Question

I’m developing messaging system and have a trouble writing down the query(MYSQL).
In the nutshell query should return an array of one last messages for each conversation user to user.

CREATE TABLE 'tbl_message'('id' int(10) AUTO_INCREMENT,
  'from_user' int(10),
  'to_user' int(10),
  'reply_to' int(10),
  'subject',
  'body' text,
  'status' tinyint(3),
  'deleted_from_sender' tinyint(3),
  'deleted_from_recipient' tinyint(3)',
  'created_on' datetime,

I wrote query:

SELECT * FROM tbl_message s1 WHERE s1.created_on IN (
   SELECT MAX(s2.created_on)
      FROM tbl_message s2 WHERE s1.from_user=".$userID." OR s2.to_user=".$userID."
      GROUP BY s2.from_user, s2.to_user) 
       AND status = 0";

That works fine but give me 2 last messages from from_user and to_user, instead of 1 last message from both. That because of GROUP BY of course, now the question is how I could find the max created_on(actually mysql datestamp) in subquery? Or any other solution will be appreciated.
Appreciate any help or advice.

After 2 days digging stackoverflow and mysql manuals hope for help from DB prof:)

UPD:
some data for example

+----+-------+--------+--------+---------------------+--------+---------+
| id | from_user | to_user | subject      | created_on          | status |
+----+-------+--------+--------+---------------------+--------+---------+
|  1 |   68      |   128   | somesubject1 | 2013-07-01 21:31:29 |      0 |
+----+-------+--------+--------+---------------------+--------+---------+
|  2 |    128    |   68    | somesubject2 | 2013-07-01 21:41:29 |      0 |
+----+-------+--------+--------+---------------------+--------+---------+
|  3 |    128    |   68    | somesubject3 | 2013-07-01 21:51:29 |      0 |
+----+-------+--------+--------+---------------------+--------+---------+
|  4 |    68     |   226   | somesubject4 | 2013-07-01 22:01:29 |      0 |
+----+-------+--------+--------+---------------------+--------+---------+

output of query

|  3 |    128    |   68    | somesubject3 | 2013-07-01 21:51:29 |      0 |
+----+-------+--------+--------+---------------------+--------+---------+
|  4 |    68     |   226   | somesubject4 | 2013-07-01 22:01:29 |      0 |

Answer 1

You could use this query to get the maximum created_on date between two users:

SELECT
  LEAST(from_user, to_user) user1,
  GREATEST(from_user, to_user) user2,
  MAX(created_on) max_created_on
FROM
  tbl_message
GROUP BY
  LEAST(from_user, to_user),
  GREATEST(from_user, to_user)

and the query you are looking for is probably this:

SELECT t.*
FROM
  tbl_message t INNER JOIN (
    SELECT
      LEAST(from_user, to_user) user1,
      GREATEST(from_user, to_user) user2,
      MAX(created_on) max_created_on
    FROM
      tbl_message
    WHERE from_user=68 or to_user=68
    GROUP BY
      LEAST(from_user, to_user),
      GREATEST(from_user, to_user)) M
  on t.created_on = M.max_created_on
     AND LEAST(t.from_user, t.to_user)=user1
     AND GREATEST(t.from_user, t.to_user)=user2

See fiddle with both queries here.