在项目中用到了timestamp这个类型，该字段本意是用于存储改行记录的创建时间的，实际上这是一个很危险的设置；
mysql官方文档上有这么一段话：
The TIMESTAMP data type provides a type that you can use to automatically mark INSERT or UPDATE operations with the current date and time. If you have multiple TIMESTAMP columns in a table, only the first one is updated automatically. (From MySQL 4.1.2 on, you can specify which TIMESTAMP column updates;
意思大概是timestamp类型会随着insert或update更新而自动更新；
下面实际来测试一个案例：
mysql> create table b(ID int ,Time timestamp);
Query OK, 0 rows affected (0.01 sec)
mysql> insert into b values(1,now());
Query OK, 1 row affected (0.00 sec)
mysql> insert into b values(2,now());
Query OK, 1 row affected (0.01 sec)
-------------------------------------------------
mysql> select * from b;
+------+---------------------+
| ID | Time |
+------+---------------------+
| 1 | 2013-07-26 13:56:11 |
| 2 | 2013-07-26 13:57:43 |
+------+---------------------+
------------------------------------------------
mysql> update b set id=3 where id=2;
------------------------------------------------
mysql> select * from b;
+------+---------------------+
| ID | Time |
+------+---------------------+
| 1 | 2013-07-26 13:56:11 |
| 3 | 2013-07-26 16:35:43 |
+------+---------------------+
可以看出timestamp自动更新了；更多timestamp的信息可以查看mysql官方文档http://dev.mysql.com/doc/refman/4.1/en/timestamp-pre-4-1.html