MySQL[笔记][视图]

视图

含义:虚拟表,和普通表一样使用
mysq5.1版本出现的新特征，时通过表动态生成的数据

EG:舞蹈班和普通班的对比

应用常见:
-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

    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

表 create table 保存了数据增删改查

#案例:查询姓张的学生名和专业名 SELECT stuname,majorname FROM stuinfo s
INNER JOIN major m ON s.majorid=m.id WHERE s.stuname LIKE ‘张%’;

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%’;

#一、创建视图
/*
语法：
create view 视图名
as
查询语句;

*/
USE myemployees;

#text
#1、查询邮箱中包含a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 departmentys d ON e.department_id = d.department_id
JOIN jobs j ON j.job_id = e.job_id;

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#②使用
SELECT * FROM myv1 WHERE email LIKE ‘%a%’;

#2.查询各部门的平均工资级别
CREATE VIEW myv2
AS
SELECT AVG(salary) ag
FROM employees
GROUP BY department_id;

SELECT myv2.ag,g.grade_level
FROM myv2
JOIN job_grades g
ON myv2.ag BETWEEN g.lowest_sal AND g.highest_sql;

#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;

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;

#三、删除视图

语法：
drop view 视图名,视图名…;

DROP VIEW myv1,myv2,myv3;

#四、查看视图

DESC myv3;

SHOW CREATE VIEW myv3;
#text
#1创建视图emp_v1,要求查询电话号码以‘011’开头的而员工姓名和工资、邮箱
CREATE OR REPLACE VIEW emp_v1
AS
SELECT last_name,salary,email
FROM employees
WHERE phone_number LIKE ‘011’;

#2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 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

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email,salary12(1+IFNULL(commission_pct,0)) “annual salary”
FROM employees;

SELECT * FROM myv1;
#1.插入

INSERT INTO myv1 VALUES(‘张飞’,‘[email protected]’,100000);

#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
/*
包含以下关键字的sql语句:分组函数、distinct、group、by、having
union或union all

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
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

);

#2常量视图
CREATE OR REPLACE VIEW myv2
AS
SELECT ‘john’ NAME;

#text
#创建表book表，字段如下:

CREATE TABLE Book(

    bid INT PRIMARY KEY,
    bname VARCHAR(20) UNIQUE NOT NULL,
    price FLOAT DEFAULT 10,
    btypeid INT,
    FOREIGN KEY(btypeid) REFERENCES bookType(id)

)

#开始事务
#向表中插入1行数据，并结束

SET autocommit=0;
INSERT INTO book(bid,bname,price,btypeid)
VALUES(1,‘小李飞刀’,100,1)

COMMIT;

#3、创建视图，实现查询价格大于100的书名和类型
CREATE VIEW myv1
AS
SELECT bname,NAME
FROM book b
INNER JOIN bookType t
ON b.btypeid=t.id
WHERE price>100;

#4、修改视图，实现查询价格在90-120之间的书名和价格

CREATE OR REPLACE VIEW myv2
AS
SELECT bname,price
FROM book
WHERE price BETWEEN 90 AND 120;

#5、删除刚才建的视图

DROP VIEW myv1

视图

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