为什么我的MySQL DISTINCT不起作用? [英] Why my mysql DISTINCT doesn't work?

问题描述

为什么下面的两个查询返回重复的member_id而不是第三个?

Why the two query below return duplicate member_id and not the third?

我需要第二个查询才能与众不同.每当我运行 GROUP BY 时，此查询的速度都非常慢，并且结果集返回的值不会与distinct相同(该值是错误的).

I need the second query to work with distinct. Anytime i run a GROUP BY, this query is incredibly slow and the resultset doesn't return the same value as distinct (the value is wrong).

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 |
+-----------+

我的餐桌样品

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;

推荐答案

它可以工作，它很脏(没有索引，没有键，临时表...)，但是它可以工作，

it works, its dirty (no index, no key, temporary table...) but it works,

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

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