Default constraint of MySQL constraint default

-- Database Constraints （ Some fields in the constraint table ）
-- Multi-table query ： Cartesian product , Internal connection , External connection （ Left lateral / Right outside ）, Subquery

-- mysql constraint
-- Default constraint ：default
-- Non empty constraint ：not null
-- Unique constraint ：unique
-- Primary key constraint ：primary key( Non empty constraint unique )------- Self growth constraints auto_increment( Record the number of records currently inserted )
-- Foreign keys ：foreign key

USE mydb_01;
-- Default constraint default

CREATE TABLE stu (
  id INT,
  NAME VARCHAR (20),
  gender VARCHAR (10)
) ;

SELECT * FROM stu;

-- Insert data normally
INSERT INTO stu (id, NAME, gender) 
VALUES
  (1, ' Zhang San ', ' male '),
  (2, ' Li Si ', ' male ');

-- If you insert data , No data is inserted into a field , The default value is null  
INSERT INTO stu (id, NAME) 
VALUES
  (3, ' Lisa ') ;
-- Add a default constraint to the current field , If you do not insert data into the current field , The default constraint will work
-- Modify the field type in the table
ALTER TABLE stu MODIFY gender VARCHAR(10) DEFAULT ' male ';

DELETE FROM stu WHERE id=3;
DROP TABLE stu;

-- Add default constraints when creating tables
CREATE TABLE stu(
    id INT,
    NAME VARCHAR(20),
    gender VARCHAR(10) DEFAULT ' male '
);