How do I select last message for each conversation? - mysql

Question

Here is how my database looks like:
table: conversations
+----+--------+--------+
| id | user_1 | user_2 |
+----+--------+--------+
| 1 | 1 | 2 |
| 2 | 2 | 3 |
| 3 | 1 | 3 |
+----+--------+--------+
table: messages
+----+--------------+------+
| id | conversation | text |
+----+--------------+------+
| 1 | 1 | hej |
| 2 | 1 | test |
| 3 | 2 | doh |
| 4 | 2 | hi |
| 5 | 3 | :) |
| 6 | 3 | :D |
+----+--------------+------+
Then when I run the followin query:
SELECT
*
FROM `messages`
INNER JOIN `conversations`
ON `conversations`.`id` = `messages`.`convesations`
GROUP BY `conversations`.`id`
ORDER BY `messages`.`id` DESC
Then I get those out from messages:
+----+--------------+------+
| id | conversation | text |
+----+--------------+------+
| 1 | 1 | hej |
| 3 | 2 | doh |
| 5 | 3 | :) |
+----+--------------+------+
But, is it somehow possible to do so that I will get the messages with the highest messages.id from that group, instead of the lowest?
EDIT: Here is the output I want from messages:
+----+--------------+------+
| id | conversation | text |
+----+--------------+------+
| 2 | 1 | test |
| 4 | 2 | hi |
| 6 | 3 | :D |
+----+--------------+------+
As those are the messages in same conversation with the highest id.

Answer 1

SELECT *
FROM conversations c
JOIN messages m
ON m.id =
(
SELECT id
FROM messages mi
WHERE mi.conversation = c.id
ORDER BY
mi.conversation DESC, mi.id DESC
LIMIT 1
)
Create an index on messages (conversation, id) for this to work fast.

Answer 2

You simply need to use nested query like this:
SELECT * FROM Messages
WHERE ID IN(
SELECT Max(m.ID) FROM Messages m
INNER JOIN conversations c
ON c.id = m.conversation
GROUP BY m.conversation
);
Output:
| ID | CONVERSATION | TEXT |
----------------------------
| 2 | 1 | test |
| 4 | 2 | hi |
| 6 | 3 | :D |
If you want data from both tables try this:
SELECT * FROM Messages m
JOIN conversations c
ON c.id = m.conversation
WHERE m.ID IN (
SELECT MAX(ID) FROM Messages
GROUP BY conversation
)
GROUP BY m.conversation;
Output:
| ID | CONVERSATION | TEXT | USER_1 | USER_2 |
----------------------------------------------
| 2 | 1 | test | 1 | 2 |
| 4 | 2 | hi | 2 | 3 |
| 6 | 3 | :D | 1 | 3 |
See this SQLFiddle

Answer 3

You are making your Join on the wrong column. 'Id' in Conversation cannot be equal to 'Id' in messages.
I thin, 'Conversation' in table messsages is 'id_conversation' right?
So, if I understood well :
SELECT *
FROM messages
INNER JOIN conversations
ON conversations.id = messages.conversation
GROUP BY conversations.id
ORDER BY messages.id DESC

Answer 4

I think you just have an incorrect table join:
SELECT *
FROM `messages`
INNER JOIN `conversations`
ON `conversations`.`id` = `messages`.`conversation`
GROUP BY `conversations`.`id`
ORDER BY `messages`.`id` DESC
Edit
You can try this:
SELECT *
FROM `messages`
WHERE `messages`.`id` IN (
SELECT MAX(id)
FROM messages
GROUP BY conversation
)

Answer 5

A couple of different approaches:
This approach relies on known but undocumented behaviour within MySQL, where the unaggregated, ungrouped values returned in a grouped query are the first in the sort order - it's fast, but should not be viewed as reliable:
SELECT * FROM
(SELECT * FROM messages
ORDER BY conversation, id desc) a
GROUP BY conversation
Alternatively, an approach that should always be reliable:
SELECT m.*, c.user_1, c.user_2 FROM messages m
JOIN (select conversation, max(id) max_id from messages group by conversation) l
ON m.id = l.max_id
JOIN conversations c
ON c.id = m.conversation
GROUP BY conversation
SQLFiddle here.