该表的功能是显示在线用户，控制多点登陆，防止异常退出

CREATE TABLE IF NOT EXISTS `employees_online` (  `id` int(10) unsigned NOT NULL AUTO_INCREMENT,  `username` varchar(20) NOT NULL COMMENT 'User ID',  `ipaddr` varchar(15) NOT NULL COMMENT 'IP Address',  `expire` datetime NOT NULL COMMENT 'Expire time',  `status` enum('Login','Logout','Offline') NOT NULL DEFAULT 'Login' COMMENT 'Current Status',  `message` varchar(255) NOT NULL COMMENT 'Leave a message',  `ctime` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP COMMENT 'Created Time',  `mtime` timestamp NULL DEFAULT NULL ON UPDATE CURRENT_TIMESTAMP COMMENT 'Modified Time',  PRIMARY KEY (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8 COMMENT='show listing of last logged in users';DELIMITER //CREATE DEFINER=`dba`@`192.168.%` EVENT `employees_online` ON SCHEDULE EVERY 15 MINUTE STARTS '2014-08-22 10:33:24' ON COMPLETION NOT PRESERVE ENABLE COMMENT 'Employees Online Logging' DO BEGIN	update employees_online set `status` = 'Offline' where expire < now() and ctime > DATE_ADD(now(), INTERVAL -15 MINUTE);END//DELIMITER ;SET @OLDTMP_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_ENGINE_SUBSTITUTION';DELIMITER //CREATE TRIGGER `employees_online_before_delete` BEFORE DELETE ON `employees_online` FOR EACH ROW BEGIN	SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Permission denied', MYSQL_ERRNO = 1001;END//DELIMITER ;SET SQL_MODE=@OLDTMP_SQL_MODE;SET @OLDTMP_SQL_MODE=@@SQL_MODE, SQL_MODE='NO_ENGINE_SUBSTITUTION';DELIMITER //CREATE TRIGGER `employees_online_before_update` BEFORE UPDATE ON `employees_online` FOR EACH ROW BEGIN	SET NEW.`id` = OLD.id;	SET NEW.`username` = OLD.username;	SET NEW.`ipaddr` = OLD.ipaddr;	SET NEW.`message` = OLD.message;	SET NEW.`ctime` = OLD.ctime;END//DELIMITER ;SET SQL_MODE=@OLDTMP_SQL_MODE;

登陆日志将永久保存，防止数据被删除由触发器employees_online_before_delete负责

防止有人撰改登陆信息，由触发器employees_online_before_update负责，主要是防止撰改登陆名与IP地址，这样讲不能从其他电脑登陆，必须用户Logout才能在其他电脑登陆。

原文出处：Netkiller 系列 手札