MySQL视图可能不一致的地方，我们如何确保它们的一致性？
在可更新视图的情况下，很有可能我们会更新视图中不可见的数据，因为我们创建的视图仅显示表的部分数据。这种更新会使视图不一致。我们可以在创建或修改视图时使用WITHCHECKOPTION来确保视图的一致性。尽管WITHCHECKOPTION子句是CREATEVIEW语句的可选部分，但使视图保持一致非常有用。
基本上，WITHCHECKOPTION子句阻止我们更新或插入通过视图不可见的行。简单来说，我们可以说，在使用WITHCHECKOPTION子句之后，MySQL确保通过视图定义确认插入或更新操作。以下是WITHCHECKOPTION子句的语法-
语法
CREATE OR REPLACE VIEW view_name AS Select_statement WITH CHECK OPTION;
示例
为了说明上述概念，我们使用表'Student_info'中的以下数据-
mysql> Select * from student_info; +------+---------+------------+------------+ | id | Name | Address | Subject | +------+---------+------------+------------+ | 101 | YashPal | Amritsar | History | | 105 | Gaurav | Chandigarh | Literature | | 125 | Raman | Shimla | Computers | | 130 | Ram | Jhansi | Computers | +------+---------+------------+------------+ 4 rows in set (0.08 sec)
现在，借助以下查询，我们将创建视图名称“Info”。在这里，我们不使用WITHCHECKOPTION。
mysql> Create OR Replace VIEW Info AS Select Id, Name, Address, Subject from student_info WHERE Subject = 'Computers'; mysql> Select * from info; +------+-------+---------+-----------+ | Id | Name | Address | Subject | +------+-------+---------+-----------+ | 125 | Raman | Shimla | Computers | | 130 | Ram | Jhansi | Computers | +------+-------+---------+-----------+ 2 rows in set (0.00 sec)
因为我们还没有使用WITHCHECKOPTION，所以即使它的定义不匹配，我们也可以在'Info'中插入/更新新行。以下查询及其结果对此进行了说明-
mysql> INSERT INTO Info(Id, Name, Address, Subject) values(132, 'Shyam','Chandigarh', 'Economics'); mysql> Select * from student_info; +------+---------+------------+------------+ | id | Name | Address | Subject | +------+---------+------------+------------+ | 101 | YashPal | Amritsar | History | | 105 | Gaurav | Chandigarh | Literature | | 125 | Raman | Shimla | Computers | | 130 | Ram | Jhansi | Computers | | 132 | Shyam | Chandigarh | Economics | +------+---------+------------+------------+ 5 rows in set (0.00 sec) mysql> Select * from info; +------+-------+---------+-----------+ | Id | Name | Address | Subject | +------+-------+---------+-----------+ | 125 | Raman | Shimla | Computers | | 130 | Ram | Jhansi | Computers | +------+-------+---------+-----------+ 2 rows in set (0.00 sec)
上面的结果集表明，新行与“Info”的定义不匹配，因此在视图中不可见。现在，在下面的查询中，我们将创建相同的视图“Info”
通过使用“WITHCHECKOPTION”-
mysql> Create OR Replace VIEW Info AS Select Id, Name, Address, Subject from student_info WHERE Subject = 'Computers' WITH CHECK OPTION;
现在，如果我们尝试插入与视图“Info”的定义匹配的行，MySQL允许我们这样做。可以从以下查询及其结果中清除它。
mysql> INSERT INTO Info(Id, Name, Address, Subject) values(133, 'Mohan','Delhi','Computers'); mysql> Select * from info; +------+-------+---------+-----------+ | Id | Name | Address | Subject | +------+-------+---------+-----------+ | 125 | Raman | Shimla | Computers | | 130 | Ram | Jhansi | Computers | | 133 | Mohan | Delhi | Computers | +------+-------+---------+-----------+ 3 rows in set (0.00 sec)
但是假设如果我们尝试插入与视图'Info'的定义不匹配的行，MySQL将不允许我们这样做并引发错误-
mysql> INSERT INTO Info(Id, Name, Address, Subject) values(134, 'Charanjeet','Amritsar','Geophysics'); ERROR 1369 (HY000): CHECK OPTION failed