How to get last record for group? - mysql

Question

I have a table called tbl_chat and tbl_post.
The tbl_chat is given as follows.
|--------------------------------------------------------------------------------|
| chat_id | message | from_user | to_user | post_id |send_date |
|--------------------------------------------------------------------------------|
| 1 | Hi | 23 | A | 35 | 2016-04-01 17:35|
| 2 | Test | 24 | A | 35 | 2016-04-02 01:35|
| 3 | Thut | A | 23 | 35 | 2016-04-02 03:35|
| 4 | test | A | 24 | 35 | 2016-04-02 12:35|
| 5 | Hi | 23 | A | 35 | 2016-04-03 17:35|
|--------------------------------------------------------------------------------|
Now, in the chat table we can see three users are interacting with each other. The Admin (A), user with id = 23 and user = 24.
So there is basically two chat thread.
One between A and 23
Another between A and 24.
I want a query which will show the two chat threads, with the last chat message. Just like in case of facebook chat list showing all the chat-threads with mentioning the last chat.
I am writing a query like this.
SELECT * FROM tbl_chat, tbl_post
WHERE tbl_post.post_id = tbl_chat.post_id
AND tbl_post.post_id = '39'
GROUP BY tbl_chat.chat_from
ORDER BY date DESC
The query has a problem. It is first retrieving all the chats and grouping it w.r.t. chat_from and then ORDERING it Descending-wise.
So first it's creating the group, and then ordering the group.
Also, the first query produces three group, taking the reply message from Admin as a separate group. Since GROUP BY chat_from.
How can I solve this issue?
EDIT:-
I would be grateful if someone can build the query in Active-Records of Codeigniter.

Answer 1

DROP TABLE IF EXISTS my_table;
CREATE TABLE my_table
(chat_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY
,message VARCHAR(20) NOT NULL
,from_user VARCHAR(12)
,to_user VARCHAR(12)
,post_id INT NOT NULL
,send_date DATETIME NOT NULL
);
INSERT INTO my_table VALUES
(1,'Hi' ,'23','A' ,35,'2016-04-01 17:35:00'),
(2,'Test','24','A' ,35,'2016-04-02 01:35:00'),
(3,'Thut','A' ,'23',35,'2016-04-02 03:35:00'),
(4,'test','A' ,'24',35,'2016-04-02 12:35:00'),
(5,'Hi' ,'23','A' ,35,'2016-04-03 17:35:00');
SELECT a.*
FROM my_table a
JOIN
( SELECT LEAST(from_user,to_user) user1
, GREATEST(from_user,to_user) user2
, MAX(send_date) send_date
FROM my_table
GROUP
BY user1
, user2
) b
ON b.user1 = LEAST(a.from_user,a.to_user)
AND b.user2 = GREATEST(a.from_user,a.to_user)
AND b.send_date = a.send_date;
+---------+---------+-----------+---------+---------+---------------------+
| chat_id | message | from_user | to_user | post_id | send_date |
+---------+---------+-----------+---------+---------+---------------------+
| 4 | test | A | 24 | 35 | 2016-04-02 12:35:00 |
| 5 | Hi | 23 | A | 35 | 2016-04-03 17:35:00 |
+---------+---------+-----------+---------+---------+---------------------+

Answer 2

You can use NOT EXISTS() :
SELECT * FROM tbl_chat
INNER JOIN tbl_post
ON tbl_post.post_id = tbl_chat.post_id
WHERE NOT EXISTS(SELECT 1 FROM tbl_chat s
WHERE tbl_chat.from_user IN(s.from_user,s.to_user)
AND tbl_chat.to_user IN(s.from_user,s.to_user)
AND tbl_chat.date < s.date)
Although date field looks like DATE type, which is curious - how would you find the difference between two messages at the same day?