MySQL - Fetching users who sent the most recent messages - mysql

Question

I have a messages table as follows:
Basically what I want is, I want to fetch n users who sent the most recent messages to a group. So it has to be grouped by from_user_id and sorted by id in descending order. I have the following query:
SELECT `users`.`id` AS `user_id`, `users`.`username`, `users`.`image`
FROM `group_messages`
JOIN `users` ON `users`.`id` = `group_messages`.`from_user_id`
WHERE `group_messages`.`to_group_id` = 31
GROUP BY `users`.`id`
ORDER BY `group_messages`.`id` DESC;
The problem with this is, when I group by user.id, the row with the smallest id field is taken into account. Therefor what I get is not in the order which id is descending.
So is there a way to group by, taking the greatest id into account ? Or should I approach it another way ?
Thanks in advance.
Edit: I think I got it.
SELECT `x`.`id`, `users`.`id` AS `user_id`, `users`.`username`, `users`.`image`
FROM (SELECT * FROM `group_messages` ORDER BY `group_messages`.`id` DESC) `x`
JOIN `users` ON `users`.`id` = `x`.`from_user_id`
WHERE `x`.`to_group_id` = 31
GROUP BY `users`.`id`
ORDER BY `x`.`id` DESC;
Just had to make a select from an already ordered list.

Answer 1

Using order by inside subquery is not a very efficient way. Try this:
users table
| uid | name | ---------- |
messages table
|id | from_uid | to_group_id | ----------- |
SELECT u.* FROM users as u JOIN (
SELECT g1.*
FROM messages g1 LEFT JOIN messages g2
ON g1.from_uid = g2.from_uid AND g1.to_groupid = g2.to_groupid
AND g1.id<g2.id
WHERE g2.id is NULL) as lastmessage
ON lastmessage.from_uid = u.uid
WHERE lastmessage.to_groupid = 1
ORDER BY lastmessage.id DESC;
Its not good to have order by in subqueries because they will make the queries slow and in your case u were running it on the whole table .
Check this out Retrieving the last record in each group.