MySQL在同場參加

消息 - 用以下結構

ID
SENDER_ID
receiver_id
MESSAGE_TEXT
用戶 - 用以下結構

ID
暱稱

我需要連接這兩個表，所以我得到結果是這樣的：

message_id, sender_nickname, receiver_nickname, message_text

sender_id始終設置。問題是我還需要郵件，因爲receiver_id爲0，所以receiver_id不能在user表中找到。

有人可以幫助我嗎？

來源

2012-01-07 user1135798

select 
m.id, 
u1.nickname as sender, 
u2.nickname as receiver, 
m.message_text 
from messages as m 
left join users as u1 on u1.id = m.sender_id 
left join users as u2 on u2.id = m.receiver_id

來源

2012-01-07 10:13:45

非常感謝您對您的解決方案它的工程！ :) – user1135798 2012-01-07 10:47:17

不客氣。 :) – 2012-01-07 10:57:12

看起來是這樣的：

SELECT m.id AS message_id 
     ,u1.nickname AS sender_nickname 
     ,u2.nickname AS receiver_nickname 
     ,m.message_text 
FROM messages m 
LEFT JOIN users u1 ON u1.id = m.sender_id 
LEFT JOIN users u2 ON u2.id = m.receiver_id

一個LEFT [OUTER] JOIN保證，如果右手錶中沒有匹配值左手錶將不會被排除在外。

來源

2012-01-07 10:13:50

非常感謝您的解決方案Erwin，它的工作原理:) – user1135798 2012-01-07 10:47:44

SELECT ... 
FROM messages 
INNER JOIN users AS senders ON senders.id = messages.sender_id 
LEFT JOIN users AS receivers ON receivers.id = messages.receiver_id

並且當沒有數據時，將receiver_id列保留爲NULL而不是INT值爲0。

來源

2012-01-07 10:14:07 Marle1

將消息表加入receiver_id上的用戶表時，應該使用左外連接。

來源

2012-01-07 10:16:59 Mithrandir

您可以使用它外連接

select 
m.id as message_id, us.nickname as sender_nickname, ur.nickname as receiver_nickname, m.message_text as message_text 
from messages m join users us on us.id = m.sender_id 
left outer join users ur.id = m.receiver_id 
where m.id = <your message id>

來源

2012-01-07 10:18:58

MySQL在同場參加

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