How to select only one row with desired values?

Question

MySQL

Hello everyone,
There is a table with users, where each user has an id,
and there is a table with messages, where each message is a string and has two fields for senders id. There are a lot of messages, some match on both id.
Example:

Where "id" is the sequence number of the message, "fsender_id" is the sender's id, and "ssender_id" is the recipient's id.
Suppose a request is made for a user with id = 1. Then all messages sent to him or him will have fsender_id = 1 or ssender_id = 1. Then you need to select from the table only those rows where fsender_id or ssender_id is equal to 1, and where the id of the second user (fsender_id or ssender_id) is not repeated. That is, according to the presented table, the result should be only rows with id 1 and 6.

Answer 1

Select through where messages where the user is the sender or recipient.
Then group "by counterparty" and select a larger departure number from the group (if you still need the last, not the first messages).

scheme

CREATE TABLE m (
  id INTEGER(11) NOT NULL AUTO_INCREMENT,
  u_from INTEGER(11) NOT NULL,
  u_to INTEGER(11) NOT NULL,
  PRIMARY KEY USING BTREE (id)
) ENGINE=InnoDB
AUTO_INCREMENT=8 CHARACTER SET 'utf8' COLLATE 'utf8_general_ci'
;

INSERT INTO `m` (id, u_from, u_to) VALUES
  (1,7,1),
  (2,7,1),
  (3,7,1),
  (4,7,1),
  (5,1,7),
  (6,1,4),
  (7,4,1);
COMMIT;

select
  max(id) id,
  if(u_from = 1, concat('TO ', u_to), concat('FROM ', u_from)) last_msg,
  u_from,
  u_to
from m
where u_from = 1 or u_to = 1
group by if(u_from = 1, u_to, u_from)
order by id

Answer 2

SELECT `user_id`, MAX(`id`)
  FROM (
    SELECT `id`, `fsender_id` AS `user_id`
      FROM `messages`
      WHERE `ssender_id` = :iserId
    UNION SELECT `id`, `ssender_id`
      FROM `messages`
      WHERE `fsender_id` = :iserId
  ) AS `t`
  GROUP BY `user_id`