Group by last occurrence - mysql

Question

i am trying to get the last rows where rec_p_id = 4 SORTED by the timestamp. Since i do not want to have all the results WHERE rec_p_id = 4, i am using GROUP BY to group it by send_p_id.
My SQL query looks like this:
SELECT *
FROM
( SELECT *
FROM chat
WHERE rec_p_id= "4"
ORDER
BY timestamp DESC) as sub
GROUP
BY send_p_id
My table looks like this:
Table chat
c_id
send_p_id
rec_p_id
timestamp
1
3
4
2020-05-01 14:46:00
2
3
4
2020-05-01 14:49:00
3
3
4
2020-05-01 14:50:00
4
7
4
2020-05-01 12:00:00
5
4
7
2020-05-01 12:10:00
6
7
4
2020-05-01 12:20:00
7
9
4
2020-05-01 16:50:00
8
9
4
2020-05-01 17:00:00
I want to get the last occurrences:
c_id
send_p_id
rec_p_id
timestamp
3
3
4
2020-05-01 14:50:00
6
7
4
2020-05-01 12:20:00
8
9
4
2020-05-01 17:00:00
But instead i get all the first ones:
c_id
send_p_id
rec_p_id
timestamp
1
3
4
2020-05-01 14:46:00
4
7
4
2020-05-01 12:00:00
7
9
4
2020-05-01 16:50:00
I saw the query i am using in this question: ORDER BY date and time BEFORE GROUP BY name in mysql
it seems to work for all of them. What am i doing wrong with my query?
Thanks in advance.

Answer 1

Looking to your expected result seems you are looking for
select max(c_id) c_id, send_p_id, min(timestamp) timestamp
from chat WHERE rec_p_id= "4"
group by send_p_id
ORDER BY c_id
Group by is for aggregated result ..
an use without aggregation function can produce unpredicatble result and in version > 5.6 can produce error

Answer 2

I used this answer and built this setup for you.
The code below is the copy of it, so that you can run it later yourself.
For the solution the example from the official manual.
CREATE TABLE chat
(
c_id INT PRIMARY KEY
, send_p_id INT
, rec_p_id INT
, timestamp DATETIME
);
INSERT INTO chat VALUES
(1, 3, 4, '2020-05-01 14:46:00')
, (2, 3, 4, '2020-05-01 14:49:00')
, (3, 3, 4, '2020-05-01 14:50:00')
, (4, 7, 4, '2020-05-01 12:00:00')
, (5, 4, 7, '2020-05-01 12:10:00')
, (6, 7, 4, '2020-05-01 12:20:00')
, (7, 9, 4, '2020-05-01 16:50:00')
, (8, 9, 4, '2020-05-01 17:00:00');
Solution:
SELECT c_id,
send_p_id,
rec_p_id,
timestamp
FROM chat AS c
WHERE timestamp=(SELECT MAX(c1.timestamp)
FROM chat AS c1
WHERE c.send_p_id = c1.send_p_id)
AND send_p_id != 4
ORDER BY timestamp;