lyt经典版MySQL基础——视图

#视图
/*
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

比如：舞蹈班和普通班的对比


    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

*/

#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 s.stuname LIKE '张%';

#一、创建视图
/*
create view 视图名
as
查询语句;

*/
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 e.job_id=j.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
CREATE VIEW myv2
AS
SELECT AVG(salary) avs,department_id
FROM employees
GROUP BY department_id;

SELECT grade_level,department_id FROM job_grades j
JOIN myv2 m ON m.avs BETWEEN j.lowest_sal AND j.highest_sal;

#3.查询平均工资最低的部门信息
SELECT * FROM departments d
JOIN myv2 m ON d.department_id=m.department_id
ORDER BY  m.avs ASC 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 avs LIMIT 1;

SELECT department_name,avs FROM
departments d JOIN myv3 m ON d.department_id=m.department_id;

#二、视图的修改
#方式一：
/*
create or replace view 视图名
as
查询语句;
*/

SELECT * FROM myv3;

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; -- 在mysql客户端里使用比较方便

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

SELECT * FROM myv1;
SELECT * FROM employees;
#1.插入
INSERT INTO myv1 VALUES('张飞','zf@qq.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';

#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; -- 不允许更新

#(2)常量视图
CREATE OR REPLACE VIEW myv2
AS
SELECT 'john' NAME;

SELECT * FROM myv2;
#更新
UPDATE myv2 SET NAME='lucy'; -- 常量视图不允许更新

#(3)select中包含子查询
CREATE OR REPLACE VIEW myv3
AS
SELECT (SELECT MAX(salary) FROM employees) 最高工资;

#更新
SELECT * FROM myv3;
UPDATE myv3 SET 最高工资=10000; -- 包含子查询的select语句不允许更新

#(4)join
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'); -- 有join的不能插入(更新)

#(5)from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; -- from一个不能更新的视图的，不能更新

#(6)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
    );

SELECT * FROM myv6;
#更新
UPDATE myv6 SET salary=10000 WHERE last_name='K_ing'; -- 不允许更新