【mysql】【分组】后取每组的top2

 1 DROP TABLE IF EXISTS `tb1`;
 2 CREATE TABLE `tb1` (
 3   `id` int(11) NOT NULL AUTO_INCREMENT,
 4   `a` varchar(255) DEFAULT NULL,
 5   `b` varchar(255) DEFAULT NULL,
 6   `content` varchar(255) DEFAULT NULL,
 7   PRIMARY KEY (`id`)
 8 )  
 9 -- ----------------------------
10 INSERT INTO `tb1` VALUES (‘1‘, ‘a‘, ‘1‘, ‘aaa‘);
11 INSERT INTO `tb1` VALUES (‘2‘, ‘a‘, ‘2‘, ‘bbb‘);
12 INSERT INTO `tb1` VALUES (‘3‘, ‘a‘, ‘3‘, ‘ccc‘);
13 INSERT INTO `tb1` VALUES (‘4‘, ‘b‘, ‘1‘, ‘ddd‘);
14 INSERT INTO `tb1` VALUES (‘5‘, ‘b‘, ‘2‘, ‘eee‘);

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