mysql数据库入门教程(11)：视图讲解大全
一.视图的介绍
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
举例说明什么是视图：假设一个年级有10个班，上面有领导来啦，说要检查舞蹈功底，学校为了应付检查，把每个班会跳舞的女孩子抽出来临时组成一个舞蹈班，这个班只在领导来啦才存在。这个班就是视图
案例说明讲解：
#案例：查询姓张的学生名和专业名
以前我们的做法
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE '张%';
现在视图的做法，把常用的封装起来
CREATE VIEW v1
AS
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`;
查询时直接输入
SELECT * FROM v1 WHERE stuname LIKE '张%';
二.视图的创建
语法：
create view 视图名
as
查询语句;
一般是复杂的查询语句，简单的用不着
案例：#1.查询姓名中包含a字符的员工名、部门名和工种信息
USE myemployees;
#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%';
其中涉及到的多表查询
mysql数据库入门教程（5）:多表操作(连接查询,子查询,分页查询,联合查询）
案例：#2.查询各部门的平均工资级别
USE myemployees;
#①创建视图查看每个部门的平均工资
#1创建
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`;
三.视图的修改
#方式一：
/*
create or replace view 视图名
as
查询语句;
*/
如果该视图存在就修改，如果不存在就创建
案例修改
原始视图
USE myemployees;
#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
视图的修改
CREATE OR REPLACE VIEW myv3
AS
SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id;
SELECT * FROM myv3
#方式二：
/*
语法：
alter view 视图名
as
查询语句;
*/
语法例子
ALTER VIEW myv3
AS
SELECT * FROM employees;
四.视图的删除
/*
语法：drop view 视图名,视图名,…;
*/
DROP VIEW emp_v1,emp_v2,myv3;
五.视图的查看
DESC 视图名
DESC myv3;
SHOW CREATE VIEW 视图名；
SHOW CREATE VIEW myv3;
`
六.视图的更新
更新 是更新视图中的数据
案例1 失败案例
首先 创建一个视图
USE myemployees;
CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email,salary*12*(1+IFNULL(commission_pct,0)) "annual salary"
FROM employees;
SELECT* FROM myv1;
数据修改、
#1.插入
INSERT INTO myv1 VALUES('张飞','[email protected]',100000);
案例2 成功案例
把案例1中的年薪去掉
创建视图
USE myemployees;
CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
FROM employees;
SELECT * FROM myv1;
插入数据
语法和表格插入一样
#1.插入
INSERT INTO myv1 VALUES('张飞','[email protected]');
#2.修改
UPDATE myv1 SET last_name = '张无忌' WHERE last_name='张飞';
#3 删除
#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
USE myemployees;
#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;
报错
#②常量视图 -查询语句后面是一个常量
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
USE myemployees;
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
USE myemployees;
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';
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