mysql-視圖

**視圖**
含義：虛擬表，和普通表一樣使用
mysql5.1版本出現的新特性，是通過表動態生成的數據

比如：舞蹈班和普通班級的對比
	創建語法的關鍵字	是否實際佔用物理空間	使用

視圖	create view		只是保存了sql邏輯	增刪改查，只是一般不能增刪改

表	create table		保存了數據		增刪改查


#案例：查詢姓張的學生名和專業名
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE '張%';

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 '張%';


一、創建視圖
/*
語法：
create view 視圖名
as
查詢語句;

*/
USE myemployees;

1.查詢姓名中包含a字符的員工名、部門名和工種信息
#①創建
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%';






2.查詢各部門的平均工資級別

#①創建視圖查看每個部門的平均工資
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`;



3.查詢平均工資最低的部門信息

SELECT * FROM myv2 ORDER BY ag LIMIT 1;

4.查詢平均工資最低的部門名和工資

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、視圖的修改

#方式一：
/*
create or replace view  視圖名
as
查詢語句;

*/
SELECT * FROM myv3 

CREATE OR REPLACE VIEW myv3
AS
SELECT AVG(salary),job_id
FROM employees
GROUP BY job_id;

方式二：
/*
語法：
alter view 視圖名
as 
查詢語句;

*/
ALTER VIEW myv3
AS
SELECT * FROM employees;

三、刪除視圖

/*

語法：drop view 視圖名,視圖名,...;
*/

DROP VIEW emp_v1,emp_v2,myv3;


四、查看視圖

DESC myv3;

SHOW CREATE VIEW myv3;


五、視圖的更新

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email,salary*12*(1+IFNULL(commission_pct,0)) "annual salary"
FROM employees;

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('張飛','[email protected]');

2.修改
UPDATE myv1 SET last_name = '張無忌' WHERE last_name='張飛';

3.刪除
DELETE FROM myv1 WHERE last_name = '張無忌';

具備以下特點的視圖不允許更新


#①包含以下關鍵字的sql語句：分組函數、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;

#②常量視圖
CREATE OR REPLACE VIEW myv2
AS

SELECT 'john' NAME;

SELECT * FROM myv2;

#更新
UPDATE myv2 SET NAME='lucy';





#③Select中包含子查詢

CREATE OR REPLACE VIEW myv3
AS

SELECT department_id,(SELECT MAX(salary) FROM employees) 最高工資
FROM departments;

#更新
SELECT * FROM myv3;
UPDATE myv3 SET 最高工資=100000;


#④join
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  = '張飛' WHERE last_name='Whalen';
INSERT INTO myv4 VALUES('陳真','xxxx');



#⑤from一個不能更新的視圖
CREATE OR REPLACE VIEW myv5
AS

SELECT * FROM myv3;

#更新

SELECT * FROM myv5;

UPDATE myv5 SET 最高工資=10000 WHERE department_id=60;



#⑥where子句的子查詢引用了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';