20.视图

声明：此MySQL基础学习源自尚硅谷。（推荐）b站官方链接：https://www.bilibili.com/video/BV1xW411u7ax?p=1

视图

含义：虚拟表，和普通表一样使用
MySQL从5.0.1版本出现的新特性。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。
 
应用场景：
	多个地方用到同样的查询结果
	该查询结果使用的sql语句较复杂
 
好处：1.重用sql语句  2.简化复杂的sql操作，不必知道它的查询细节  3.保护数据，提高安全性

案例：查询姓张的学生名和专业名

-- 原始操作
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE '张%';
 
-- 使用视图操作
CREATE VIEW v1
AS
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`;
 
-- 查询视图
SELECT * FROM v1 WHERE stuname LIKE '张%';

1.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`;

2.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;

3.删除视图

语法：
drop view 视图名1,视图名2,...;

DROP VIEW emp_v1,emp_v2,myv3;

4.查看视图

# 方式1
DESC myv3;
# 方式2
SHOW CREATE VIEW myv3;

5.视图的更新

更新了视图中数据，原始表中也会更新视图中的数据

视图一般用于查询的，而不是更新的，一般视图也不会更新数据，由于视图更新限制条件，一般也不能更新

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

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; # 存在group by，不允许更新
 
SELECT * FROM myv1;
 
#更新失败
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;
 
#更新失败
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;
 
SELECT * FROM myv3;
 
#更新失败
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;
 
SELECT * FROM myv4;
 
#更新
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;
 
SELECT * FROM myv5;
 
#更新失败
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
);
 
SELECT * FROM myv6;
 
#更新失败
UPDATE myv6 SET salary=10000 WHERE last_name = 'k_ing';

视图与实际表对比

	创建语法关键字	是否实际占用物理空间	使用
视图	create view	只保存SQL逻辑	增删改查，一般不能增删改
表	create table	保存数据	增上改查