mysql - mysql DISTINCT 关键字未显示不同的行

Question

询问：

mysql> SELECT DISTINCT (stories.id), stories.news_type, story_keywords.keyword 
FROM (`stories`) LEFT JOIN `story_keywords` ON `stories`.`id`
=`story_keywords`.`story_id` 
WHERE `image_full_url` != '' AND `order` != 0 
AND `news_type` IN ('movie_review', 'movie') 
AND `keyword` IN ('topnews', 'toptablet') 
ORDER BY `created` DESC, `order` DESC LIMIT 5;

结果：（请注意 ID 不是不同的）

+--------+--------------+-----------+
| id     | news_type    | keyword   |
+--------+--------------+-----------+
| 100895 | movie_review | topnews   |
| 100895 | movie_review | toptablet |
| 100798 | movie_review | toptablet |
| 100060 | movie_review | toptablet |
| 100060 | movie_review | topnews   |
+--------+--------------+-----------+
5 rows in set (0.00 sec)

Answer 1

select distinct适用于被提取的行，而不是该行中的单个字段。结果集中的每一行都是不同的。

Answer 2

DISTINCT应用于整行，因为这些keyword列对于它们匹配条件的不同行有多个值DISTINCT。如果要返回一行，则需要确定keyword要返回的值。

如果要将值连接在一起，则可以使用GROUP_CONCAT()：

SELECT stories.id, stories.news_type, GROUP_CONCAT(story_keywords.keyword)
FROM (`stories`) 
LEFT JOIN `story_keywords` 
  ON `stories`.`id`=`story_keywords`.`story_id` 
WHERE `image_full_url` != '' AND `order` != 0 
  AND `news_type` IN ('movie_review', 'movie') 
  AND `keyword` IN ('topnews', 'toptablet') 
GROUP BY stories.id, stories.news_type
ORDER BY `created` DESC, `order` DESC LIMIT 5;

数据会像这样返回：

| id     | news_type    |   keyword           |
+--------+--------------+---------------------+
| 100895 | movie_review | topnews,toptablet   |
| 100798 | movie_review | toptablet           |
| 100060 | movie_review | toptablet, topnews  |

Answer 3

DISTINCT 选项删除重复的记录，而不是字段值。如果你想选择不同的 id，那么你可以使用这个查询 -

SELECT
  stories.id,
  GROUP_CONCAT(stories.news_type),
  GROUP_CONCAT(story_keywords.keyword)
FROM (`stories`)
LEFT JOIN `story_keywords`
ON `stories`.`id` = `story_keywords`.`story_id`
WHERE
  `image_full_url` != ''
  AND `order` != 0
  AND `news_type` IN ('movie_review', 'movie')
  AND `keyword` IN ('topnews', 'toptablet')
GROUP BY
  stories.id
ORDER BY
  `created` DESC
, `order` DESC
LIMIT
  5;