在不同的列上連接兩次相同的表

Question

我有一張user表和一張complaint表。

complaint表具有以下結構：

[opened_by]   [complaint_text]   [closed_by]
 (user_id)         (text)         (user_id)
 (user_id)         (text)         (user_id)
 (user_id)         (text)         (user_id)

所有用戶，包括抱怨者和投訴解決者都位於表user 。

如何編寫查詢以顯示兩列的用戶名？

這給了我一個：

SELECT user.username, complaint.complaint_text
FROM complaint
LEFT JOIN user ON user.user_id=complaint.opened_by

但我不知道怎么寫它所以_by列顯示用戶名而不是ID。

Answer 1

SELECT 
     complaint.complaint_text, 
     A.username, 
     B.username
FROM 
     complaint 
     LEFT JOIN user A ON A.user_id=complaint.opened_by 
     LEFT JOIN user B ON B.user_id=complaint.closed_by

Answer 2

我更喜歡子查詢，因為我覺得它們更容易理解......

SELECT (SELECT name
            FROM user
            WHERE user_id = opened_by) AS opener,
       (SELECT name
            FROM user
            WHERE user_id = closed_by) AS closer,
       complaint_text
    FROM complaint;

如果您有任何性能問題，則查詢優化器通常會重寫子查詢。

Answer 3

SELECT user1.username AS opened_by_username, complaint.complaint_text, user2.username AS closed_by_username
FROM user AS user1, complaint, user as user2
WHERE user1.user_id = complaint.opened_by
AND user2.user_id = complaint.closed_by

使用別名再次加入它（這就是用戶作為user2的東西）

Answer 4

使用此查詢：

SELECT opener.username as opened_by, complaint.complaint_text, closer.username as closed_by
FROM complaint
LEFT JOIN user as opener ON opener.user_id=complaint.opened_by
LEFT JOIN user as closer ON closer.user_id=complaint.closed_by