视图

一、含义

mysql5.1版本出现的新特性，本身是一个虚拟表，它的数据来自于表，通过执行时动态生成。
好处：
1、简化sql语句
2、提高了sql的重用性
3、保护基表的数据，提高了安全性

#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%';

二、创建视图

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`;

三、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：
alter view 视图名
as
查询语句

ALTER VIEW myv3
AS
SELECT * FROM employees;

四、删除视图

drop view 视图1，视图2,...;

ROP VIEW emp_v1,emp_v2,myv3;

五、查看视图

desc 视图名;
show create view 视图名;

DESC myv3;

SHOW CREATE VIEW myv3;

六、使用

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

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

2.修改
update

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

3.删除
delete

DELETE FROM myv1 WHERE last_name = '张无忌';

4.查看
select
注意：视图一般用于查询的，而不是更新的，所以具备以下特点的视图都不允许更新
①包含分组函数、group by、distinct、having、union、

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;

②join

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');

③常量视图

CREATE OR REPLACE VIEW myv2
AS

SELECT 'john' NAME;

SELECT * FROM myv2;

#更新
UPDATE myv2 SET NAME='lucy';

④where后的子查询用到了from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';

⑤用到了不可更新的视图

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;

七、视图和表的对比

类型	创建语法的关键字	是否占用物理空间	使用
视图	create view	占用较小，只保存sql逻辑	一般用于查询
表	create table	保存实际的数据	增删改查