概念

虚拟表，和普通表一样使用。比如：舞蹈班和普通班级的对比。

特点

临时的，mysql5.1版本出现的新特性，是通过表动态生成的数据。

案例

-- 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%';

创建视图

-- 语法：
create view 视图名
as
查询语句;

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`;

修改视图

#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;

删除视图

/*
语法：drop view 视图名,视图名,...;
*/

DROP VIEW emp_v1,emp_v2,myv3;

查看视图

DESC myv3;

SHOW CREATE VIEW myv3;

更新视图

数据准备

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;

插入

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

修改

UPDATE myv1 SET last_name = '张无忌' WHERE last_name='张飞';

删除

DELETE FROM myv1 WHERE last_name = '张无忌';

不允许更新

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。 ```sql 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;

2. 常量视图
```sql
CREATE OR REPLACE VIEW myv2
AS

SELECT 'john' NAME;

SELECT * FROM myv2;

#更新
UPDATE myv2 SET NAME='lucy';

1. Select中包含子查询 ```sql 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;

4. join
```sql
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');

1. from一个不能更新的视图 ```sql 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;

6. where子句的子查询引用了from子句中的表
```sql
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';