Select MySQL Group By - mysql

Question

I have a table chat in mysql, it this the record of conversations between users.
The fields are.
id, id_from, id_to, message
Example (My Data in DB):
1, 50, 10, 'Hello'
2, 10, 50 'Nice?'
3, 50, 10, 'Yeah, And you?'
4, 10, 50, 'Me too'
5, 1, 20, 'Hello'
6, 20, 1, 'Nice?'
7, 1, 20, 'Yeah, And you?'
8, 20, 1, 'Me too'
8, 50, 1, 'Hey .....'
9, 1, 50, "Enhancements ??? '
10, 50, 1 ':)'
11, 1, 50, 'LOL'
[User]
id
name
id = Paul 50
id = 1 Samuel
id = Donald 20
id = 10 Max
What I need is via SELECT, return the last conversation that took place between people.
For example, in my select, I want to know the last two conversations (showing only the last message of each conversation) that id had 50 (Paul).
How Poderi do this via MySQL?
The result would be like this:
4, 10, 50, 'Me too'
11, 1, 50, 'LOL'

Answer 1

You can get the ids of the messages using:
select least(id_from, id_to), greatest(id_from, id_to), max(id)
from messages m
where 50 in (id_from, id_to)
group by least(id_from, id_to), greatest(id_from, id_to)
To get the messages, you can use in or join. Here is one method:
select m.*
from messages m
where m.id in (select max(m2.id)
from messages m2
where 50 in (m2.id_from, m2.id_to)
group by least(m2.id_from, m2.id_to), greatest(m2.id_from, m2.id_to)
);