SQL基础学习笔记14视图

视图

内容源自尚硅谷SQL学习视频，将笔记重点按内容归纳整理

/*
含义：虚拟表，和普通表一样使用
mysql5.1版本出现的新特性，是通过表动态生成的数据

比如：舞蹈班和普通班级的对比

​ 创建语法的关键字 是否实际占用物理空间 使用

视图 create view 只是保存了sql逻辑 增删改查，只是一般不能增删改

表 create table 保存了数据 增删改查

*/

#案例：查询姓张的学生名和专业名

SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE '张%';

CREATE VIEW v1
AS
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`;

SELECT * FROM v1 WHERE stuname LIKE '张%';

一、创建视图

/*
语法：
create view 视图名
as
查询语句;

*/
USE myemployees;

#1.查询姓名中包含a字符的员工名、部门名和工种信息

#①创建
CREATE VIEW myv1
AS

SELECT last_name,department_name,job_title
FROM employees e
JOIN departments d ON e.department_id  = d.department_id
JOIN jobs j ON j.job_id  = e.job_id;


#②使用
SELECT * FROM myv1 WHERE last_name LIKE '%a%';

#2.查询各部门的平均工资级别

#①创建视图查看每个部门的平均工资
CREATE VIEW myv2
AS
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id;

#②使用
SELECT myv2.`ag`,g.grade_level
FROM myv2
JOIN job_grades g
ON myv2.`ag` BETWEEN g.`lowest_sal` AND g.`highest_sal`;

#3.查询平均工资最低的部门信息

SELECT * FROM myv2 ORDER BY ag LIMIT 1;

#4.查询平均工资最低的部门名和工资

CREATE VIEW myv3
AS
SELECT * FROM myv2 ORDER BY ag LIMIT 1;


SELECT d.*,m.ag
FROM myv3 m
JOIN departments d
ON m.`department_id`=d.`department_id`;

二、视图的修改

方式一：

/*
create or replace view 视图名
as
查询语句;

*/

SELECT * FROM myv3 

CREATE OR REPLACE VIEW myv3
AS
SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id;

方式二：

/*
语法：
alter view 视图名
as
查询语句;

*/

ALTER VIEW myv3
AS
SELECT * FROM employees;

三、删除视图

/*

语法：drop view 视图名,视图名,…;
*/

DROP VIEW emp_v1,emp_v2,myv3;

四、查看视图

DESC myv3;

SHOW CREATE VIEW myv3;

五、视图的更新

#一般的select语句可以进行增删改：

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
FROM employees;


SELECT * FROM myv1;
SELECT * FROM employees;

#1.插入
INSERT INTO myv1 VALUES('张飞','[email protected]');

#2.修改
UPDATE myv1 SET last_name = '张无忌' WHERE last_name='张飞';

#3.删除
DELETE FROM myv1 WHERE last_name = '张无忌';


#具备以下特点的视图不允许更新

#①包含以下关键字的sql语句：分组函数、distinct、group by、having、union或者union all
CREATE OR REPLACE VIEW myv1
AS
SELECT MAX(salary) m,department_id
FROM employees
GROUP BY department_id;

#更新:不能执行
UPDATE myv1 SET m=9000 WHERE department_id=10;


#②常量视图
CREATE OR REPLACE VIEW myv2
AS
SELECT 'john' NAME;

#更新：不能更新
UPDATE myv2 SET NAME='lucy';


#③Select中包含子查询
CREATE OR REPLACE VIEW myv3
AS
SELECT department_id,(SELECT MAX(salary) FROM employees) 最高工资
FROM departments;

#更新：不能更新
SELECT * FROM myv3;
UPDATE myv3 SET 最高工资=100000;


#④join
CREATE OR REPLACE VIEW myv4
AS
SELECT last_name,department_name
FROM employees e
JOIN departments d
ON e.department_id  = d.department_id;

#更新：能更新不能插入
SELECT * FROM myv4;
UPDATE myv4 SET last_name  = '张飞' WHERE last_name='Whalen';
INSERT INTO myv4 VALUES('陈真','xxxx');


#⑤from一个不能更新的视图
CREATE OR REPLACE VIEW myv5
AS
SELECT * FROM myv3;

#更新：不能更新
SELECT * FROM myv5;
UPDATE myv5 SET 最高工资=10000 WHERE department_id=60;



#⑥where子句的子查询引用了from子句中的表
CREATE OR REPLACE VIEW myv6
AS

SELECT last_name,email,salary
FROM employees
WHERE employee_id IN(
	SELECT  manager_id
	FROM employees
	WHERE manager_id IS NOT NULL
);

#更新：不能更新
SELECT * FROM myv6;
UPDATE myv6 SET salary=10000 WHERE last_name = 'k_ing';

练习：

#一、创建视图emp_v1,要求查询电话号码以‘011’开头的员工姓名和工资、邮箱
CREATE OR REPLACE VIEW emp_v1
AS
SELECT last_name,salary,email
FROM employees
WHERE phone_number LIKE '011%';


#二、创建视图emp_v2，要求查询部门的最高工资高于12000的部门信息
CREATE OR REPLACE VIEW emp_v2
AS
SELECT MAX(salary) mx_dep,department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary)>12000;

SELECT d.*,m.mx_dep
FROM departments d
JOIN emp_v2 m
ON m.department_id = d.`department_id`;

测试：表的管理和事务：

#1.创建book表，字段如下：
bid整型，要求主键
bname 字符型，唯一键，并非空
price浮点型，默认值10
btypeId类型编号，要求引用bookType表的id字段

已知bookType表，字段如下：
id
name

create table book(
	bid int primary key,
    bname varchar(20) unique not null,
    price float default 10,
    btypeId int,
    foreign key (btypeId) references bookType(id)
)

#2.开启事务：向表中插入一行数据并结束
set autocommit=0;
insert into book(bid,bnaem,price,btypeId) 
values(1,'小李飞刀',100,1);
rollback;

#创建视图,实现查询价格大于100的书名和类型名
create view myv1   
as      
select bname,name   
from book b  
join bookType t on b.btypeId = t.btypeId	
where price>100;

#修改视图，实现查询价格在90-120之间的书名和价格：
create or replace view myv1
as 
select bnama,price
from book
where price between 90 and 120

#s删除刚才建的视图
drop view myv1