Question

I have this in a MySQL db:

table Message
    sender_id int
    recipient_id int
    created datetime
    [title, body, etc... omitted]

Is it possible to get from this, in a single query, a list of all the users who have been communicating with a given user with id N (meaning N appears in either sender_id or recipient_id), ordered by created, including the most recent value of created, without duplicates (no sender/recipient pairs N, M and M, N)?

I’m pretty sure the answer is No, but I’m having trouble convincing myself. Afternoon mental dullness.

If No, what’s the most efficient alternative you would recommend?

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EDIT: holy crap, Stack Overflow is hopping these days. 8 answers before I realize I forgot to specify that I would like to have the most-recent value of created present along with the other user ID in each row.

Answer 1

Use:

SELECT x.friend,
       MAX(x.created)
  FROM (SELECT t.recipient_id 'friend',
               t.created
          FROM MESSAGE t
         WHERE t.sender_id = @id 
        UNION
        SELECT r.sender_id 'friend',
               r.created
          FROM MESSAGE r
         WHERE r.recipient_id = @id) x
GROUP BY x.friend