【mysql】【分组】后取每组的top2

 DROP TABLE IF EXISTS `tb1`;
 CREATE TABLE `tb1` (
   `id` int(11) NOT NULL AUTO_INCREMENT,
   `a` varchar(255) DEFAULT NULL,
   `b` varchar(255) DEFAULT NULL,
   `content` varchar(255) DEFAULT NULL,
   PRIMARY KEY (`id`)
 )
 -- ----------------------------
 INSERT INTO `tb1` VALUES ('', 'a', '', 'aaa');
 INSERT INTO `tb1` VALUES ('', 'a', '', 'bbb');
 INSERT INTO `tb1` VALUES ('', 'a', '', 'ccc');
 INSERT INTO `tb1` VALUES ('', 'b', '', 'ddd');
 INSERT INTO `tb1` VALUES ('', 'b', '', 'eee');

代码:

 select *
 from tb1 k
 where 2>(select count(*) from tb1 where k.a=a and id>k.id);

效果:

原理:

待续...