mysql - 按连接表的多行排序连接查询

Question

我有一个posts 表和一个包含meta_key 和meta_value 列的postmeta 表。我在下面包含了一个简化的表格结构(带有一些演示数据)。

CREATE TABLE `posts` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `title` varchar(200) NOT NULL DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;

INSERT INTO `posts` (`id`, `title`) VALUES
    (1,'First post'),
    (2,'Second post'),
    (3,'Third post'),
    (4,'Fourth post');

CREATE TABLE `postmeta` (
  `meta_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `post_id` int(11) NOT NULL,
  `meta_key` varchar(200) NOT NULL DEFAULT '',
  `meta_value` text NOT NULL,
  PRIMARY KEY (`meta_id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

INSERT INTO `postmeta` (`meta_id`, `post_id`, `meta_key`, `meta_value`) VALUES
    (1,1,'rating','80'),
    (2,1,'total_votes','500'),
    (3,2,'rating','80'),
    (4,2,'total_votes','501'),
    (5,3,'rating','95'),
    (6,3,'total_votes','200');

帖子需要按评分排序。没有任何评级的帖子也需要包括在内，因此 LEFT JOIN。没问题:

SELECT *
FROM posts p
LEFT JOIN postmeta m ON p.id = m.post_id AND m.meta_key = 'rating'
ORDER BY m.meta_value DESC

但是，对于具有相同评级的帖子，我想根据存储在 postmeta 表的另一行中的总票数进一步排序。在上面的示例中，第一个和第二个帖子的评分为 80，但由于第二个帖子的投票更多，因此它应该排在第一个帖子之前。您将如何在单个查询中实现这一点？

Answer 1

SELECT * <-- you'll want to specify columns here
FROM posts p
LEFT JOIN postmeta m ON p.id = m.post_id AND m.meta_key = 'rating'
LEFT JOIN postmeta mv ON p.id = mv.post_id AND mv.meta_key = 'total_votes'
ORDER BY m.meta_value DESC, mv.meta_value DESC

SQL Fiddle Example

输出:

| ID |       TITLE | META_ID | POST_ID | META_KEY | META_VALUE |
----------------------------------------------------------------
|  3 |  Third post |       5 |       3 |   rating |         95 |
|  2 | Second post |       3 |       2 |   rating |         80 |
|  1 |  First post |       1 |       1 |   rating |         80 |
|  4 | Fourth post |  (null) |  (null) |   (null) |     (null) |