MySQL基础-视图

视图

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

通俗点：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

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

示例:

查询姓张的学生名和专业名

第一步：
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE '张%';

第二步：
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%';

 

视图与表的区别

	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

练习

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;

例如：

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;

 

三、删除视图

语法：

DROP VIEW emp_v1,emp_v2,myv3;

delete 和 truncate 在事务中的区别

set autocommit = 0 ;
start trasaction ;
delect from account ;
rollback ;//删除可回滚

set autocommit = 0 ;
start trasaction ;
truncate from account ;
rollback ;//删除不可回滚

 

四、查看视图

DESC myv3;

SHOW CREATE VIEW myv3;

 

练习：

一、创建视图emp_v1,要求查询电话号码以‘011’开头的员工姓名和工资、邮箱

CREATE OR REPLACE VIEW emp_v1
AS
SELECT last_name,salary,email
FROM employees
WHERE phone_number LIKE '011%';

二、创建视图emp_v2，要求查询部门的最高工资高于12000的部门信息

CREATE OR REPLACE VIEW emp_v2
AS
SELECT MAX(salary) mx_dep,department_id
FROM employees
GROUP BY department_id
HAVING MAX(salary)>12000;


SELECT d.*,m.mx_dep
FROM departments d
JOIN emp_v2 m
ON m.department_id = d.`department_id`;

 

五、视图的更新

显示姓名、邮箱、年薪

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

 

例如：

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

1.插入 不仅插入视图中，还插入原始table中

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

SELECT * FROM myv1;
SELECT * FROM employees;//均存在

2.修改，对原始图修改

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

3.删除 对原始图修改

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';