在可更新视图的情况下，我们很可能更新通过视图不可见的数据，因为我们创建的视图仅显示表的部分数据。这种更新使得视图不一致。我们在创建或修改视图时可以通过使用WITH CHECK OPTION来保证视图的一致性。虽然WITH CHECK OPTION子句是CREATE VIEW语句的可选部分，但它对于使视图保持一致非常有用。
基本上，WITH CHECK OPTION子句可以防止我们更新或插入以下行：通过视图不可见。简单来说，我们可以说，使用WITH CHECK OPTION子句后，MySQL确保插入或更新操作得到视图定义的确认。以下是WITH CHECK OPTION 子句的语法 -
CREATE OR REPLACE VIEW view_name AS Select_statement WITH CHECK OPTION;
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”。这里我们没有使用WITH CHECK OPTION。
mysql> Create OR Replace VIEW Info AS Select Id, Name, Address, Subject from student_info WHERE Subject = 'Computers'; Query OK, 0 rows affected (0.46 sec) 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，我们没有使用WITH CHECK OPTION，所以我们可以在“Info”中插入/更新新行，即使它与其定义不匹配。下面的查询及其结果说明了这一点 -
mysql> INSERT INTO Info(Id, Name, Address, Subject) values(132, 'Shyam','Chandigarh', 'Economics'); Query OK, 1 row affected (0.37 sec) 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)
上面的结果集显示新行与“Info”的定义不匹配，因此它在视图中不可见。现在，在以下查询中，我们将创建相同的视图“Info”
通过使用“WITH CHECK OPTION” -
mysql> Create OR Replace VIEW Info AS Select Id, Name, Address, Subject from student_info WHERE Subject = 'Computers' WITH CHECK OPTION; Query OK, 0 rows affected (0.06 sec)
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。可以从下面的查询及其结果中清除它。
mysql> INSERT INTO Info(Id, Name, Address, Subject) values(133, 'Mohan','Delhi','Computers'); Query OK, 1 row affected (0.07 sec) 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抛出错误 -
mysql> INSERT INTO Info(Id, Name, Address, Subject) values(134, 'Charanjeet','Amritsar','Geophysics'); ERROR 1369 (HY000): CHECK OPTION failed
以上是MySQL视图在哪些地方可能会不一致，如何保证它们的一致性？的详细内容。更多信息请关注PHP中文网其他相关文章！
