mysql> SELECT VERSION();
+------------+
| VERSION()  |
+------------+
| 5.5.29-log |
+------------+
1 row in set (0.00 sec)

如今有这种需求，一张表中有一个字段created_at记录创建该条记录的时间戳，另一个字段updated_at记录更新该条记录的时间戳。
我们尝试下面几个语句。

第一个，測试通过。

CREATE TABLE temp
(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

第二个，測试不通过。报ERROR 1293 (HY000)错误。（完整错误信息：ERROR 1293 (HY000): Incorrect table definition; there can be only one TIMESTAMP column with CURRENT_TIMESTAMP in DEFAULT or ON UPDATE clause）

CREATE TABLE temp
(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT CURRENT_TIMESTAMP,updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

MySQL 5.5.29中有这种奇葩限制，不明白为什么。既然有这种限制。那么仅仅有绕道而行。如今尝试给出例如以下几种解决的方法。

第一种，created_at使用DEFAULT CURRENT_TIMESTAMP或者DEFAULT now()，updated_at使用触发器。
详细解决方法例如以下：
1.temp表结构例如以下：

CREATE TABLE temp
(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT CURRENT_TIMESTAMP,updated_at timestamp NULL
);

2.插入測试数据：

mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('robin',now(),now());
Query OK, 1 row affected (0.03 sec)mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('wentasy',now(),now());
Query OK, 1 row affected (0.01 sec)mysql> SELECT * FROM temp;
+----+---------+---------------------+---------------------+
| id | name    | created_at          | updated_at          |
+----+---------+---------------------+---------------------+
|  1 | robin   | 2014-09-01 14:00:39 | 2014-09-01 14:00:39 |
|  2 | wentasy | 2014-09-01 14:01:11 | 2014-09-01 14:01:11 |
+----+---------+---------------------+---------------------+
2 rows in set (0.00 sec)

3.在temp上创建触发器，实现更新时记录更新时间；

delimiter |
DROP TRIGGER IF EXISTS tri_temp_updated_at;
CREATE TRIGGER tri_temp_updated_at BEFORE UPDATE ON temp
FOR EACH ROW
BEGINSET NEW.updated_at = now();
END;
|
delimiter ;

4.測试。

mysql> UPDATE temp SET name='robinwen' WHERE id=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0#能够看到已经记录了第一条数据的更新时间
mysql> SELECT * FROM temp;
+----+----------+---------------------+---------------------+
| id | name     | created_at          | updated_at          |
+----+----------+---------------------+---------------------+
|  1 | robinwen | 2014-09-01 14:00:39 | 2014-09-01 14:03:05 |
|  2 | wentasy  | 2014-09-01 14:01:11 | 2014-09-01 14:01:11 |
+----+----------+---------------------+---------------------+
2 rows in set (0.00 sec)

另外一种，created_at使用触发器。updated_at使用DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP或者DEFAULT now() ON UPDATE now()。

详细解决方法例如以下：
1.temp表结构例如以下：

CREATE TABLE temp
(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL,updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

2.在temp上创建触发器。实现插入数据记录创建时间。

delimiter |
DROP TRIGGER IF EXISTS tri_temp_created_at;
CREATE TRIGGER tri_temp_created_at BEFORE INSERT ON temp
FOR EACH ROW
BEGINIF new.created_at IS NULLTHENSET new.created_at=now();END IF;
END;
|
delimiter ;

3.插入測试数据：

mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('robin',now(),now());
Query OK, 1 row affected (0.01 sec)mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('wentasy',now(),now());
Query OK, 1 row affected (0.01 sec)mysql> SELECT * FROM temp;
+----+---------+---------------------+---------------------+
| id | name    | created_at          | updated_at          |
+----+---------+---------------------+---------------------+
|  1 | robin   | 2014-09-01 14:08:36 | 2014-09-01 14:08:36 |
|  2 | wentasy | 2014-09-01 14:08:44 | 2014-09-01 14:08:44 |
+----+---------+---------------------+---------------------+
2 rows in set (0.00 sec)

4.測试。

mysql> UPDATE temp SET name='robinwen' WHERE id=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0#能够看到已经记录了第一条数据的更新时间
mysql> SELECT * FROM temp;
+----+----------+---------------------+---------------------+
| id | name     | created_at          | updated_at          |
+----+----------+---------------------+---------------------+
|  1 | robinwen | 2014-09-01 14:08:36 | 2014-09-01 14:09:09 |
|  2 | wentasy  | 2014-09-01 14:08:44 | 2014-09-01 14:08:44 |
+----+----------+---------------------+---------------------+
2 rows in set (0.00 sec)

第三种，created_at指定timestamp DEFAULT '0000-00-00 00:00:00'，updated_at指定DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP或者timestamp DEFAULT now() ON UPDATE now()。
详细解决方法例如以下：
1.temp表结构例如以下：

CREATE TABLE temp
(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT '0000-00-00 00:00:00',updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);

2.插入測试数据：

mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('robin',now(),now());
Query OK, 1 row affected (0.01 sec)mysql> INSERT INTO temp(name,created_at,updated_at) VALUES('wentasy',now(),now());
Query OK, 1 row affected (0.01 sec)mysql> SELECT * FROM temp;
+----+---------+---------------------+---------------------+
| id | name    | created_at          | updated_at          |
+----+---------+---------------------+---------------------+
|  1 | robin   | 2014-09-01 14:10:43 | 2014-09-01 14:10:43 |
|  2 | wentasy | 2014-09-01 14:10:57 | 2014-09-01 14:10:57 |
+----+---------+---------------------+---------------------+
2 rows in set (0.00 sec)

3.測试。

mysql> UPDATE temp SET name='robinwen' WHERE id=1;
Query OK, 1 row affected (0.01 sec)
Rows matched: 1  Changed: 1  Warnings: 0#能够看到已经记录了第一条数据的更新时间
mysql> SELECT * FROM temp;
+----+----------+---------------------+---------------------+
| id | name     | created_at          | updated_at          |
+----+----------+---------------------+---------------------+
|  1 | robinwen | 2014-09-01 14:10:43 | 2014-09-01 14:11:24 |
|  2 | wentasy  | 2014-09-01 14:10:57 | 2014-09-01 14:10:57 |
+----+----------+---------------------+---------------------+
2 rows in set (0.00 sec)

第四种，更换MySQL版本号，MySQL 5.6已经去除了此限制。

我们能够看下MySQL 5.5和5.6帮助文档对于这个问题的解释。

From the MySQL 5.5 documentation:

One TIMESTAMP column in a table can have the current timestamp as the default value for initializing the column, as the auto-update value, or both. It is not possible to have the current timestamp be the default value for one column and the auto-update value for another column.

Changes in MySQL 5.6.5:

Previously, at most one TIMESTAMP column per table could be automatically initialized or updated to the current date and time. This restriction has been lifted. Any TIMESTAMP column definition can have any combination of DEFAULT CURRENT_TIMESTAMP and ON UPDATE CURRENT_TIMESTAMP clauses. In addition, these clauses now can be used with DATETIME column definitions. For more information, see Automatic Initialization and Updating for TIMESTAMP and DATETIME.

我们确定下MySQL的版本号。

mysql> SELECT VERSION();
+---------------------------------------+
| VERSION()                             |
+---------------------------------------+
| 5.6.20-enterprise-commercial-advanced |
+---------------------------------------+
1 row in set (0.00 sec)

我们把文首測试不通过的SQL语句在MySQL 5.6下运行。能够看到没有不论什么错误。

CREATE TABLE temp
(id INT(11) PRIMARY KEY AUTO_INCREMENT,name VARCHAR(10),created_at timestamp NULL DEFAULT CURRENT_TIMESTAMP,updated_at timestamp NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
);
Query OK, 0 rows affected (0.28 sec)

接着我们插入測试语句，并作測试。

mysql> INSERT INTO temp(name) VALUES('robin');
Query OK, 1 row affected (0.07 sec)mysql> INSERT INTO temp(name) VALUES('wentasy');
Query OK, 1 row affected (0.00 sec)mysql> SELECT * FROM temp;
+----+---------+---------------------+---------------------+
| id | name    | created_at          | updated_at          |
+----+---------+---------------------+---------------------+
|  1 | robin   | 2014-09-01 15:05:57 | 2014-09-01 15:05:57 |
|  2 | wentasy | 2014-09-01 15:06:02 | 2014-09-01 15:06:02 |
+----+---------+---------------------+---------------------+
2 rows in set (0.01 sec)mysql> UPDATE temp SET name='robinwen' WHERE id=1;
Query OK, 1 row affected (0.02 sec)
Rows matched: 1  Changed: 1  Warnings: 0#能够看到已经记录了第一条数据的更新时间
mysql> SELECT * FROM temp;
+----+----------+---------------------+---------------------+
| id | name     | created_at          | updated_at          |
+----+----------+---------------------+---------------------+
|  1 | robinwen | 2014-09-01 15:05:57 | 2014-09-01 15:06:45 |
|  2 | wentasy  | 2014-09-01 15:06:02 | 2014-09-01 15:06:02 |
+----+----------+---------------------+---------------------+
2 rows in set (0.00 sec)

总结
本文介绍的方法归根结底。就两条，一是建表语句指定默认值和更新动作。二是使用触发器插入默认值和更新时间。面对当前无法更改的事实，仅仅能採取折中的办法或者牺牲很多其它来弥补。另一条值得注意的是，遇到问题多想想不同的解决的方法。尽可能地列出全部可能或者可行的方案。这样一来让自己学到很多其它，二来能够锻炼思维的广度，三来多种方案能够弥补某种方案在特定环境下不可行的不足。

Good Luck!

Robin

2014年9月1日