MySQL视图(view)

view acts as a virtual table. Views are available in binary releases from 5.0.1 and up. 视图是从5.0 开始支持，是一张虚拟

从5.0 开始就支持）
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 是派生列

UPDATE v SET col1 = 0;# 允许

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'

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