一、视图的概念

MySQL从5.0.1版本开始提供视图功能。一种虚拟存在的表，行和列的数据来自定义视图的查询中使用的表 ，并且是在使用视图时动态生成的，只保存了sql逻辑，不 保存查询结果

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

二、创建视图

语法：
create view 视图名
as
查询语句;

案例：

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`;

三、修改视图

3-1、方式一

create or replace view 视图名
as
查询语句;

3-2、方式一案例

SELECT * FROM myv3 

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

3-3、方式二：

语法：
alter view 视图名
as
查询语句;

3-4、方式二案例：

ALTER VIEW myv3
AS
SELECT * FROM employees;

四、删除视图

语法：drop view 视图名,视图名,...;

DROP VIEW emp_v1,emp_v2,myv3;

五、查看视图

DESC myv3;

SHOW CREATE VIEW myv3;

六、视图的更新

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

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','zf@qq.com');

#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';

注意 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;

SELECT * FROM myv1;

#更新
UPDATE myv1 SET m=9000 WHERE department_id=10;

#结果不允许更新

②常量视图

CREATE OR REPLACE VIEW myv2
AS
SELECT 'john' NAME;

SELECT * FROM myv2;

#更新
UPDATE myv2 SET NAME='lucy';

#结果不允许更新

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
SELECT * FROM myv3;
UPDATE myv3 SET 最高工资=100000;

#结果不允许更新

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
SELECT * FROM myv4;
UPDATE myv4 SET last_name  = '张飞' WHERE last_name='Whalen';
INSERT INTO myv4 VALUES('陈真','xxxx');

#结果不允许更新

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;

#结果不允许更新

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';

#结果不允许更新

案例

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