写入值

DROP TABLE IF EXISTS `tp_users`;

CREATE TABLE `tp_users` (
  `user_id` int(11) unsigned NOT NULL AUTO_INCREMENT COMMENT '表id',
  `openid` varchar(100) NOT NULL DEFAULT '' COMMENT '第三方唯一标示',
  PRIMARY KEY (`user_id`)
) ENGINE=MyISAM AUTO_INCREMENT=18 DEFAULT CHARSET=utf8 CHECKSUM=1;

/*Data for the table `tp_users` */

insert  into `tp_users`(`user_id`,`openid`) values (12,'123');
insert  into `tp_users`(`user_id`,`openid`) values (13,'123');
insert  into `tp_users`(`user_id`,`openid`) values (15,'235');
insert  into `tp_users`(`user_id`,`openid`) values (16,'235');
insert  into `tp_users`(`user_id`,`openid`) values (17,'21');

如图

查出重复

SELECT openid,
   user_id
FROM tp_users WHERE openid IN(SELECT openid
               FROM   tp_users
               GROUP  BY openid
               HAVING COUNT(openid) > 1)

如图