SQL在条件或其他条件下左联接

Question

i've got a question:

In a query i need to check for replies and interactions, if these are present. i want to know which user it was that made the reply or interaction and include it in my row. I currently have this:

SELECT * FROM `alerts` a 
LEFT JOIN `interactions` i ON a.alerts = i.alert_id 
LEFT JOIN `reply` r ON a.alerts = r.alert_id 
LEFT JOIN `users` u ON u.id = r.user_id OR i.user_id 
WHERE (a.user_id = '0' AND r.user_id != '') 
OR (a.user_id = '0' AND i.user_id != '')

This somewhat returns what i wan't except that the With the interaction table returns duplicate alert id, The first said that the police posted the interaction on that alert which is a.user_id 0 and the other returns correct with the user's name. Can someone help me?

Answer 1

In your situation you have for an alert: 1. An user that is referenced by the table reply 2. An user that is referenced by the table interactions

As both users have a different role in your query, I suggest you the following query:

SELECT *
FROM alerts A
LEFT JOIN interactions I ON I.alert_id = A.alerts
LEFT JOIN users UI ON UI.id = I.user_id
LEFT JOIN reply R ON R.alert_id = A.alerts
LEFT JOIN users UR ON UR.id = R.user_id
WHERE A.user_id = '0'

By doing so you can then display the desired fields in your SELECT statement.

Hope this will help.

Answer 2

You have your r and i user ID references in the where clause turning it from a left-join to an inner join. Move those conditions to the join portion, then test for NOT NULL for either of them.

SELECT * 
   FROM `alerts` a 
      LEFT JOIN `interactions` i 
         ON a.alerts = i.alert_id 
      LEFT JOIN `reply` r 
         ON a.alerts = r.alert_id 
        AND r.user_id != ''
      LEFT JOIN `users` u 
         ON ( u.id = r.user_id 
           OR u.id = i.user_id )
        AND i.user_id != ''
   WHERE 
      a.user_id = '0'
      AND ( NOT r.user_id IS NULL
           OR NOT i.user_id IS NULL )