`created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP
`updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Updated At',

`created_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Created At',
`updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Updated At',

mysql> CREATE TABLE mt8
    -> (
    ->   billy int,
    ->   created_at timestamp NOT NULL DEFAULT '0000-00-00 00:00:00' COMMENT 'Created At',
    ->   updated_at timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP COMMENT 'Created At'
    -> );
Query OK, 0 rows affected (0.16 sec)

mysql>
mysql>
mysql> DELIMITER $$
mysql> create trigger my_ins8_trigger
    -> before insert on mt8
    -> for each row
    -> begin
    ->   set new.created_at = current_timestamp;
    -> end $$
Query OK, 0 rows affected (0.08 sec)

mysql> DELIMITER ;
mysql>
mysql>
mysql> insert into mt8 values(123, null, null);
Query OK, 1 row affected (0.08 sec)

mysql> insert into mt8 values(454, null, null);
Query OK, 1 row affected (0.05 sec)

mysql> insert into mt8 values(1676, null, null);
Query OK, 1 row affected (0.06 sec)

mysql> insert into mt8 (billy) values(34334);
Query OK, 1 row affected (0.07 sec)

mysql> insert into mt8 (billy) values(3506);
Query OK, 1 row affected (0.05 sec)

mysql> insert into mt8 (billy) values(3435454);
Query OK, 1 row affected (0.05 sec)

mysql> select * from mt8;
+---------+---------------------+---------------------+
| billy   | created_at          | updated_at          |
+---------+---------------------+---------------------+
|     123 | 2014-06-12 01:53:40 | 2014-06-12 01:53:40 |
|     454 | 2014-06-12 01:53:40 | 2014-06-12 01:53:40 |
|    1676 | 2014-06-12 01:53:40 | 2014-06-12 01:53:40 |
|   34334 | 2014-06-12 01:53:56 | 2014-06-12 01:53:56 |
|    3506 | 2014-06-12 01:53:56 | 2014-06-12 01:53:56 |
| 3435454 | 2014-06-12 01:53:56 | 2014-06-12 01:53:56 |
+---------+---------------------+---------------------+
6 rows in set (0.00 sec)

mysql> update mt8 set billy = 455 where billy = 454;
Query OK, 1 row affected (0.06 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from mt8;
+---------+---------------------+---------------------+
| billy   | created_at          | updated_at          |
+---------+---------------------+---------------------+
|     123 | 2014-06-12 01:53:40 | 2014-06-12 01:53:40 |
|     455 | 2014-06-12 01:53:40 | 2014-06-12 01:54:26 |
|    1676 | 2014-06-12 01:53:40 | 2014-06-12 01:53:40 |
|   34334 | 2014-06-12 01:53:56 | 2014-06-12 01:53:56 |
|    3506 | 2014-06-12 01:53:56 | 2014-06-12 01:53:56 |
| 3435454 | 2014-06-12 01:53:56 | 2014-06-12 01:53:56 |
+---------+---------------------+---------------------+
6 rows in set (0.00 sec)
mysql>