SQL select latest message of every conversation received AND sent - mysql

Question

This question SQL select only rows with max value on a column doesn't solve my problem although it has been marked as duplicate.
It assumes my columns from_id and to_id are primary keys, when they don't have such constraint (see code provided bellow). If they were primary keys, I couldn't store my messages in the same table. As a result the SQL query of this answer prints all duplicates multiple times, which is not what I want. Please see expected behaviour bellow.
Expected behaviour : I need to select the latest message from all conversations, regardless of whether the user is only sender, recipient, or both. Each conversation/thread should only be displayed once.
Example : when querying this table, my SQL statement should only output msg3 and msg4, ignoring all the previous messages John and Alice exchanged.
Here is the closest query I could write. Problem is this query only selects conversations where user received a message. I'm stuck adding conversations where user is only sender (he didn't get any reply) to the selection.
SELECT * FROM messages where `to_id` = '1' GROUP BY `from_id` ORDER BY `send_date` ASC
Here are users and messages tables:
CREATE TABLE users (
id INT(11) AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(128) NOT NULL
);
CREATE TABLE messages (
id INT(11) AUTO_INCREMENT PRIMARY KEY,
to_id INT(11) NOT NULL, //recipient id to match to current user id
from_id INT(11) NOT NULL, //sender id to match to current user id
send_date DATETIME DEFAULT CURRENT_TIMESTAMP,
content TEXT
);
Question: How can I do this using a single SQL query ? Or should I change my data structure using three tables instead of one ?

Answer 1

I would first get the ids. You can do this using least() and greatest():
select least(m.to_id, m.from_id) as id1,
greatest(m.to_id, m.from_id) as id2, max(m.id) as max_id
from messages m
group by id1, id2;
You can then get the complete information about the message by joining back:
select m.*
from messages m
where m.id in (select max(m.id) as max_id
from messages m
group by least(m.to_id, m.from_id), greatest(m.to_id, m.from_id)
);
Note: In older versions of MySQL, putting the subquery in the from clause and using join is much more efficient.