MySQL 基础+高级篇- 数据库 -sql -尚硅谷 139-145视图的学习

#视图
/*
含义：虚拟表，和普通表一样使用
mysql15.1版本出现的新特性，是通过表动态生成的数据
比如：不存在的班级和存在的班级的对比
*/
#创建视图
/*
语法create view as
*/
USE myemployees;
CREATE VIEW  myv2
AS 
SELECT b.email,b.`last_name`,a.`department_name`,c.`job_title`
FROM `departments` a,`employees` b,`jobs` c
WHERE a.`department_id`=b.`department_id` AND b.`job_id`=c.`job_id`;
SELECT * FROM myv2;
SELECT * FROM myv2 WHERE last_name LIKE '%a%';

CREATE VIEW myv3
AS
SELECT AVG(salary) AS '每个部门的平均工资', FROM 
GROUP BY `department_id`;
SELECT * FROM myv3;
SELECT * FROM myv3 ORDER BY '每个部门的平均工资';

CREATE OR REPLACE VIEW myv3
AS 
SELECT AVG(salary),`department_id`
FROM `employees` GROUP BY `department_id`;
DESC myv3;
#方法二
/*
语法：
alter view 视图名
as 查询语句
*/
CREATE VIEW myv3
AS 
SELECT * FROM`employees`;
#三，视图的删除
DROP VIEW myv1,myv2,myv3;
SHOW CREATE VIEW myv3;
CREATE VIEW emp_v2
AS 
SELECT MAX(salary),department_id
FROM `employees` GROUP BY `department_id`
HAVING MAX(salary)>12000;
SELECT * FROM `emp_v2`;
#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,12*salary*(1+IFNULL(`commission_pct`,0)) AS 'annuak salary'
FROM `employees`;
SELECT * FROM myv1;
#1.插入
INSERT INTO myv1 (last_name,email) VALUES("张飞","[email protected]");
#2.修改
UPDATE myv1 SET last_name='张无忌' WHERE last_name='Gietz';
#3.删除
DELETE FROM myv1  WHERE last_name='张无忌';
#具备一下特点的视图不允许更新

#1:包含以下关键词的sql语句：分组函数，distinct,group,by,having,union或者union all

#2:常量视图不能更新

#3:select 中包含子查询
#4:join,连接的语法
#5:from一个不能更新的视图
#6:where 子句的子查询引用了from子句中的表

#视图和表的对比:
/*
         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，一般不能增删改
  表      create table         保存了数据             增删改查