LEFT JOIN 返回不相关的数据，它应该是 NULL答案

【问题标题】：LEFT JOIN returning non-related data where it should be NULLLEFT JOIN 返回不相关的数据，它应该是 NULL
【发布时间】：2019-08-08 18:22:07
【问题描述】：

我有 7 个表要连接在一起。

发帖

+------+--------+--------+-----------------------+-----------------------+-----------+---------+
| Id   | commid | author |        created        |        updated        |   title   | content |
+------+--------+--------+-----------------------+-----------------------+-----------+---------+
|   80 |     16 |    7   | 2019-07-27 23:11:07   | 2019-07-27 23:11:07   | Lorem     | Ipsum   |
|   79 |     16 |    7   | 2019-07-27 23:09:47   | 2019-07-27 23:09:47   | Lorem     | Ipsum   |
|   78 |     16 |    7   | 2019-07-27 23:09:09   | 2019-07-27 23:09:09   | Lorem     | Ipsum   |
|   77 |     16 |    7   | 2019-07-27 23:08:36   | 2019-07-27 23:08:36   | Lorem     | Ipsum   |
|   76 |     16 |    7   | 2019-07-27 23:03:17   | 2019-07-27 23:03:17   | Lorem     | Ipsum   |
|   75 |     16 |    7   | 2019-07-27 22:38:48   | 2019-07-27 22:38:48   |           |         |
+------+--------+--------+-----------------------+-----------------------+-----------+---------+

社区

+----+----------------+
| ID |     title      |
+----+----------------+
| 16 | The Scary Door |
+----+----------------+

社区标签类别

+-----------+----------+
| Community | Category |
+-----------+----------+
| 16        | 5        |
+-----------+----------+
| 16        | 18       |
+-----------+----------+
| 16        | 19       |
+-----------+----------+
| 16        | 20       |
+-----------+----------+

标签类别

+-------------+----+
| category    | Id |
+-------------+----+
| character   | 5  |
+-------------+----+
| focus       | 18 |
+-------------+----+
| warning     | 19 |
+-------------+----+
| Time-period | 20 |
+-------------+----+

标签分类

+----------+-----+
| category | Tag |
+----------+-----+
| 5        | 146 |
+----------+-----+
| 19       | 147 |
+----------+-----+
| 20       | 148 |
+----------+-----+

标签

+---------------+-----+
| name          | Id  |
+---------------+-----+
| spider        | 146 |
+---------------+-----+
| arachnophobia | 147 |
+---------------+-----+
| Victorian era | 148 |
+---------------+-----+

发布标签

+------+-----+
| post | Tag |
+------+-----+
| 80   | 146 |
+------+-----+
| 80   | 147 |
+------+-----+
| 80   | 148 |
+------+-----+

有些帖子在某些表中没有相应的信息，因此我尝试使用 LEFT JOIN。这是我一直在尝试使用的查询：

SELECT p.id, c.id as 'commid', c.title AS 'community', p.author, p.created, p.updated, tc.category, t.name AS 'tag', p.title, p.content
FROM Post p
INNER JOIN Community c
ON p.community=c.id
AND c.id=16
LEFT JOIN CommunityTagCategories cc
ON c.id=cc.community
LEFT JOIN TagCategory tc
ON cc.category=tc.id
LEFT JOIN TagCategorised ct
ON tc.id=ct.category
LEFT JOIN Tag t
ON ct.tag=t.id
LEFT JOIN PostTags pt
ON t.id=pt.tag
GROUP BY p.id, t.id
ORDER BY p.created DESC;

由于只有 post 80 有任何关联的标签，因此 post 80 上的三个标签应该有三个条目，而其他没有标签的帖子应该有一个条目。像这样。我实际上并不关心带有 NULL 标签的帖子的类别是什么，我只想为这些帖子留一行。

我想要的结果

+----+--------+----------------+--------+---------------------+---------------------+-------------+---------------+-------+----------+
| Id | commid |   community    | author |       created       |       updated       |  category   |      tag      | title | content  |
+----+--------+----------------+--------+---------------------+---------------------+-------------+---------------+-------+----------+
| 80 |     16 | The Scary Door |      7 | 2019-07-27 23:11:07 | 2019-07-27 23:11:07 | character   | spider        | Lorem | Ipsum    |
| 80 |     16 | The Scary Door |      7 | 2019-07-27 23:11:07 | 2019-07-27 23:11:07 | warning     | arachnophobia | Lorem | Ipsum    |
| 80 |     16 | The Scary Door |      7 | 2019-07-27 23:11:07 | 2019-07-27 23:11:07 | time-period | victorian era | Lorem | Ipsum    |
| 79 |     16 | The Scary Door |      7 | 2019-07-27 23:09:47 | 2019-07-27 23:09:47 |             | NULL          | Lorem | Ipsum    |
| 78 |     16 | The Scary Door |      7 | 2019-07-27 23:09:09 | 2019-07-27 23:09:09 |             | NULL          | Lorem | Ipsum    |
| 77 |     16 | The Scary Door |      7 | 2019-07-27 23:08:36 | 2019-07-27 23:08:36 |             | NULL          | Lorem | Ipsum    |
| 76 |     16 | The Scary Door |      7 | 2019-07-27 23:03:17 | 2019-07-27 23:03:17 |             | NULL          | Lorem | Ipsum    |
| 75 |     16 | The Scary Door |      7 | 2019-07-27 22:38:48 | 2019-07-27 22:38:48 |             | NULL          |       |          |
+----+--------+----------------+--------+---------------------+---------------------+-------------+---------------+-------+----------+

但是，当我运行查询时，我得到 24 个结果，其中每个帖子的数据重复 4 行。每个标签包括 NULL。像这样（但每篇文章，不仅仅是我在这里展示的两个帖子）。它还显示了一个未用于任何帖子 (Focus) 的标签类别，我不确定为什么会出现。

实际结果

+----+--------+----------------+--------+---------------------+---------------------+-------------+---------------+-------+----------+
| Id | commid |   community    | author |       created       |       updated       |  category   |      tag      | title | content  |
+----+--------+----------------+--------+---------------------+---------------------+-------------+---------------+-------+----------+
| 80 |     16 | The Scary Door |      7 | 2019-07-27 23:11:07 | 2019-07-27 23:11:07 | character   | spider        | Lorem | Ipsum    |
| 80 |     16 | The Scary Door |      7 | 2019-07-27 23:11:07 | 2019-07-27 23:11:07 | focus       |               | Lorem | Ipsum    |
| 80 |     16 | The Scary Door |      7 | 2019-07-27 23:11:07 | 2019-07-27 23:11:07 | warning     | arachnophobia | Lorem | Ipsum    |
| 80 |     16 | The Scary Door |      7 | 2019-07-27 23:11:07 | 2019-07-27 23:11:07 | time-period | victorian era | Lorem | Ipsum    |
| 79 |     16 | The Scary Door |      7 | 2019-07-27 23:09:47 | 2019-07-27 23:09:47 | character   | spider        | Lorem | Ipsum    |
| 79 |     16 | The Scary Door |      7 | 2019-07-27 23:09:47 | 2019-07-27 23:09:47 | focus       |               | Lorem | Ipsum    |
| 79 |     16 | The Scary Door |      7 | 2019-07-27 23:09:47 | 2019-07-27 23:09:47 | warning     | arachnophobia | Lorem | Ipsum    |
| 79 |     16 | The Scary Door |      7 | 2019-07-27 23:09:47 | 2019-07-27 23:09:47 | time-period | victorian era | Lorem | Ipsum    |
+----+--------+----------------+--------+---------------------+---------------------+-------------+---------------+-------+----------+

【问题讨论】：

不确定这是否相关，但您是否尝试过通过 WHERE 子句而不是 Inner Join 过滤社区 ID？
不可能重新构造您的语句，您的列与您给定的表不对应。这就是我得到的选择 pt.post,p.Id,c.Community, p.misc,c.title,tcd.Tag, t.name from posts p inner join Communnity c on p.community = c.Community left join CommunnityTagCategories ctc on p.community = ctc.Community Left Join TagCategory tc on ctc.Category = tc.id Left Join TagCategorised tcd ON tcd.category = tc.ID Left join Tag t ON t.Id = tcd.Tag Left join PostTags pt ON pt.Tag = t.Id GROUP BY p.id, t.id;
我尝试将 WHERE c.id=16 放在最后的 GROUP BY 部分之前，但它给了我相同的 24 个结果。
@nbk 抱歉，我意识到我在社区表 (community->id) 中写错了一个列名。 misc 列只是 Post 表中的各个列（作者、创建、更新、标题、内容等，我刚刚截断为 misc。我尝试了您的查询，您将 Community 的 LEFT JOIN 更改为 CommunityTagCategories ON p.community= ctc.id 而不是 c.id=ctc.id 但它给了我相同的结果。
如果你能告诉我你得到了什么，请在 dbfiddle 中告诉我dbfiddle.uk/…

标签： mysql

【解决方案1】：

经过深思熟虑。我提供了一个解决方案，可以让您如愿以偿结果

Select 
  p.Id, max(p.author) author, max(p.community) community, max(p.created) created
  ,max(p.title) title , max(p.content) content
  ,GROUP_CONCAT(DISTINCT (cttc.category)) category
  , GROUP_CONCAT(DISTINCT (ctatc.name)) Tag
From 
  Post p 
  inner Join Communnity c On c.Id = p.CommId
  right Join (
    Select cc.Community, tc.category, tc.Id
    From 
      TagCategory tc 
      inner join CommunnityTagCategories cc 
    ON cc.Category = tc.Id) cttc  
  ON p.Commid = cttc.Community
Left join (
  Select 
    p.post,t.name,ct.category
    From 
      PostTags p 
      inner join Tag t ON t.Id = p.Tag 
      inner join TagCategorised ct ON ct.Tag = t.Id
  ) ctatc  
  ON ctatc.category = cttc.id and ctatc.post = p.id
GROUP BY p.Id;

它让你

Id作者社区创建的标题内容类别标签

75 7 可怕的门 2019-07-27 22:38:48 警告，字符，时间段，焦点
76 7 可怕的门 2019-07-27 23:03:17 Lorem Ipsum 角色，时间段，焦点，警告
77 7 可怕的门 2019-07-27 23:08:36 Lorem Ipsum 焦点,警告,字符,时间段
78 7 可怕的门 2019-07-27 23:09:09 Lorem Ipsum 警告，字符，时间段，焦点
79 7 可怕的门 2019-07-27 23:09:47 Lorem Ipsum 角色，时间段，焦点，警告
80 7 可怕的门 2019-07-27 23:11:07 Lorem Ipsum 警告，角色，时间段，焦点蜘蛛恐惧症，蜘蛛，维多利亚时代

DBfille example

【讨论】：

哇，这真是太好了！谢谢你。它需要一些更改 PHP 代码才能使用这种新格式，但它似乎得到了我想要的一切。再次感谢:)

【解决方案2】：

问题似乎出在数据库设计上，所以我决定尝试通过创建一个 NULL 标签并链接到没有标签的帖子（以及将 NULL 标签链接到该社区的随机标签类别）来解决这个问题）。

当我这样做时，它与这个 mysql 查询一起工作：

SELECT p.id, p.title, p.content, p.author, p.created, p.updated, ct.category, t.name AS 'tag' 
FROM Post p, Community c, CommunityTagCategories cc, TagCategory ct, TagCategorised tc, Tag t, PostTags pt 
WHERE c.id=18 AND p.community=c.id AND c.id=cc.community AND cc.category=ct.id AND ct.id=tc.category AND tc.tag=t.id AND t.id=pt.tag AND pt.post=p.id 
ORDER BY p.created DESC;

【讨论】：