MySQL的两个左连接区分同一列

Question

In the following query, I am selecting the one column from users and am expecting a join to take place on both follow_reconciliations user_id and followed_user_id column.

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) 

But, I need to know which query the ig_user_id comes from. How can I rename ig_user_id as an alias for each join? What is the best approach to this?

EDIT: Visually, this is how I want the results to return

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

This query might be more in the direction of this. It returns the correct results as two rows but they are not renamed as two aliases.

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) 

Second Edit: This does the trick.

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

If I understand correctly (do I?) you only need one join:

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

This gives me the results I am looking for:

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)

If anyone has any performance suggestions, please chime in.

Answer 3

if i understand your question.. i'm sory if wrong..

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)

this query provide you to rename or aliasing every column on table.. :)

CMIIW