MySQL去重保留最大的那条记录（取最新的记录）

以用户登录日志表为例，取用户最近登录的设备

 1 set names utf8mb4;
 2 set foreign_key_checks = 0;
 3 
 4 -- ----------------------------
 5 -- table structure for t_login_log
 6 -- ----------------------------
 7 drop table if exists `t_login_log`;
 8 create table `t_login_log` (
 9   `id` int(11) not null auto_increment,
10   `user_id` int(11) not null comment '用户id',
11   `device_name` varchar(32) collate utf8mb4_bin not null comment '登录设备',
12   `login_time` datetime not null default current_timestamp on update current_timestamp comment '登录时间',
13   primary key (`id`)
14 ) engine=innodb auto_increment=9 default charset=utf8mb4 collate=utf8mb4_bin;
15 
16 -- ----------------------------
17 -- records of t_login_log
18 -- ----------------------------
19 begin;
20 insert into `t_login_log` values (1, 1121, 'iphone 6s', '2019-07-01 19:20:25');
21 insert into `t_login_log` values (2, 2120, 'vivo x20', '2019-06-28 16:21:11');
22 insert into `t_login_log` values (3, 1607, 'huawei p30', '2019-07-04 19:21:59');
23 insert into `t_login_log` values (4, 2120, 'vivo x20', '2019-06-30 19:22:34');
24 insert into `t_login_log` values (5, 2120, 'vivo x20', '2019-07-04 19:23:07');
25 insert into `t_login_log` values (6, 1121, 'ipad mini', '2019-07-03 19:23:25');
26 insert into `t_login_log` values (7, 1607, 'iphone 8 plus', '2019-06-30 19:24:06');
27 insert into `t_login_log` values (8, 1970, 'mi8', '2019-07-03 19:25:00');
28 commit;
29 
30 set foreign_key_checks = 1; 

自连接，取最新的记录

1 select * from t_login_log order by user_id;
2 
3 select 
4 	t1.* 
5 from t_login_log t1
6 	left join t_login_log t2 on t1.user_id = t2.user_id and t1.login_time < t2.login_time 
7 where t2.id is null; 

效果