選擇某些標籤的帖子及其作者

Question

你怎麼能加入這些5桌一起：

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

（我也有一個表稱爲author_tags（TAG_ID，AUTHOR_ID），但我不認爲這裏需要的多數民衆贊成）。

我想選擇標記某個標記及其作者的所有線程。

下面的代碼返回#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' 

編輯：

這工作：

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 

然而，每當我試圖加入的作者與他們的線程，它拋出＃1066錯誤。

Answer 1

您已經加入tag表兩次，（因此錯誤），並沒有加入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

爲什麼你的JOIN中有一個標籤表？這就是爲什麼你所得到的錯誤：

JOIN tag ON thread_tags.tag_id = tag.id 

你也有表標籤瀏覽：

FROM tag 

變量表出現了兩次。

Answer 3

查詢中有兩次標記表。也許這就是問題所在。

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' 

順便說一句 - 是不是更好地存儲author_id`` in線程表？