MySQL之——GROUPBY分组取字段最大值，假设有一个业务场景，需要查询用户登录记录信息，其中表结构如下：

CREATE TABLE `tb` (  `id` int(11) NOT NULL AUTO_INCREMENT,  `uid` int(11) NOT NULL,  `ip` varchar(16) NOT NULL,  `login_time` datetime,  PRIMARY KEY (`id`),  KEY (`uid`));

再来点测试数据：

INSERT INTO tb SELECT null, 1001, '192.168.1.1', '2017-01-21 16:30:47';INSERT INTO tb SELECT null, 1003, '192.168.1.153', '2017-01-21 19:30:51';INSERT INTO tb SELECT null, 1001, '192.168.1.61', '2017-01-21 16:50:41';INSERT INTO tb SELECT null, 1002, '192.168.1.31', '2017-01-21 18:30:21';INSERT INTO tb SELECT null, 1002, '192.168.1.66', '2017-01-21 19:12:32';INSERT INTO tb SELECT null, 1001, '192.168.1.81', '2017-01-21 19:53:09';INSERT INTO tb SELECT null, 1001, '192.168.1.231', '2017-01-21 19:55:34';

表数据情况：

+----+------+---------------+---------------------+| id | uid  | ip            | login_time          |+----+------+---------------+---------------------+| 1  | 1001 | 192.168.1.1   | 2017-01-21 16:30:47 || 2  | 1003 | 192.168.1.153 | 2017-01-21 19:30:51 || 3  | 1001 | 192.168.1.61  | 2017-01-21 16:50:41 || 4  | 1002 | 192.168.1.31  | 2017-01-21 18:30:21 || 5  | 1002 | 192.168.1.66  | 2017-01-21 19:12:32 || 6  | 1001 | 192.168.1.81  | 2017-01-21 19:53:09 || 7  | 1001 | 192.168.1.231 | 2017-01-21 19:55:34 |+----+------+---------------+---------------------+

如果只需要针对用户查出其最后登录的时间，可以简单写出：

SELECT uid, max(login_time)FROM tbGROUP BY uid;

+------+---------------------+| uid  | max(login_time)       |+------+---------------------+| 1001 | 2017-01-21 19:55:34 || 1002 | 2017-01-21 19:12:32 || 1003 | 2017-01-21 19:30:51 |+------+---------------------+

若还需要查询用户最后登录时的其他信息，就不能用这种sql写了：

-- 错误写法SELECT uid, ip, max(login_time)FROM tbGROUP BY uid;-- 错误写法

这样的语句是非SQL标准的，虽然能够在MySQL数据库中执行成功，但返回的却是未知的
（如果sql_mode开启了only_full_group_by，则不会执行成功。）
可能ip字段会取uid分组前的第一个row的值，显然不是所需信息
写法1
写一个子查询：

SELECT a.uid, a.ip, a.login_timeFROM tb aWHERE a.login_time in (SELECT max(login_time)FROM tbGROUP BY uid);

写法2
再或者换一个写法：

SELECT a.uid, a.ip, a.login_timeFROM tb aWHERE a.login_time = (SELECT max(login_time)FROM tbWHERE a.uid = uid);

顺便测了一下
在5.6以前的版本中，写法②这条sql在大数据量的情况下，执行计划不理想，目测性能不佳。
在5.6及以后的版本中，写法②这条sql会快很多，执行计划也有了改变
5.5.50：

+----+--------------------+-------+------+---------------+------+---------+------+------+-------------+| id | select_type        | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |+----+--------------------+-------+------+---------------+------+---------+------+------+-------------+| 1  | PRIMARY            | a     | ALL  | NULL             | NULL  | NULL      | NULL | 7    | Using where || 2  | DEPENDENT SUBQUERY | tb    | ALL  | uid           | NULL  | NULL      | NULL | 7    | Using where |+----+--------------------+-------+------+---------------+------+---------+------+------+-------------+

5.6.30：

+----+--------------------+-------+------+---------------+------+---------+------------+------+-------------+| id | select_type        | table  | type | possible_keys | key  | key_len | ref       | rows  | Extra      |+----+--------------------+-------+------+---------------+------+---------+------------+------+-------------+| 1  | PRIMARY            | a     | ALL  | NULL              | NULL | NULL      | NULL        | 7    | Using where || 2  | DEPENDENT SUBQUERY | tb    | ref  | uid           | uid  | 4       | test.a.uid | 1    | NULL           |+----+--------------------+-------+------+---------------+------+---------+------------+------+-------------+

写法3
直接改成join性能会更加好：

SELECT a.uid, a.ip, a.login_timeFROM (SELECT uid, max(login_time) login_timeFROM tbGROUP BY uid) b JOIN tb a ON a.uid = b.uid AND a.login_time = b.login_time;

当然，结果都相同：

+------+---------------+---------------------+| uid  | ip            | login_time          |+------+---------------+---------------------+| 1003 | 192.168.1.153 | 2017-01-21 19:30:51 || 1002 | 192.168.1.66  | 2017-01-21 19:12:32 || 1001 | 192.168.1.231 | 2017-01-21 19:55:34 |+------+---------------+---------------------+

注：如果要分组取最小值直接改对应函数和符号就行了。