[打卡]张博研——自增列

#自增长列 AUTO_INCREMENT

CREATE DATABASE dbtest;

USE dbtest;

SELECT DATABASE();

#在create table 时添加

CREATE TABLE test1(

id INT PRIMARY KEY AUTO_INCREMENT,

last_name VARCHAR(15)


);

DESC test1;

INSERT into test1
(last_name)
VALUES('Sam');

SELECT * FROM test1;

#结论：当我们向主键（含有增长列）的字段上增加0或NULL时，实际上会自动往上添加指定的字段的数据

INSERT into test1
(id,last_name)
VALUES(0,'Tom');

INSERT into test1
(id,last_name)
VALUES(NULL,'LiLi');

#开发中，一旦主键作用的字段上声明有（增长列）则我们在添加数据时，就不要给主键对应的字段去赋值

INSERT into test1(id,last_name)
VALUES(30,'meiyangyang');

INSERT into test1(id,last_name)
VALUES(-30,'meiyangyang');

#在ALTER TABLE 时添加（很少）

CREATE TABLE test4(

id int PRIMARY KEY,

last_name VARCHAR(10)

);

DESC test4;

ALTER TABLE test4
MODIFY id INT AUTO_INCREMENT;

#在ALTER TABLE时删除

ALTER TABLE test4
MODIFY id INT;

USE atguigudb;

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
);