我有这个问题加入多个表并按表对这些表进行分组。我今天在工作上花了几个小时没有成功，我想我对连接或多列分组有误解。
我在这篇文章的末尾包含了一个SQL转储，因此您可以自己查看数据库 - 它可能比我的帖子更好地解释......！
基本上，这是一个WordPress博客，需要从数据库中取出三个“最新帖子”。复杂功能只是每个类别的一个帖子 - 因此您无法提取三个最新帖子并将它们全部归入同一类别。
我认为这对于帖子ID和类别ID（term_id）是一个简单的'group by'，然后是LIMIT 3？
在这个例子中，我将尝试从独特的类别中提取5篇最新文章。
所以有四个表：
您将在下面看到表格和演示数据。很快，我想到的 - 显然是错误的 - 将是这样的查询：
SELECT * FROM wp_posts
INNER JOIN wp_term_relationships ON wp_term_relationships.object_id = wp_posts.id
INNER JOIN wp_term_taxonomy ON wp_term_taxonomy.term_taxonomy_id = wp_term_relationships.term_taxonomy_id
INNER JOIN wp_terms ON wp_terms.term_id = wp_term_taxonomy.term_id
WHERE wp_term_taxonomy.taxonomy = 'category'
AND wp_posts.post_type = 'post'
GROUP BY wp_term_taxonomy.term_id wp_posts.id,
ORDER BY post_date DESC
LIMIT 5
通过表格中的以下数据，我希望上面的查询能够返回Training Puppy，Bathroom Makeover，Post Nachos和Sewing Buttons。最后两个将被忽略，因为它们是重复的post id或term id。但相反，我得到了这四个，然后是改变保险丝。
查询我想要的数据的正确方法是什么？
表wp_posts
+----+-------------------+---------------------+-----------+
| id | post_title | post_date | post_type |
+----+-------------------+---------------------+-----------+
| 5 | Boiling Eggs | 2009-06-11 22:57:07 | post |
| 6 | Boiling Eggs | 2009-06-11 22:57:05 | revision |
| 2 | About | 2009-06-11 22:55:37 | page |
| 7 | Changing Fuses | 2009-06-11 22:57:19 | post |
| 8 | Changing Fuses | 2009-06-11 22:57:15 | revision |
| 9 | Sewing Buttons | 2009-06-11 22:57:28 | post |
| 10 | Sewing Buttons | 2009-06-11 22:57:25 | revision |
| 11 | Posh Nachos | 2009-06-11 22:57:35 | post |
| 12 | Posh Nachos | 2009-06-11 22:57:34 | revision |
| 13 | Bathroom Makeover | 2009-06-11 22:57:44 | post |
| 14 | Bathroom Makeover | 2009-06-11 22:57:41 | revision |
| 15 | Training Puppy | 2009-06-11 22:57:51 | post |
| 16 | Training Puppy | 2009-06-11 22:57:48 | revision |
| 17 | Boiling Eggs | 2009-06-11 22:57:07 | revision |
| 18 | Training Puppy | 2009-06-11 22:57:51 | revision |
| 19 | Bathroom Makeover | 2009-06-11 22:57:44 | revision |
| 20 | Posh Nachos | 2009-06-11 22:57:35 | revision |
+----+-------------------+---------------------+-----------+
表wp_terms
+---------+---------------+---------------+------------+
| term_id | name | slug | term_group |
+---------+---------------+---------------+------------+
| 1 | Uncategorized | uncategorized | 0 |
| 2 | Blogroll | blogroll | 0 |
| 3 | Food | foot | 0 |
| 4 | DIY | diy | 0 |
| 5 | Crafts | crafts | 0 |
| 6 | Pets | pets | 0 |
| 7 | puppy | puppy | 0 |
| 8 | dog | dog | 0 |
| 9 | training | training | 0 |
| 10 | bathroom | bathroom | 0 |
| 11 | rennovate | rennovate | 0 |
| 12 | mexican | mexican | 0 |
| 13 | snack | snack | 0 |
| 14 | fast food | fast-food | 0 |
+---------+---------------+---------------+------------+
表wp_term_taxonomy
+------------------+---------+---------------+-------------+--------+-------+
| term_taxonomy_id | term_id | taxonomy | description | parent | count |
+------------------+---------+---------------+-------------+--------+-------+
| 1 | 1 | category | | 0 | 0 |
| 2 | 2 | link_category | | 0 | 7 |
| 3 | 3 | category | | 0 | 2 |
| 4 | 4 | category | | 0 | 2 |
| 5 | 5 | category | | 0 | 1 |
| 6 | 6 | category | | 0 | 1 |
| 8 | 7 | post_tag | | 0 | 1 |
| 9 | 8 | post_tag | | 0 | 1 |
| 10 | 9 | post_tag | | 0 | 1 |
| 11 | 10 | post_tag | | 0 | 1 |
| 12 | 11 | post_tag | | 0 | 1 |
| 13 | 12 | post_tag | | 0 | 1 |
| 14 | 13 | post_tag | | 0 | 1 |
| 15 | 14 | post_tag | | 0 | 1 |
+------------------+---------+---------------+-------------+--------+-------+
表wp_term_relationships
+-----------+------------------+------------+
| object_id | term_taxonomy_id | term_order |
+-----------+------------------+------------+
| 1 | 2 | 0 |
| 2 | 2 | 0 |
| 3 | 2 | 0 |
| 4 | 2 | 0 |
| 5 | 2 | 0 |
| 6 | 2 | 0 |
| 7 | 2 | 0 |
| 6 | 1 | 0 |
| 18 | 1 | 0 |
| 8 | 1 | 0 |
| 7 | 4 | 0 |
| 10 | 1 | 0 |
| 9 | 5 | 0 |
| 12 | 1 | 0 |
| 11 | 3 | 0 |
| 14 | 1 | 0 |
| 13 | 4 | 0 |
| 16 | 1 | 0 |
| 15 | 6 | 0 |
| 17 | 1 | 0 |
| 5 | 3 | 0 |
| 15 | 8 | 0 |
| 15 | 9 | 0 |
| 15 | 10 | 0 |
| 19 | 1 | 0 |
| 13 | 11 | 0 |
| 13 | 12 | 0 |
| 20 | 1 | 0 |
| 11 | 13 | 0 |
| 11 | 14 | 0 |
| 11 | 15 | 0 |
+-----------+------------------+------------+
SQL Dump
CREATE TABLE IF NOT EXISTS `wp_posts` (
`ID` bigint(20) unsigned NOT NULL auto_increment,
`post_author` bigint(20) unsigned NOT NULL default '0',
`post_date` datetime NOT NULL default '0000-00-00 00:00:00',
`post_date_gmt` datetime NOT NULL default '0000-00-00 00:00:00',
`post_content` longtext NOT NULL,
`post_title` text NOT NULL,
`post_excerpt` text NOT NULL,
`post_status` varchar(20) NOT NULL default 'publish',
`comment_status` varchar(20) NOT NULL default 'open',
`ping_status` varchar(20) NOT NULL default 'open',
`post_password` varchar(20) NOT NULL default '',
`post_name` varchar(200) NOT NULL default '',
`to_ping` text NOT NULL,
`pinged` text NOT NULL,
`post_modified` datetime NOT NULL default '0000-00-00 00:00:00',
`post_modified_gmt` datetime NOT NULL default '0000-00-00 00:00:00',
`post_content_filtered` text NOT NULL,
`post_parent` bigint(20) unsigned NOT NULL default '0',
`guid` varchar(255) NOT NULL default '',
`menu_order` int(11) NOT NULL default '0',
`post_type` varchar(20) NOT NULL default 'post',
`post_mime_type` varchar(100) NOT NULL default '',
`comment_count` bigint(20) NOT NULL default '0',
PRIMARY KEY (`ID`),
KEY `post_name` (`post_name`),
KEY `type_status_date` (`post_type`,`post_status`,`post_date`,`ID`),
KEY `post_parent` (`post_parent`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=21 ;
--
-- Dumping data for table `wp_posts`
--
INSERT INTO `wp_posts` (`ID`, `post_author`, `post_date`, `post_date_gmt`, `post_content`, `post_title`, `post_excerpt`, `post_status`, `comment_status`, `ping_status`, `post_password`, `post_name`, `to_ping`, `pinged`, `post_modified`, `post_modified_gmt`, `post_content_filtered`, `post_parent`, `guid`, `menu_order`, `post_type`, `post_mime_type`, `comment_count`) VALUES
(5, 1, '2009-06-11 22:57:07', '2009-06-11 21:57:07', 'fadgagadf', 'Boiling Eggs', '', 'publish', 'open', 'open', '', 'boiling-eggs', '', '', '2009-06-11 22:58:03', '2009-06-11 21:58:03', '', 0, 'http://project1.searbe.co.uk/?p=5', 0, 'post', '', 0),
(6, 1, '2009-06-11 22:57:05', '2009-06-11 21:57:05', '', 'Boiling Eggs', '', 'inherit', 'open', 'open', '', '5-revision', '', '', '2009-06-11 22:57:05', '2009-06-11 21:57:05', '', 5, 'http://project1.searbe.co.uk/?p=6', 0, 'revision', '', 0),
(2, 1, '2009-06-11 22:55:37', '2009-06-11 21:55:37', 'This is an example of a WordPress page, you could edit this to put information about yourself or your site so readers know where you are coming from. You can create as many pages like this one or sub-pages as you like and manage all of your content inside of WordPress.', 'About', '', 'publish', 'open', 'open', '', 'about', '', '', '2009-06-11 22:55:37', '2009-06-11 21:55:37', '', 0, 'http://project1.searbe.co.uk/?page_id=2', 0, 'page', '', 0),
(7, 1, '2009-06-11 22:57:19', '2009-06-11 21:57:19', 'fgsgfafga', 'Changing Fuses', '', 'publish', 'open', 'open', '', 'changing-fuses', '', '', '2009-06-11 22:57:19', '2009-06-11 21:57:19', '', 0, 'http://project1.searbe.co.uk/?p=7', 0, 'post', '', 0),
(8, 1, '2009-06-11 22:57:15', '2009-06-11 21:57:15', '', 'Changing Fuses', '', 'inherit', 'open', 'open', '', '7-revision', '', '', '2009-06-11 22:57:15', '2009-06-11 21:57:15', '', 7, 'http://project1.searbe.co.uk/?p=8', 0, 'revision', '', 0),
(9, 1, '2009-06-11 22:57:28', '2009-06-11 21:57:28', 'ghsghgs', 'Sewing Buttons', '', 'publish', 'open', 'open', '', 'sewing-buttons', '', '', '2009-06-11 22:57:28', '2009-06-11 21:57:28', '', 0, 'http://project1.searbe.co.uk/?p=9', 0, 'post', '', 0),
(10, 1, '2009-06-11 22:57:25', '2009-06-11 21:57:25', '', 'Sewing Buttons', '', 'inherit', 'open', 'open', '', '9-revision', '', '', '2009-06-11 22:57:25', '2009-06-11 21:57:25', '', 9, 'http://project1.searbe.co.uk/?p=10', 0, 'revision', '', 0),
(11, 1, '2009-06-11 22:57:35', '2009-06-11 21:57:35', 'hhjhdh', 'Posh Nachos', '', 'publish', 'open', 'open', '', 'posh-nachos', '', '', '2009-06-11 22:59:34', '2009-06-11 21:59:34', '', 0, 'http://project1.searbe.co.uk/?p=11', 0, 'post', '', 0),
(12, 1, '2009-06-11 22:57:34', '2009-06-11 21:57:34', '', 'Posh Nachos', '', 'inherit', 'open', 'open', '', '11-revision', '', '', '2009-06-11 22:57:34', '2009-06-11 21:57:34', '', 11, 'http://project1.searbe.co.uk/?p=12', 0, 'revision', '', 0),
(13, 1, '2009-06-11 22:57:44', '2009-06-11 21:57:44', 'hjhjdhjdjdh', 'Bathroom Makeover', '', 'publish', 'open', 'open', '', 'bathroom-makeover', '', '', '2009-06-11 22:59:19', '2009-06-11 21:59:19', '', 0, 'http://project1.searbe.co.uk/?p=13', 0, 'post', '', 0),
(14, 1, '2009-06-11 22:57:41', '2009-06-11 21:57:41', '', 'Bathroom Makeover', '', 'inherit', 'open', 'open', '', '13-revision', '', '', '2009-06-11 22:57:41', '2009-06-11 21:57:41', '', 13, 'http://project1.searbe.co.uk/?p=14', 0, 'revision', '', 0),
(15, 1, '2009-06-11 22:57:51', '2009-06-11 21:57:51', 'hjhjdjdy', 'Training Puppy', '', 'publish', 'open', 'open', '', 'training-puppy', '', '', '2009-06-11 22:59:05', '2009-06-11 21:59:05', '', 0, 'http://project1.searbe.co.uk/?p=15', 0, 'post', '', 0),
(16, 1, '2009-06-11 22:57:48', '2009-06-11 21:57:48', '', 'Training Puppy', '', 'inherit', 'open', 'open', '', '15-revision', '', '', '2009-06-11 22:57:48', '2009-06-11 21:57:48', '', 15, 'http://project1.searbe.co.uk/?p=16', 0, 'revision', '', 0),
(17, 1, '2009-06-11 22:57:07', '2009-06-11 21:57:07', 'fadgagadf', 'Boiling Eggs', '', 'inherit', 'open', 'open', '', '5-revision-2', '', '', '2009-06-11 22:57:07', '2009-06-11 21:57:07', '', 5, 'http://project1.searbe.co.uk/?p=17', 0, 'revision', '', 0),
(18, 1, '2009-06-11 22:57:51', '2009-06-11 21:57:51', 'hjhjdjdy', 'Training Puppy', '', 'inherit', 'open', 'open', '', '15-revision-2', '', '', '2009-06-11 22:57:51', '2009-06-11 21:57:51', '', 15, 'http://project1.searbe.co.uk/?p=18', 0, 'revision', '', 0),
(19, 1, '2009-06-11 22:57:44', '2009-06-11 21:57:44', 'hjhjdhjdjdh', 'Bathroom Makeover', '', 'inherit', 'open', 'open', '', '13-revision-2', '', '', '2009-06-11 22:57:44', '2009-06-11 21:57:44', '', 13, 'http://project1.searbe.co.uk/?p=19', 0, 'revision', '', 0),
(20, 1, '2009-06-11 22:57:35', '2009-06-11 21:57:35', 'hhjhdh', 'Posh Nachos', '', 'inherit', 'open', 'open', '', '11-revision-2', '', '', '2009-06-11 22:57:35', '2009-06-11 21:57:35', '', 11, 'http://project1.searbe.co.uk/?p=20', 0, 'revision', '', 0);
-- --------------------------------------------------------
--
-- Table structure for table `wp_terms`
--
CREATE TABLE IF NOT EXISTS `wp_terms` (
`term_id` bigint(20) unsigned NOT NULL auto_increment,
`name` varchar(200) NOT NULL default '',
`slug` varchar(200) NOT NULL default '',
`term_group` bigint(10) NOT NULL default '0',
PRIMARY KEY (`term_id`),
UNIQUE KEY `slug` (`slug`),
KEY `name` (`name`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=15 ;
--
-- Dumping data for table `wp_terms`
--
INSERT INTO `wp_terms` (`term_id`, `name`, `slug`, `term_group`) VALUES
(1, 'Uncategorized', 'uncategorized', 0),
(2, 'Blogroll', 'blogroll', 0),
(3, 'Food', 'foot', 0),
(4, 'DIY', 'diy', 0),
(5, 'Crafts', 'crafts', 0),
(6, 'Pets', 'pets', 0),
(7, 'puppy', 'puppy', 0),
(8, 'dog', 'dog', 0),
(9, 'training', 'training', 0),
(10, 'bathroom', 'bathroom', 0),
(11, 'rennovate', 'rennovate', 0),
(12, 'mexican', 'mexican', 0),
(13, 'snack', 'snack', 0),
(14, 'fast food', 'fast-food', 0);
-- --------------------------------------------------------
--
-- Table structure for table `wp_term_relationships`
--
CREATE TABLE IF NOT EXISTS `wp_term_relationships` (
`object_id` bigint(20) unsigned NOT NULL default '0',
`term_taxonomy_id` bigint(20) unsigned NOT NULL default '0',
`term_order` int(11) NOT NULL default '0',
PRIMARY KEY (`object_id`,`term_taxonomy_id`),
KEY `term_taxonomy_id` (`term_taxonomy_id`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8;
--
-- Dumping data for table `wp_term_relationships`
--
INSERT INTO `wp_term_relationships` (`object_id`, `term_taxonomy_id`, `term_order`) VALUES
(1, 2, 0),
(2, 2, 0),
(3, 2, 0),
(4, 2, 0),
(5, 2, 0),
(6, 2, 0),
(7, 2, 0),
(6, 1, 0),
(18, 1, 0),
(8, 1, 0),
(7, 4, 0),
(10, 1, 0),
(9, 5, 0),
(12, 1, 0),
(11, 3, 0),
(14, 1, 0),
(13, 4, 0),
(16, 1, 0),
(15, 6, 0),
(17, 1, 0),
(5, 3, 0),
(15, 8, 0),
(15, 9, 0),
(15, 10, 0),
(19, 1, 0),
(13, 11, 0),
(13, 12, 0),
(20, 1, 0),
(11, 13, 0),
(11, 14, 0),
(11, 15, 0);
-- --------------------------------------------------------
--
-- Table structure for table `wp_term_taxonomy`
--
CREATE TABLE IF NOT EXISTS `wp_term_taxonomy` (
`term_taxonomy_id` bigint(20) unsigned NOT NULL auto_increment,
`term_id` bigint(20) unsigned NOT NULL default '0',
`taxonomy` varchar(32) NOT NULL default '',
`description` longtext NOT NULL,
`parent` bigint(20) unsigned NOT NULL default '0',
`count` bigint(20) NOT NULL default '0',
PRIMARY KEY (`term_taxonomy_id`),
UNIQUE KEY `term_id_taxonomy` (`term_id`,`taxonomy`),
KEY `taxonomy` (`taxonomy`)
) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=16 ;
--
-- Dumping data for table `wp_term_taxonomy`
--
INSERT INTO `wp_term_taxonomy` (`term_taxonomy_id`, `term_id`, `taxonomy`, `description`, `parent`, `count`) VALUES
(1, 1, 'category', '', 0, 0),
(2, 2, 'link_category', '', 0, 7),
(3, 3, 'category', '', 0, 2),
(4, 4, 'category', '', 0, 2),
(5, 5, 'category', '', 0, 1),
(6, 6, 'category', '', 0, 1),
(8, 7, 'post_tag', '', 0, 1),
(9, 8, 'post_tag', '', 0, 1),
(10, 9, 'post_tag', '', 0, 1),
(11, 10, 'post_tag', '', 0, 1),
(12, 11, 'post_tag', '', 0, 1),
(13, 12, 'post_tag', '', 0, 1),
(14, 13, 'post_tag', '', 0, 1),
(15, 14, 'post_tag', '', 0, 1);
答案 0 :(得分：3)
更新：我为理解您原来的问题而道歉。我浏览了它，并认为这是我经常看到的“每组中的前N个”问题的另一个例子。
以下是您原始问题的解决方案，这是最近的五个帖子，每个类别最多只有一个帖子：
SELECT t.name, p1.post_title, p1.post_date
FROM wp_term_taxonomy x
INNER JOIN wp_terms t ON (t.term_id = x.term_id)
INNER JOIN wp_term_relationships r1 ON (x.term_taxonomy_id = r1.term_taxonomy_id)
INNER JOIN wp_posts p1 ON (r1.object_id = p1.id AND p1.post_type = 'post')
LEFT OUTER JOIN (wp_term_relationships r2
INNER JOIN wp_posts p2 ON (r2.object_id = p2.id AND p2.post_type = 'post'))
ON (x.term_taxonomy_id = r2.term_taxonomy_id AND p1.post_date < p2.post_date)
WHERE x.taxonomy = 'category'
AND r2.term_taxonomy_id IS NULL
GROUP BY x.term_id
ORDER BY p1.post_date DESC
LIMIT 5;
这是我测试中的输出：
+--------+-------------------+---------------------+
| name | post_title | post_date |
+--------+-------------------+---------------------+
| Pets | Training Puppy | 2009-06-11 22:57:51 |
| DIY | Bathroom Makeover | 2009-06-11 22:57:44 |
| Food | Posh Nachos | 2009-06-11 22:57:35 |
| Crafts | Sewing Buttons | 2009-06-11 22:57:28 |
+--------+-------------------+---------------------+
PS：非常感谢您在原始问题中发布DDL和INSERT语句！没有多少人问过SQL问题。
以下是我的第一个答案，基于我对该问题的错误理解：
因此，您希望所有帖子都包含少于五个其他帖子，这些帖子具有相同的类别字词和更新的post_date？
您需要加入posts表两次：
SELECT x.*, t.*, r1.*, p1.*
FROM wp_term_taxonomy x
INNER JOIN wp_terms t ON (t.term_id = x.term_id)
INNER JOIN wp_term_relationships r1 ON (r1.term_taxonomy_id = x.term_taxonomy_id)
INNER JOIN wp_posts p1 ON (p1.id = r1.object_id)
INNER JOIN wp_term_relationships r2 ON (r2.term_taxonomy_id = x.term_taxonomy_id)
INNER JOIN wp_posts p2 ON (p2.id = r2.object_id AND p2.post_date > p1.post_date)
WHERE x.taxonomy = 'category'
AND p1.post_type = 'post' AND p2.post_type = 'post'
GROUP BY x.term_id, p1.id
HAVING COUNT(*) < 5;
答案 1 :(得分：0)
这是一个更简单的版本，但感谢你的想法法案：
SELECT wt.name, wp.post_title, wp.post_date
FROM wp_term_relationships AS wtr
INNER JOIN wp_posts AS wp ON ( wtr.object_id = wp.id )
INNER JOIN wp_term_taxonomy AS wtt ON ( wtr.term_taxonomy_id = wtt.term_taxonomy_id)
INNER JOIN wp_terms AS wt ON ( wtt.term_id = wt.term_id )
WHERE wp.post_type = 'post'
and wtt.taxonomy = 'category'
group by wtt.term_id
order by wp.post_date desc;