mysql相关 timestamp遇到的小问题

DROP TABLE IF EXISTS `user_log`;
CREATE TABLE `user_log` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键id',
  `o_id` char(24) NOT NULL COMMENT '订单id号',
  `uid` int(10) unsigned NOT NULL COMMENT '用户ID',
  `note` varchar(255) NOT NULL DEFAULT '' COMMENT '取消原因',
  `time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  PRIMARY KEY (`id`)

) ENGINE=MyISAM DEFAULT CHARSET=utf8 COMMENT='订单表';

 `time` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
注意这句话