How to group by rows but with the most recent date? - mysql

Question

In my MySQL database I have a table like this used for storing conversation messages from any people
id int(11) id of the message
from member_id int(11) id of the person the message was sent from
to member_id int(11) id of the person the message was sent to
date sent datetime date of when it was sent
active tinyint(1) if the message is deleted
text longtext the text of the message
from_read tinyint(1) boolean to know if the person who sent it read it
to_read tinyint(1) boolean to know if the person who it got sent to read it
So for example, it could have like:
from_member_id to_member_id date sent
1 2 june 12
1 3 june 13
2 3 june 14
3 1 june 9
So we have a conversation between person 1 and 2, 1 and 3, 2 and 3.
I am trying to get a select statement which will give me the most recent message that the current user is involved with from every conversation that user is in. So if 1 is logged in then I would expect to get 2 rows. The first row in the result set would be the second row above (july 13) because its the most recent, then then the second row in the result set would be the first row above (june 12), which are the most recent from 1's two conversations. The result set also needs to be sorted by date sent, so newer conversations are listed on top.
What I am trying to do is like the texting in android phones, where you see the list of conversations, and the most recent message in each listing.
This is my sql query
SELECT *
FROM (
SELECT *
FROM message
WHERE `from member_id`=1 OR `to member_id`=1
ORDER BY IF(`from member_id`=1, `to member_id`, `from member_id`)
) as t
GROUP BY IF(`from member_id`=1, `to member_id`, `from member_id`)
I just hardcoded 1 for now to be the current user. What I am doing is, sorting them by the id of the other person which I can check using the if statement, then grouping that result so I try to get the recent one from each conversation.
The problem is that when grouping, each group can have more than 1 rows, and it just seems to pick some random row. How can I get it to pick the row that has the most recent date sent value?

Answer 1

Are you looking for something like this?
SELECT m.*
FROM message m JOIN
(
SELECT from_member_id, to_member_id, MAX(date_sent) date_sent
FROM message
WHERE from_member_id = 1
GROUP BY from_member_id, to_member_id
) q
ON m.from_member_id = q.from_member_id
AND m.to_member_id = q.to_member_id
AND m.date_sent = q.date_sent
ORDER BY date_sent DESC
Sample output:
| FROM_MEMBER_ID | TO_MEMBER_ID | DATE_SENT |
----------------------------------------------
| 1 | 3 | 2013-06-13 |
| 1 | 2 | 2013-06-12 |
Here is SQLFiddle demo
UPDATE
SELECT m.*
FROM message m JOIN
(
SELECT LEAST(from_member_id, to_member_id) least_id,
GREATEST(from_member_id, to_member_id) greatest_id,
MAX(date_sent) date_sent
FROM message
WHERE from_member_id = 1
OR to_member_id = 1
GROUP BY LEAST(from_member_id, to_member_id),
GREATEST(from_member_id, to_member_id)
) q
ON LEAST(m.from_member_id, m.to_member_id) = q.least_id
AND GREATEST(m.from_member_id, m.to_member_id) = q.greatest_id
AND m.date_sent = q.date_sent
ORDER BY date_sent DESC
Sample output:
| FROM_MEMBER_ID | TO_MEMBER_ID | DATE_SENT |
----------------------------------------------
| 3 | 1 | 2013-06-14 |
| 1 | 2 | 2013-06-12 |
Here is SQLFiddle demo

Answer 2

SELECT
*
FROM message m INNER JOIN
(
SELECT
from_menber_id,
MAX(date_sent) AS sentdate
FROM message s
GROUP BY from_menber_id
) AS a
ON m.date_sent = a.sentdate AND a.from_menber_id = m.from_menber_id