[打卡]自增列 xgl

#自增长列 auto_increment
#在 CREATE TABLE 时创建
CREATE DATABASE dbtest;

use dbtest;
use employees;

SELECT DATABASE();

CREATE TABLE stu_1(
id int PRIMARY key auto_increment,
last_name VARCHAR(15)
CONSTRAINT
);
desc stu_1;
#当我们向主键（含有自增列）的字段上添加0，null时
#实际上会自动往上添加指定的字段的数值
#开发中，一旦主键含有自增列时，则我们添加数据时，就不要给主键对应的字段去赋值
INSERT into stu_1(id,last_name)
VALUES(-40,'aada');

SELECT * from stu_1;

#在 ALTER TABLE 时添加 auto_increment
CREATE TABLE stu_2(
id int PRIMARY key,
last_name VARCHAR(15)
);
CREATE TABLE stu_3(
id int,
last_name VARCHAR(10)
);

ALTER TABLE stu_3
MODIFY id int PRIMARY key;


desc stu_2;
#在 ALTER TABLE 时添加

ALTER TABLE stu_2
MODIFY id int auto_increment;

#在 ALTER TABLE 时删除
ALTER TABLE stu_2
MODIFY id int;

UPDATE employees
set employee_id=100
where employee_id=2;

SELECT * from employees

SELECT last_name,salary
from employees
where department_id =(
SELECT department_id
from employees
where last_name='Zlotkey');


SELECT employee_id,last_name,salary
FROM employees
where salary >(
SELECT AVG(salary)
from employees
);

CREATE TABLE stu_1(
id int ,
salary VARCHAR(10),
last_name VARCHAR(15)
);

INSERT into stu_1 (id,salary,last_name)
VALUES(1,12123,'qasa');

ALTER TABLE stu_1
MODIFY salary int;