MySQL进阶16 - 视图的创建/修改/删除/更新--可更新性的不适用条件

#进阶16 : 视图
/*
含义: 虚拟表,和普通表一样使用;(从5.1开始使用的:)是通过表动态生成的数据
创建语法:
    create view 视图名
    as
    查询语句;
----------
作用: 重用SQL语句,简化复杂的sql操作,不必知道它的查询细节;
    保护数据,提高安全性;
-------------------------
视图的修改:
   方式1: create or replace view 视图名
       as
       查询语句;
   方法2: alter view 视图名
      as
      查询语句;
----------------------------
三:视图的删除 : 用户可以一次删除一个或者多个视图，前提是必须有该视图的drop权限。
语法: drop view 视图名,视图名,视图名;
-----------------------------
四:查看视图
desc v2;
-----------------------------
#五: 视图的更新
    CREATE OR REPLACE VIEW myv1
    AS
    SELECT last_name,email FROM employees;

    SELECT * FROM myv1 ORDER BY last_name DESC;
    #1.插入
    INSERT INTO myv1 VALUES('张飞','zafqq.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';
--------------------------------------------
#六 视图的可更新性
  视图的可更新性和视图中查询的定义有关系，以下类型的视图是不能更新的。
    • 包含以下关键字的sql语句：分组函数、distinct、group  by 、having、union或者union all
    • 常量视图 • Select中包含子查询
    • join
    • from一个不能更新的视图
    • where子句的子查询引用了from子句中的表
---------------------------------------------
视图 : create view   没有占用多少物理空间,只保存sql逻辑
表   : create table  占用了

*/
      #案例():查询姓张的学生名和专业名
USE student;
SELECT studentname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`=m.`majorid`
WHERE s.`studentname` LIKE '张%';
    #demo1:  创建视图 ;
CREATE VIEW v1
AS
SELECT studentname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`=m.`majorid`
WHERE s.`studentname` LIKE '张%';
        #然后从视图中查找出全部信息;
SELECT * FROM v1
WHERE studentname LIKE '张%';
#----------------------------
    #1.查询邮箱中包含a字符的 员工名、部门名和工种信息 

CREATE VIEW v2
AS
SELECT e.`last_name`,e.`department_id`,j.`job_id`,j.job_title
FROM employees e,jobs j
WHERE e.`job_id`=j.`job_id` AND email LIKE '%a%';
    #② 使用视图
SELECT * FROM v2 WHERE last_name LIKE '%a%';

    #2.查询各部门的平均工资的级别
CREATE VIEW v3
AS
SELECT AVG(salary) ag,department_id
FROM employees
GROUP BY department_id;

SELECT v3.department_id,g.`grade_level`
FROM v3
JOIN job_grades g
ON v3.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 v3 ORDER BY ag LIMIT 1;

    #4.查询平均工资最低的部门名和工资

SELECT *
FROM `departments`
WHERE departments.department_id = (
  SELECT department_id FROM v3
  ORDER BY ag LIMIT 1
);
    #5.视图的修改(REPLACE)--方式1
CREATE OR REPLACE VIEW v3
AS
SELECT MIN(salary) min_ag,department_id
FROM employees
GROUP BY department_id;

SELECT * FROM v3;
    #6.视图的修改(alter)--方式2
ALTER VIEW v3
AS
SELECT MAX(salary) max_ag,department_id
FROM employees
GROUP BY department_id;

    #平均1.00 -- 2400次
SELECT ('28-29两天scanner sum'),(
865-833+621-523+230-175+3+ 58-41+87-66+4+48-32+7+2+20+31-14+11-9
),(0.054+0.073);

#五: 视图的更新
CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
FROM employees;

SELECT * FROM myv1 ORDER BY last_name DESC;
#1.插入
INSERT INTO myv1 VALUES('张飞','zafqq.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';