
people_num INT(10) DEFAULT '',
CONSTRAINT dpt_pk PRIMARY KEY (dpt_name)
); CREATE TABLE employee
(
id INT(10) PRIMARY KEY,
name CHAR(20),
age INT(10),
salary INT(10) NOT NULL,
phone INT(12) NOT NULL,
in_dpt CHAR(20) NOT NULL,
UNIQUE (phone),
CONSTRAINT emp_fk FOREIGN KEY (in_dpt) REFERENCES department(dpt_name)
); CREATE TABLE project
(
proj_num INT(10) NOT NULL,
proj_name CHAR(20) NOT NULL,
start_date DATE NOT NULL,
end_date DATE DEFAULT '2015-04-01',
of_dpt CHAR(20) REFERENCES department(dpt_name),
CONSTRAINT proj_pk PRIMARY KEY (proj_num,proj_name)
);

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