一.視圖的介紹

含義：虛擬表，和普通表一樣使用
mysql5.1版本出現(xiàn)的新特性，是通過表動態(tài)生成的數(shù)據(jù)

舉例說明什么是視圖：假設一個年級有10個班，上面有領導來啦，說要檢查舞蹈功底，學校為了應付檢查，把每個班會跳舞的女孩子抽出來臨時組成一個舞蹈班，這個班只在領導來啦才存在。這個班就是視圖

案例說明講解：

#案例：查詢姓張的學生名和專業(yè)名
以前我們的做法

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

現(xiàn)在視圖的做法，把常用的封裝起來

CREATE VIEW v1 AS SELECT stuname,majorname FROM stuinfo s INNER JOIN major m ON s.`majorid`= m.`id`;

查詢時直接輸入

SELECT * FROM v1 WHERE stuname LIKE '張%';

二.視圖的創(chuàng)建

語法：
create view 視圖名
as
查詢語句;

一般是復雜的查詢語句，簡單的用不著

案例：#1.查詢姓名中包含a字符的員工名、部門名和工種信息

USE myemployees;#①創(chuàng)建 CREATE VIEW myv1 ASSELECT 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%';

其中涉及到的多表查詢
mysql數(shù)據(jù)庫入門教程（5）:多表操作(連接查詢,子查詢,分頁查詢,聯(lián)合查詢）

案例：#2.查詢各部門的平均工資級別

USE myemployees; #①創(chuàng)建視圖查看每個部門的平均工資 #1創(chuàng)建 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`;

三.視圖的修改

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

如果該視圖存在就修改，如果不存在就創(chuàng)建
案例修改
原始視圖

USE myemployees; #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`;

修改

視圖的修改CREATE OR REPLACE VIEW myv3 AS SELECT AVG(salary),job_id FROM employees GROUP BY job_id;SELECT * FROM myv3

#方式二：
/*
語法：
alter view 視圖名
as
查詢語句;
*/

語法例子

ALTER VIEW myv3 AS SELECT * FROM employees;

四.視圖的刪除

/*
語法：drop view 視圖名,視圖名,…;
*/

DROP VIEW emp_v1,emp_v2,myv3;

五.視圖的查看

DESC 視圖名

DESC myv3;

SHOW CREATE VIEW 視圖名；

SHOW CREATE VIEW myv3;

`

六.視圖的更新

更新 是更新視圖中的數(shù)據(jù)

案例1 失敗案例
首先 創(chuàng)建一個視圖

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

數(shù)據(jù)修改、

#1.插入INSERT INTO myv1 VALUES('張飛','zf@qq.com',100000);

案例2 成功案例
把案例1中的年薪去掉
創(chuàng)建視圖

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

插入數(shù)據(jù)
語法和表格插入一樣

#1.插入INSERT INTO myv1 VALUES('張飛','zf@qq.com');

#2.修改

UPDATE myv1 SET last_name = '張無忌' WHERE last_name='張飛';

#3 刪除

#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(shù)、distinct、group by、having、union或者union all

USE myemployees;#①包含以下關鍵字的sql語句：分組函數(shù)、distinct、group by、having、union或者union allCREATE 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;

報錯

#②常量視圖 -查詢語句后面是一個常量

CREATE OR REPLACE VIEW myv2 ASSELECT 'john' NAME;SELECT * FROM myv2;#更新 UPDATE myv2 SET NAME='lucy';

#③Select中包含子查詢

USE myemployees; CREATE OR REPLACE VIEW myv3 ASSELECT 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

USE myemployees; CREATE OR REPLACE VIEW myv4 ASSELECT 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 ASSELECT * 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 ASSELECT last_name,email,salary FROM employees WHERE employee_id IN(SELECT manager_idFROM employeesWHERE manager_id IS NOT NULL );#更新 SELECT * FROM myv6; UPDATE myv6 SET salary=10000 WHERE last_name = 'k_ing';

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