同時連接幾個表：

Question

我使用下面的SQL語句：

SELECT reply.id, reply.content, author.username 
FROM thread, reply, author 
JOIN thread_reply ON thread.id = thread_reply.thread_id 
JOIN reply ON thread_reply.reply_id = reply.id 
JOIN author_reply ON thread.id = author_reply.thread_id 
JOIN author ON author_reply.author_id = author.id 
WHERE thread.id = '40' 

我有follwing表：

thread_reply: thread_id, reply_id 

reply: id, content, created (timestamp) 

author: id, username, password_hash, salt #etc 

thread: id, content, created 

author_reply: author_id, reply_id 

我不斷收到以下錯誤：

#1066 - Not unique table/alias: 'reply' 

哦，我正在使用MySQL。

Answer 1

SELECT reply.id, reply.content, author.username 
FROM thread 
INNER JOIN thread_reply ON thread.id = thread_reply.thread_id 
INNER JOIN reply ON thread_reply.reply_id = reply.id 
INNER JOIN author_reply ON thread.id = author_reply.thread_id 
INNER JOIN author ON author_reply.author_id = author.id 
WHERE thread.id = '40' 

Answer 2

你有一個隱含的CROSS JOIN和reply之間thread，author原始查詢，並且加入第二次相同的表不走樣他們。

使用此：

SELECT reply.id, reply.content, author.username 
FROM thread t 
JOIN thread_reply tr 
ON  tr.thread_id = t.id 
JOIN reply r 
ON  r.id = tr.reply_id 
JOIN author_reply ar 
ON  ar.reply_id = r.id 
JOIN author a 
ON  a.id = ar.author_id 
WHERE thread.id = '40' 

Answer 3

要包括的FROM子句中的表，然後加入到他們以及 - 我想你想簡單地FROM thread，然後你的加入。

Answer 4

你，說

JOIN author_reply ON thread.id = author_reply.thread_id 

有根據給出的定義在author_reply表中沒有thread_id，則該行得到了一個錯誤。

Answer 5

這工作：

SELECT reply.id, reply.content, author.username 
FROM thread 
JOIN thread_reply ON thread.id = thread_reply.thread_id 
JOIN reply ON thread_reply.reply_id = reply.id 
JOIN author_reply ON reply.id = author_reply.reply_id 
JOIN author ON author_reply.author_id = author.id 
WHERE thread.id = '40'