SELECT

SELECT tid FROM `join` group by tid order by count(*) desc limit 2;

CREATE TABLE `join` (
  `id` int(25) NOT NULL auto_increment,
  `uid` int(25) NOT NULL,
  `tid` int(25) NOT NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 AUTO_INCREMENT=7 ;

--
-- 导出表中的数据 `join`
--

INSERT INTO `join` VALUES (1, 1, 1);
INSERT INTO `join` VALUES (2, 1, 2);
INSERT INTO `join` VALUES (3, 2, 2);
INSERT INTO `join` VALUES (4, 3, 1);
INSERT INTO `join` VALUES (5, 3, 3);
INSERT INTO `join` VALUES (6, 4, 2);