在不同的列上加入同一個表兩次

Question

我有一個user表和complaint表。

的complaint表的結構如下：

[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，無論是抱怨和投訴解析器位於表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 

，但我不知道怎麼寫這樣既​​欄顯示用戶名，而不是標識。

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

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 

再次使用別名加入它（這就是用戶如user2的東西，是關於什麼的）

Answer 3

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 

Answer 4

我更喜歡子查詢，因爲我覺得他們更容易理解...

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，子查詢通常會被查詢優化器重寫。