宣告：此MySQL基礎學習源自尚矽谷。（推薦）b站官方連結：https://www.bilibili.com/video/BV1xW411u7ax?p=1

檢視

含義：虛擬表，和普通表一樣使用
MySQL從5.0.1版本出現的新特性。一種虛擬存在的表，行和列的資料來自定義檢視的查詢中使用的表，並且是在使用檢視時動態生成的，只儲存了sql邏輯，不儲存查詢結果。

應用場景：
	多個地方用到同樣的查詢結果
	該查詢結果使用的sql語句較複雜

好處：1.重用sql語句  2.簡化複雜的sql操作，不必知道它的查詢細節  3.保護資料，提高安全性

案例：查詢姓張的學生名和專業名

-- 原始操作
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`
WHERE s.`stuname` LIKE '張%';

-- 使用檢視操作
CREATE VIEW v1
AS
SELECT stuname,majorname
FROM stuinfo s
INNER JOIN major m ON s.`majorid`= m.`id`;

-- 查詢檢視
SELECT * FROM v1 WHERE stuname LIKE '張%';
 

1.建立檢視

語法：
create view 檢視名
as
查詢語句;

1.查詢姓名中包含a字元的員工名、部門名和工種資訊

#①建立
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建立檢視檢視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資訊

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`;

2.檢視的修改

方式一：

create or replace view  檢視名
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 檢視名
as 
查詢語句;

ALTER VIEW myv3
AS
SELECT * FROM employees;

3.刪除檢視

語法：
drop view 檢視名1,檢視名2,...;

DROP VIEW emp_v1,emp_v2,myv3;

4.檢視檢視

# 方式1
DESC myv3;
# 方式2
SHOW CREATE VIEW myv3;

5.檢視的更新

更新了檢視中資料，原始表中也會更新檢視中的資料

檢視一般用於查詢的，而不是更新的，一般檢視也不會更新資料，由於檢視更新限制條件，一般也不能更新

CREATE OR REPLACE VIEW myv1
AS
SELECT last_name,email
FROM employees;

SELECT * FROM myv1;

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';

具備以下特點的檢視不允許更新⭐

#①包含以下關鍵字的sql語句：分組函式、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; # 存在group by，不允許更新

SELECT * FROM myv1;

#更新失敗
UPDATE myv1 SET m=9000 WHERE department_id=10;

#②常量檢視
CREATE OR REPLACE VIEW myv2
AS
SELECT 'john' NAME;

SELECT * FROM myv2;

#更新失敗
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;

SELECT * FROM myv3;

#更新失敗
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;

SELECT * FROM myv4;

#更新
UPDATE myv4 SET last_name  = '張飛' WHERE last_name='Whalen';
INSERT INTO myv4 VALUES('陳真','xxxx');

#⑤from一個不能更新的檢視
CREATE OR REPLACE VIEW myv5
AS
SELECT * FROM myv3;

SELECT * FROM myv5;

#更新失敗
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
);

SELECT * FROM myv6;

#更新失敗
UPDATE myv6 SET salary=10000 WHERE last_name = 'k_ing';

檢視與實際表對比