Question

我有一张user表和一张complaint表。

complaint表具有以下结构：

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

所有用户，投诉人和投诉解析器都位于表user中。

如何编写查询以显示两列的用户名？

这给了我一个：

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

但我不知道如何编写它，因此_by列都显示用户名而不是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

我更喜欢子查询，因为我觉得它们更容易理解......

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;

如果您有任何性能问题，则查询优化器通常会重写子查询。

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

使用别名再次加入它（这就是用户作为user2的内容）

Answer 4

使用此查询：

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

在不同的列上连接两次相同的表

4 个答案: