一.视图的介绍

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

举例说明什么是视图：假设一个年级有10个班，上面有领导来啦，说要检查舞蹈功底，学校为了应付检查，把每个班会跳舞的女孩子抽出来临时组成一个舞蹈班，这个班只在领导来啦才存在。这个班就是视图

案例说明讲解：

#案例：查询姓张的学生名和专业名
以前我们的做法

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

现在视图的做法，把常用的封装起来

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

查询时直接输入

SELECt * FROM v1 WHERe stuname LIKE '张%';

二.视图的创建

语法：
create view 视图名
as
查询语句;

一般是复杂的查询语句，简单的用不着

案例：#1.查询姓名中包含a字符的员工名、部门名和工种信息

USE myemployees;

#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%';

其中涉及到的多表查询
mysql数据库入门教程（5）:多表操作(连接查询,子查询,分页查询,联合查询）

案例：#2.查询各部门的平均工资级别

USE myemployees;
#①创建视图查看每个部门的平均工资
#1创建
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`;

三.视图的修改

#方式一：

如果该视图存在就修改，如果不存在就创建
案例修改
原始视图

USE myemployees;
#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`;

修改

视图的修改

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

SELECt * FROM myv3 

#方式二：

语法例子

ALTER VIEW myv3
AS
SELECt * FROM employees;

四.视图的删除

DROp VIEW emp_v1,emp_v2,myv3;

五.视图的查看

DESC 视图名

DESC myv3;

SHOW CREATE VIEW 视图名；

SHOW CREATE VIEW myv3;

`

六.视图的更新

更新 是更新视图中的数据

案例1 失败案例
首先 创建一个视图

USE myemployees;

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email,salary*12*(1+IFNULL(commission_pct,0)) "annual salary"
FROM employees;

SELECt* FROM myv1;

数据修改、

#1.插入

INSERT INTO myv1 VALUES('张飞','zf@qq.com',100000);

案例2 成功案例
把案例1中的年薪去掉
创建视图

USE myemployees;

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


SELECt * FROM myv1;

插入数据
语法和表格插入一样

#1.插入

INSERT INTO myv1 VALUES('张飞','zf@qq.com');

#2.修改

UPDATe myv1 SET last_name = '张无忌' WHERe last_name='张飞';

#3 删除

#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

USE myemployees;

#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;

SELECt * FROM myv1;

#更新
UPDATe myv1 SET m=9000 WHERe department_id=10;

报错

#②常量视图 -查询语句后面是一个常量

CREATE OR REPLACE VIEW myv2
AS

SELECT 'john' NAME;

SELECT * FROM myv2;

#更新
UPDATe myv2 SET NAME='lucy';

#③Select中包含子查询

USE myemployees;
CREATE OR REPLACE VIEW myv3
AS

SELECT department_id,(SELECT MAX(salary) FROM employees) 最高工资
FROM departments;

#更新
SELECt * FROM myv3;
UPDATe myv3 SET 最高工资=100000;

#④join

USE myemployees;
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

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

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
SELECt * FROM myv6;
UPDATE myv6 SET salary=10000 WHERe last_name = 'k_ing';

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