Question

我想根据类别ID从数据库中检索最后三个帖子，例如

post_id         category_id
   1                 1
   2                 4
   3                 3
   4                 4
   5                 2
   6                 1
   7                 3
   8                 2
   9                 4
  10                 1
  11                 2
  12                 3

我想要回复像这样的东西

post_id         category_id
   1                 1
   6                 1
  10                 1
   5                 2
   8                 2
  11                 2
   7                 3
   3                 3
  12                 3
   4                 4
   2                 4
   9                 4

我已经提出了这个问题，但是如何限制每个category_id返回3个？

SELECT
     `posts`.`id`,
     `posts`.`title`,
     `posts`.`slug`,
     `posts`.`image`,
     `posts`.`status`
FROM 
     `posts`
ORDER BY
     `posts`.`id`

如何限制为每个category_id返回3条记录，当我使用LIMIT 3查询时只返回3条记录。

Answer 1

您可以从每个category_id

中读取3个新的post_id

SELECT
    p1.`post_id`,
    p1.`category_id`
FROM
    post p1
JOIN post p2 ON p1.`category_id` = p2.`category_id`
AND p2.`post_id` >= p1.`post_id`
GROUP BY
    p1.`post_id`,
    p1.`category_id`
HAVING
    COUNT(*) <= 3
ORDER BY
    `category_id`,
    `post_id`

SQL FIDDLE：LIVE DE

Answer 2

$sql = "SELECT `posts`.* FROM `posts` WHERE `posts`.`category_id` = 1 ORDER BY `posts`.`id` DESC LIMIT 3"

然后运行

$sql = "SELECT `posts`.* FROM `posts` WHERE `posts`.`category_id` = 2 ORDER BY `posts`.`id` DESC LIMIT 3"

然后运行

$sql = "SELECT `posts`.* FROM `posts` WHERE `posts`.`category_id` = 3 ORDER BY `posts`.`id` DESC LIMIT 3"

然后那个，但别忘了

$sql = "SELECT `posts`.* FROM `posts` WHERE `posts`.`category_id` = 4 ORDER BY `posts`.`id` DESC LIMIT 3"`

然后你已到达目的地

最好只分开运行查询。

从数据库中为每个id重新传输3个最后记录

2 个答案: