[打卡]打卡1234

#AUTO_INCREMENT 自增列

CREATE DATABASE dbtest;

USE dbtest;

SELECT DATABASE();

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;
#当我们主键含有自增列的字段添加0或null时，实际上会自动往上添加指定的字段的数值,
INSERT INTO test1
(id,last_name)
VALUES(0,'Tam');

INSERT INTO test1
(id,last_name)
VALUES(NULL,'lili');

#开发中，一旦主键作用声明有自增列时，则我们在添加数据时就不要给字段对应的字段去赋值。


INSERT INTO test1
(id,last_name)
VALUES(-20,'lucky');

#ALTER TABLE时添加自增列（很少）

CREATE TABLE test2(

id INT PRIMARY KEY,

last_name VARCHAR(10)

);

DESC test2;

ALTER TABLE test2
MODIFY id INT AUTO_INCREMENT;


#ALTER TABLE时删除自增列


DESC test2;

ALTER TABLE test2
MODIFY id INT;

USE company;

#作业

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


);