join inbox and sent box table - php

Question

I am creating message conversation script in PHP mysqli. I have two table inbox and sent box, this two table same columns I want to join this two table.and i want to get last message between two users.
inbox table
id from_id to_id msg sent_date
1 2 3 hi how are you? 2019-12-05 04:14:20
2 3 2 fine 2019-12-05 05:15:58
3 2 3 hi 2019-12-05 03:20:34
4 5 2 hi 2019-12-05 08:30:40
sentbox table
id from_id to_id msg sent_date
1 2 3 hi how are you? 2019-12-05 04:14:20
2 3 2 fine 2019-12-05 05:15:58
3 2 3 hi 2019-12-05 03:20:34
4 5 2 hi 2019-12-05 08:30:40
Here is my source code
<?php
if (isset($_SESSION['userid'])) {
$session_id = $_SESSION['userid'];
}
$sql = "SELECT *,
(SELECT username FROM users WHERE userid=from_id) AS from_username,
(SELECT username FROM users WHERE userid=to_id) AS to_username,
(SELECT username FROM users WHERE userid=?) AS my_username,
(SELECT profile_pic FROM users WHERE userid=from_id) AS from_profile_pic,
(SELECT profile_pic FROM users WHERE userid=to_id) AS to_profile_pic,
(SELECT profile_pic FROM users WHERE userid=?) AS my_profile_pic
FROM inbox WHERE from_id = ? OR to_id = ? ORDER BY id DESC";
if ($stmt = $con->prepare($sql)) {
$stmt->bind_param('iiii', $session_id, $session_id, $session_id, $session_id);
$stmt->execute();
}

Answer 1

What about this?
SELECT * from inbox i
inner join sentbox s on i.from_id = s.to_id
inner join users u on u.user_id = i.from_id
where i.from_id = 'your desired id here'
order by i.sent_date DESC limit 1;
I believe this would give you the latest communication between 2 users.
Having said that, it feels like it would be better to go with a simpler design where every communication is a transaction and you store it in a single table, with FROM and TO fields and the time of the communication. No need to do a JOIN between 2 tables here. You're duplicating data here.