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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 | [email protected] | 1 | 1002 | | 1076 | Firrelli | Jeff | VP Marketing | x9273 | [email protected] | 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','[email protected]',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 | [email protected] | 1 | 1002 | IT Manager | | 1702 | Gerard | Martin | x2312 | [email protected] | 4 | 1102 | Sales Rep | | 1625 | Kato | Yoshimi | x102 | [email protected] | 5 | 1621 | Sales Rep | | 1621 | Nishi | Mami | x101 | [email protected] | 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','[email protected]',1,1703); 这时mysql会拒绝插入并发出以下错误消息:
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','[email protected]',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 | [email protected] | 1 | 1002 | | 1076 | Firrelli | Jeff | VP Marketing | x9273 | [email protected] | 1 | 1002 | | 1704 | Minsu | John | SVP Marketing | x9112 | [email protected] | 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:《MySQL查询技巧大全》、《MySQL事务操作技巧汇总》、《MySQL存储过程技巧大全》、《MySQL数据库锁相关技巧汇总》及《MySQL常用函数大汇总》 希望本文所述对大家MySQL数据库计有所帮助。 |
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