Getting last row of each group in mysql - mysql

Question

I have two tables as following:
messages
+----+--------+-----------------+
| id | sender | body |
+----+--------+-----------------+
| 11 | 4 | test msg one |
| 12 | 4 | test msg |
| 13 | 1 | this is test |
| 14 | 4 | WANT TO SHOW G1 |
| 15 | 4 | WANT TO SHOW G2 |
+----+--------+-----------------+
message_receivers
+----------+------------+
| receiver | message_id |
+----------+------------+
| 1 | 11 |
| 1 | 12 |
| 4 | 13 |
| 1 | 14 |
| 3 | 15 |
+----------+------------+
I have written following query
SELECT
`messages`.`id`,
`messages`.`body` ,
`messages`.`sender`,
MAX(`messages`.`id`) AS MID,
IF(`messages`.`sender`>`message_receivers`.`receiver`,`CONCAT_WS`(',',`messages`.`sender`,`message_receivers`.`receiver`),
`CONCAT_WS`(',',`message_receivers`.`receiver`,`messages`.`sender`)) AS `conc`
FROM
`messages`
JOIN `message_receivers` ON `messages`.`id` = `message_receivers`.`message_id`
WHERE
`message_receivers`.`receiver` = '4'
OR `messages`.`sender` = '4'
GROUP BY conc
which give me the following result
+----+-----------------+--------+------+------+
| id | body | sender | MID | conc |
+----+-----------------+--------+------+------+
| 11 | test msg one | 4 | 14 | 4,1 |
| 15 | WANT TO SHOW G2 | 4 | 15 | 4,3 |
+----+-----------------+--------+------+------+
But I want
+----+-----------------+--------+------+------+
| id | body | sender | MID | conc |
+----+-----------------+--------+------+------+
| 14 | WANT TO SHOW G1 | 4 | 14 | 4,1 |
| 15 | WANT TO SHOW G2 | 4 | 15 | 4,3 |
+----+-----------------+--------+------+------+
What should I do?
Thanks in advance.

Answer 1

Using a sub query to get the max message, and then joining that back to get the other fields:-
SELECT messages.id, messages.body, messages.sender, Sub1.MID, Sub1.conc
FROM messages
INNER JOIN message_receivers
ON messages.id = message_receivers.message_id
INNER JOIN
(
SELECT
IF(messages.sender>message_receivers.receiver,
CONCAT_WS(',',messages.sender,message_receivers.receiver),
CONCAT_WS(',',message_receivers.receiver,messages.sender)) AS conc,
MAX(messages.id) AS MID
FROM messages
JOIN message_receivers
ON messages.id = message_receivers.message_id
WHERE message_receivers.receiver = '4'
OR messages.sender = '4'
GROUP BY conc
) Sub1
ON Sub1.MID = messages.id
AND Sub1.conc = IF(messages.sender>message_receivers.receiver,
CONCAT_WS(',',messages.sender,message_receivers.receiver),
CONCAT_WS(',',message_receivers.receiver,messages.sender))
SQL fiddle for it - http://sqlfiddle.com/#!2/8ee98/2

Answer 2

The standard solution is to use an uncorrelated subquery, as follows:
SELECT x.*
FROM my_table
JOIN
( SELECT grouping_column
, MAX(ordering_column) max_ordering_column
FROM my_table
GROUP
BY grouping_column
) y
ON y.grouping_column = x.grouping_column
AND y.max_ordering_column = x.ordering_column;
or, where a key is formed on multiple columns...
SELECT x.*
FROM my_table
JOIN
( SELECT grouping_column1
, grouping_column2
, MAX(ordering_column) max_ordering_column
FROM my_table
GROUP
BY grouping_column1
, grouping_coulmn2
) y
ON y.grouping_column1 = x.grouping_column1
AND y.grouping_column2 = x.grouping_column2
AND y.max_ordering_column = x.ordering_column;