视图

/*
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

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email,salary12(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';

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;

②常量视图

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

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

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';