实现中的例子用触发器审计记录，既然你了解了触发器的基本原理，让我们来看一个稍稍复杂的例子。我们常用触发器来建立一个自动“审计记录”，以记录各种用户对数据库的更改。为了解审计记录的实际应用，请看下面的表格（表格名：accounts），它列出了一个用户的三个银行账户余额。
mysql> SELECT * FROM accounts; +----+------------+---------+ | id | label| balance | +----+------------+---------+ |1 | Savings #1 |500 | |2 | Current #1 |2000 | |3 | Current #2 |3500 | +----+------------+---------+ 3 rows in set (0.00 sec)
然后，检查触发器是否被激活：
mysql> SHOW TRIGGERS \G *************************** 1. row *************************** ?Trigger: t1 ?Event: UPDATE ?Table: accounts Statement: INSERT INTO audit (id, balance, user, time) VALUES (OLD.id, NEW.balance, CURRENT_USER(), NOW()) Timing: AFTER ?Created: NULL Sql_mode: 1 row in set (0.01 sec)
再来看最后的结果：
mysql> UPDATE accounts SET balance = 500 WHERE id = 1; Query OK, 1 row affected (0.00 sec) Rows matched: 1?Changed: 1?Warnings: 0 mysql> UPDATE accounts SET balance = 900 WHERE id = 3; Query OK, 1 row affected (0.01 sec) Rows matched: 1?Changed: 1?Warnings: 0 mysql> UPDATE accounts SET balance = 1900 WHERE id = 1; Query OK, 1 row affected (0.00 sec) Rows matched: 1?Changed: 1?Warnings: 0
注意，对accounts表格所作的改变已被记录到audit表格中，将来如果出现问题，我们可以方便地从中进行恢复。
mysql> SELECT * FROM audit; +------+---------+----------------+---------------------+ | id| balance | user| time| +------+---------+----------------+---------------------+ |1 |500 | root@localhost | 2006-04-22 12:52:15 | |3 |900 | root@localhost | 2006-04-22 12:53:15 | |1 |1900 | root@localhost | 2006-04-22 12:53:23 | +------+---------+----------------+---------------------+ 3 rows in set (0.00 sec)
如上面的例子所示，触发器是一个强大的新功能，它大大增强了RDBMS的自动化程度。自己去试验，练习吧！
