What are the constraints in MySQL? (instance verification)

1、 Default constraint

We set default values for fields , Is the default constraint . Verified as follows :

（1） Create a table

CREATE TABLE demo.importhead
(
listnumber INT,
-- We are in the field importype Defined as INT After type , Added default value 1.
importtype INT DEFAULT 1,
quantity DECIMAL(10,3),
recordingdate DATETIME
);

（2） insert data , Do not add... To the default field

INSERT INTO demo.importhead
(
listnumber,
--  Here we do not insert fields importtype Value 
quantity,
recordingdate
)
VALUES
(
3456,
10.23,
'2020-12-10'
);

（3） Query data

SELECT * FROM demo.importhead

  result ： Field importtype The value of is already 1 , We have implemented the definition of the default value of the field as 1 Purpose .

2、 Primary key constraint

Primary key constraint is to define a primary key in a table to uniquely determine the identifier of each row of data in the table . Verified as follows ：

（1） Create a table

CREATE TABLE demo.goodsmaster
(
  barcode TEXT,
  goodsname TEXT,
  price DOUBLE,
  itemnumber INT PRIMARY KEY AUTO_INCREMENT
);

（2） insert data

INSERT INTO demo.goodsmaster( barcode, goodsname, price)VALUES ( '0001', ' book ', 0.47);
INSERT INTO demo.goodsmaster( barcode, goodsname, price)VALUES ( '0002', ' pen ', 0.44);
INSERT INTO demo.goodsmaster( barcode, goodsname, price)VALUES ( '0003', ' pen ', 0.19);

（3） Query table structure and data

DESCRIBE demo.goodsmaster;
SELECT * from demo.goodsmaster;

 （4） Conclusion ： If a field is set as a primary key and automatically added , The value of this field is unique , After new data is inserted, the value of this field is incremented .

3、 Foreign key constraints

To be added

4、 Non empty constraint

A non NULL constraint indicates that the field value cannot be null . Verified as follows ：

（1） Create table

CREATE TABLE demo.test3
(
apple INT NOT NULL,
price DECIMAL(10,3)
);

（2） insert data

INSERT INTO demo.test3( apple, price) VALUES ( 1, 3.19);

（3） View table structure and data

DESCRIBE demo.test3;
SELECT * FROM demo.test3;

 （4） Insert data again ,apple Field is empty

 （5） Conclusion ： Indicates that a field is not empty , So when adding data , This field must have a value , Otherwise, the system will prompt an error .

5、 Uniqueness constraint

The uniqueness constraint means that the value of this field cannot be repeated , Otherwise, the system will prompt an error . Compared with the primary key constraint , The uniqueness constraint is weaker . Verified as follows ：

（1） Create table

CREATE TABLE test4
(
id INT(11) PRIMARY KEY,
name VARCHAR(22) UNIQUE,
location VARCHAR(50)
);

（2） insert data

INSERT INTO demo.test4( id, name,location) VALUES (3,'weibo','charchar');

（3） View table structure and data

desc test4;
SELECT * FROM demo.test4;

（4） summary ： In a table , We can specify that multiple fields satisfy the uniqueness constraint , The primary key constraint can only have one . Fields that satisfy the primary key constraint , Automatically satisfy non null constraints , But the fields that satisfy the uniqueness constraint , Can be null .

6、 Self increasing constraint