mysql视图之确保视图的一致性(with check option)操

编程学习 2021-07-05 14:37www.dzhlxh.cn编程入门
这篇文章主要介绍了mysql视图之确保视图的一致性(with check option)操作,结合实例形式详细分析了视图的一致性操作原理、实现技巧与操作注意事项,需要的朋友可以参考下

本文实例讲述了mysql视图之确保视图的一致性(with check option)操作。分享给大家供大家参考,具体如下:

我们有的时候,会创建一个视图来显示表的部分数据。我们知道,简单视图是的,因此可以更新通过视图不可见的数据,但是此更新会使的视图不一致。为了确保视图的一致性,在创建或修改视图时使用WITH CHECK OPTION可更新子句。我们来看下WITH CHECK OPTION可更新子句的语法结构:

CREATE OR REPLACE VIEW view_name 
AS
 select_statement
 WITH CHECK OPTION;

我们要注意,将分号(;)放在WITH CHECK OPTION子句的末尾,而不是在select语句的末尾来定义视图。完事我们来尝试根据employees表创建一个名为vps的视图,以显示其职位为VP的员工,例如VP Marketing和 VP Sales:

CREATE OR REPLACE VIEW vps AS
  SELECT 
    employeeNumber,
    lastname,
    firstname,
    jobtitle,
    extension,
    email,
    officeCode,
    reportsTo
  FROM
    employees
  WHERE
    jobTitle LIKE '%VP%';

接下来,我们使用以下语句从vps视图中查询数:

mysql> SELECT * FROM vps;
+----------------+----------+-----------+--------------+-----------+----------------------+------------+-----------+
| employeeNumber | lastname | firstname | jobtitle   | extension | email        | officeCode | reportsTo |
+----------------+----------+-----------+--------------+-----------+----------------------+------------+-----------+
|      1056 | Hill   | Mary   | VP Sales   | x4611   | mary.hill@yiibai.com | 1     |   1002 |
|      1076 | Firrelli | Jeff   | VP Marketing | x9273   | jfirrelli@yiibai.com | 1     |   1002 |
+----------------+----------+-----------+--------------+-----------+----------------------+------------+-----------+
2 rows in set

因为vps是一个简单的视图,因此它是可更新的,所以,我们通过vps视图将一行员工数据信息插入:

INSERT INTO vps(employeeNumber,firstname,lastname,jobtitle,extension,email,officeCode,reportsTo)
values(1703,'Lily','Bush','IT Manager','x9111','lilybush@yiiibai.com',1,1002);

我们要注意,新创建的员工通过vps视图不可见,因为她的职位是IT经理,而不是VP。使用以下SELECT语句来验证它:

SELECT * FROM employees WHERE employeeNumber=1703;

执行上面语句,得到以下结果:

+----------------+-----------+-----------+-----------+-----------------------+------------+-----------+----------------------+
| employeeNumber | lastName | firstName | extension | email         | officeCode | reportsTo | jobTitle       |
+----------------+-----------+-----------+-----------+-----------------------+------------+-----------+----------------------+
|      1703 | Bush   | Lily   | x9111   | lilybush@yiiibai.com | 1     |   1002 | IT Manager      |
|      1702 | Gerard  | Martin  | x2312   | mgerard@gmail.com   | 4     |   1102 | Sales Rep      |
|      1625 | Kato   | Yoshimi  | x102   | ykato@gmail.com    | 5     |   1621 | Sales Rep      |
|      1621 | Nishi   | Mami   | x101   | mnishi@gmail.com   | 5     |   1056 | Sales Rep      |

但这可能不是我们想要的,因为通过vps视图暴露VP员工,而不是其他员工,所以,为了确保视图的一致性,用户只能显示或更新通过视图可见的数据,则在创建或修改视图时使用WITH CHECK OPTION:

CREATE OR REPLACE VIEW vps AS
  SELECT 
    employeeNumber,
    lastname,
    firstname,
    jobtitle,
    extension,
    email,
    officeCode,
    reportsTo
  FROM
    employees
  WHERE
    jobTitle LIKE '%VP%' 
WITH CHECK OPTION;

我们要注意在CREATE OR REPLACE语句的结尾处加上WITH CHECK OPTION子句,完事再次通过vps视图将一行插入employees表中,如下所示:

INSERT INTO vps(employeeNumber,firstname,lastname,jobtitle,extension,email,officeCode,reportsTo)
VALUES(1704,'John','Minsu','IT Staff','x9112','johnminsu@yiibai.com',1,1703);

这时mysql会拒绝插入并发出以下错误消息:

Error Code: 1369 - CHECK OPTION failed 'luyaran.vps'

我们可以通过vps视图将一个职位为SVP Marketing的员工插入employees表,看看mysql是否允许这样做:

INSERT INTO vps(employeeNumber,firstname,lastname,jobtitle,extension,email,officeCode,reportsTo)
VALUES(1704,'John','Minsu','SVP Marketing','x9112','johnminsu@classicmodelcars.com',1,1076);

mysql发出1行受影响(Query OK, 1 row affected),我们可以通过根据vps视图查询数据来再次验证插入操作:

SELECT * FROM vps;

如上查询结果所示,它的确按预期工作了:

mysql> SELECT * FROM vps;
+----------------+----------+-----------+---------------+-----------+--------------------------------+------------+-----------+
| employeeNumber | lastname | firstname | jobtitle   | extension | email             | officeCode | reportsTo |
+----------------+----------+-----------+---------------+-----------+--------------------------------+------------+-----------+
|      1056 | Hill   | Mary   | VP Sales   | x4611   | mary.hill@yiibai.com      | 1     |   1002 |
|      1076 | Firrelli | Jeff   | VP Marketing | x9273   | jfirrelli@yiibai.com      | 1     |   1002 |
|      1704 | Minsu  | John   | SVP Marketing | x9112   | johnminsu@classicmodelcars.com | 1     |   1076 |
+----------------+----------+-----------+---------------+-----------+--------------------------------+------------+-----------+
3 rows in set

好啦,本次记录就到这里了。

更多关于MySQL相关内容感兴趣的读者可查看本站专题:《》、《》、《》、《》及《》

希望本文所述对大家MySQL数据库计有所帮助。

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