选择某些标记的帖子及其作者

Question

如何将这 5 个表连接在一起：（

tag: id, name
author: username, id
thread_tags: thread_id, tag_id
thread: id, content
author_threads: author_id, thread_id

我还有一个名为author_tags（tag_id，author_id）的表，但我认为这里不需要）。

我想选择标记特定标签及其作者的所有线程。

以下代码返回 #1066 - Not unique table/alias: 'tag'

SELECT thread.content, author.username
FROM tag
JOIN thread_tags ON thread.id = thread_tags.thread_id
JOIN tag ON thread_tags.tag_id = tag.id
JOIN author_threads ON author.id = author_threads.author_id
JOIN author ON author_threads.thread_id = thread.id
WHERE tag.name = 'arsenal'

编辑：

这有效：

SELECT thread.content
FROM tag
JOIN thread_tags ON tag.id = thread_tags.tag_id
JOIN thread ON thread.id = thread_tags.thread_id
WHERE tag.name =  'tagged'
LIMIT 0 , 30

但是，每当我尝试将作者与其线程连接时，它都会抛出 #1066错误。

Answer 1

您已加入 tag 表两次（因此出现错误），但尚未加入 thread 表。

SELECT thread.content, author.username
FROM tag
  JOIN thread_tags
    ON tag.id = thread_tags.tag_id
  JOIN thread                                  --join thread (not tag again)
    ON thread.id = thread_tags.thread_id
  JOIN author_threads
    ON author_threads.thread_id = thread.id     --error here too, in your query
  JOIN author
    ON author.id = author_threads.thread_id     --error here too, in your query
WHERE tag.name = 'arsenal'

Answer 2

为什么你的 JOIN 中有一个标签表？这就是您收到错误的原因：

JOIN tag ON thread_tags.tag_id = tag.id

您这里也有表标签：

FROM tag

标签表出现了两次。

Answer 3

您的查询中有两次标签表。也许这就是问题所在。

Answer 4

SELECT thread.content, author.username
FROM thread
LEFT JOIN thread_tags ON thread.id = thread_tags.thread_id
LEFT JOIN tag ON thread_tags.tag_id = tag.id
LEFT JOIN author_threads ON author.id = author_threads.author_id
LEFT JOIN author ON author_threads.thread_id = thread.id
WHERE tag.name = 'arsenal'

顺便说一句 - 将 author_id`` 存储在thread` 表中不是更好吗？