mysql - 如何为每个类别选择准确数量的文章？

Question

这是我的表格:

CREATE TABLE IF NOT EXISTS `category` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(50) NOT NULL,
  `post_limit` int(11) NOT NULL,
  PRIMARY KEY (`id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=5 ;

INSERT INTO `category` (`id`, `title`, `post_limit`) VALUES
(1, 'News', 2),
(2, 'Sport', 2),
(3, 'Science', 1),
(4, 'Games', 1);

CREATE TABLE IF NOT EXISTS `article` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `title` varchar(50) NOT NULL,
  `category_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `category_id` (`category_id`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 AUTO_INCREMENT=11 ;

INSERT INTO `article` (`id`, `title`, `category_id`) VALUES
(1, 'news article 1', 1),
(2, 'news article 2', 1),
(3, 'news article 3', 1),
(4, 'sports article 1', 2),
(5, 'sports article 2', 2),
(6, 'sports article 3', 2),
(7, 'Science article 1', 3),
(8, 'Science article 2', 3),
(9, 'games article 1', 4),
(10, 'games article 2', 4);

我需要做的是选择 10 篇文章 (ORDER BY article.id DESC) 但要记住每个类别都有 post_limit，所以例如我们不能对于 category_id=1 5 个帖子，如果 post_limit=2。

提前谢谢你。

更新 1: 结果应该是:

10  games article 2     4
8   science article 2   3
6   sports article 3    2
5   sports article 2    2
3   news article 3      1

Answer 1

将每个类别的计数限制为该类别的post_limit:

select c.id, c.title, least(post_limit, count(a.id))
from category c
left join article a on category_id = c.id
group by 1, 2 -- or "group by c.id, c.title" if you prefer the verbose style

参见 SQLFiddle .