Question

在以下查询中，我从用户中选择了一列，并期望在follow_reconciliations user_id和followed_user_id列上进行连接。

SELECT u.ig_user_id FROM users u 
LEFT JOIN follow_reconciliations f ON (f.followed_user_id = u.user_id) 
LEFT JOIN follow_reconciliations f2 ON (f2.user_id = u.user_id)

但是，我需要知道ig_user_id来自哪个查询。如何将ig_user_id重命名为每个连接的别名？最好的办法是什么？

编辑：从视觉上看，这就是我希望结果返回的方式

+------------+---------------------+
| ig_user_id | followed_ig_user_id |
+------------+---------------------+
| 15325      | 5295                |
+------------+---------------------+

此查询可能更符合此方向。它将正确的结果作为两行返回，但它们不会重命名为两个别名。

SELECT u.ig_user_id AS ig_user_id FROM users u LEFT JOIN follow_reconciliations f ON (f.user_id = u.user_id) 
UNION
SELECT u2.ig_user_id AS followed_ig_user_id FROM users u2 LEFT JOIN follow_reconciliations f2 ON (f2.followed_user_id = u2.user_id)

第二次编辑：这就是诀窍。

SELECT u.ig_user_id AS ig_user_id, u2.ig_user_id AS followed_user_id
FROM follow_reconciliations f
LEFT JOIN users u ON (u.user_id = f.user_id)
LEFT JOIN users u2 ON (f.followed_user_id = u2.user_id)

Answer 1

如果我理解正确（是吗？）你只需要一次加入：

SELECT 
  ig_user_id,
  f.followed_user_id as followed_ig_user_id
FROM users u
LEFT JOIN follow_reconciliations f USING (user_id)

Answer 2

这给了我正在寻找的结果：

SELECT u.ig_user_id AS ig_user_id, u2.ig_user_id AS followed_user_id
FROM follow_reconciliations f
LEFT JOIN users u ON (u.user_id = f.user_id)
LEFT JOIN users u2 ON (f.followed_user_id = u2.user_id)

如果有人有任何表现建议，请加入。

Answer 3

如果我理解你的问题......如果错误的话，我就知道了......

SELECT u.ig_user_id,xx.followed_user_id FROM users u LEFT JOIN (SELECT * FROM follow_reconciliations f) as xx ON (xx.followed_user_id = u.user_id) LEFT JOIN (SELECT * FROM follow_reconciliations f2) as yy ON (yy.user_id = u.user_id)

此查询允许您重命名或别名表上的每一列。:)

CMIIW

MySQL两个左连接区分同一列

3 个答案: