MySQL視圖(view)

一.A view acts as a virtual table. Views are available in binary releases from 5.0.1 and up.
視圖是從5.0 開始支援，是一張虛擬表，在二進位的5.0.1以及更高的版本都支援。

二.Updatable and Insertable Views—update和insert 視圖（從5.0 開始就支援）
A view that has a mix of simple column references and derived columns is not insertable, but it can be updatable if you update only those columns that are not derived. Consider this view:

CREATE VIEW v AS SELECT col1, 1 AS col2 FROM t; #col2 是衍生的資料行

This view is not insertable because col2 is derived from an expression. But it is updatable if the update does not try to update col2. This update is permissible:
UPDATE v SET col1 = 0;# 允許

This update is not permissible because it attempts to update a derived column:
UPDATE v SET col2 = 0;# 不允許
3.The WITH CHECK OPTION clause can be given for an updatable view to prevent inserts or updates to rows except those for which the WHERE clause in the select_statement is true.
In a WITH CHECK OPTION clause for an updatable view, the LOCAL and CASCADED keywords determine the scope of check testing when the view is defined in terms of another view. The LOCAL keyword restricts the CHECK OPTION only to the view being defined。CASCADED causes the checks for underlying views to be evaluated as well. When neither keyword is given, the default is CASCADED. Consider the definitions for the following table and set of views:
mysql> CREATE TABLE t1 (a INT);
mysql> CREATE VIEW v1 AS SELECT * FROM t1 WHERE a < 2
-> WITH CHECK OPTION;
mysql> CREATE VIEW v2 AS SELECT * FROM v1 WHERE a > 0
-> WITH LOCAL CHECK OPTION;
mysql> CREATE VIEW v3 AS SELECT * FROM v1 WHERE a > 0
-> WITH CASCADED CHECK OPTION;
Here the v2 and v3 views are defined in terms of another view, v1. v2 has a LOCAL check option, so inserts are tested only against the v2 check. v3 has a CASCADED check option, so inserts are tested not only against its own check, but against those of underlying views. The following statements illustrate these differences:
mysql> INSERT INTO v2 VALUES (2);
Query OK, 1 row affected (0.00 sec)
mysql> INSERT INTO v3 VALUES (2);
ERROR 1369 (HY000): CHECK OPTION failed 'test.v3'

V3 建立在v1上，且用WITH CASCADED CHECK OPTION; 對下層的視圖v1 的定義檢查，違反了v1的定義，報錯！！

相關閱讀：MySQL視圖表建立與修改