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。