php – 为什么我的mysql DISTINCT不起作用？

我需要第二个查询才能使用distinct.无论何时我运行GROUP BY,此查询都非常慢,并且结果集不会返回与distinct相同的值(值是错误的).

SELECT member_id,id 
FROM ( SELECT * FROM table1 ORDER BY created_at desc ) as u 
LIMIT 5

+-----------+--------+
| member_id | id     |
+-----------+--------+
|     11333 | 313095 |
|    141831 | 313094 |
|    141831 | 313093 |
|     12013 | 313092 |
|     60821 | 313091 |
+-----------+--------+

SELECT distinct member_id,id 
FROM ( SELECT * FROM table1 ORDER BY created_at desc ) as u 
LIMIT 5

+-----------+--------+
| member_id | id     |
+-----------+--------+
|     11333 | 313095 |
|    141831 | 313094 |
|    141831 | 313093 |
|     12013 | 313092 |
|     60821 | 313091 |
+-----------+--------+

SELECT distinct member_id
    FROM ( SELECT * FROM table1 ORDER BY created_at desc ) as u 
    LIMIT 5

+-----------+
| member_id |
+-----------+
|     11333 |
|    141831 |
|     12013 |
|     60821 |
|     64980 |
+-----------+

我的表样本

CREATE TABLE `table1` (
  `id` int(11) NOT NULL AUTO_INCREMENT,`member_id` int(11) NOT NULL,`s_type_id` int(11) NOT NULL,`created_at` datetime DEFAULT NULL,PRIMARY KEY (`id`),KEY `s_FI_1` (`member_id`),KEY `s_FI_2` (`s_type_id`)
) ENGINE=InnoDB AUTO_INCREMENT=313096 DEFAULT CHARSET=utf8;