MySQL视图在哪些地方可能会不一致，如何保证它们的一致性？

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;

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”。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

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