SQL - get latest records from table where field is unique

Question

I have a table of data as follows

id    status    conversation_id    message_id    date_created
1     1         1                  72            2012-01-01 00:00:00
2     2         1                  87            2012-03-03 00:00:00
3     2         2                  95            2012-05-05 00:00:00

I want to get all the rows from the table in date_created DESC order, but only one row per conversation_id . So in the case of the example data above, I would want to get the rows with id 2 and 3.

Any advice is much appreciated.

Answer 1

SELECT t.id, t.status, t.conversation_id, t.message_id, t.date_created
    FROM YourTable t
        INNER JOIN (SELECT conversation_id, MAX(date_created) AS MaxDate
                        FROM YourTable
                        GROUP BY conversation_id) q
            ON t.conversation_id = q.conversation_id
                AND t.date_created = q.MaxDate
    ORDER BY t.date_created DESC;

Answer 2

See SQL Fiddle

SELECT T.*
FROM T
WHERE NOT EXISTS (
  SELECT * 
  FROM T AS _T
  WHERE _T.conversation_id = T.conversation_id
  AND (
    _T.date_created > T.date_created
    OR
    _T.date_created = T.date_created AND _T.id > T.id) 
)
ORDER BY T.date_created DESC

gets

ID      STATUS  CONVERSATION_ID   MESSAGE_ID    DATE_CREATED
3         2         2                95         May, 05 2012 
2         2         1                87         March, 03 2012 

Answer 3

Creation and inserts borrowed from @Incidently:

CREATE TABLE T
(id int, status int, conversation_id int, message_id int, date_created datetime);

insert into T (id, status, conversation_id, message_id, date_created)
values(1,     1,         1,                  72,            '2012-01-01 00:00:00');
insert into T (id, status, conversation_id, message_id, date_created)
values(2,     2,         1,                  87,            '2012-03-03 00:00:00');
insert into T (id, status, conversation_id, message_id, date_created)
values(3,     2 ,        2 ,                 95 ,           '2012-05-05 00:00:00');

Supposing the id is the primary key then this solves the problem pointed by @Incidentally in @Joe's answer:

select T.*
from 
    T
    inner join (
        select conversation_id, max(id) as id
        from T
        group by conversation_id
    ) s on s.id = T.id
order by T.date_created desc, T.conversation_id
;
+------+--------+-----------------+------------+---------------------+
| id   | status | conversation_id | message_id | date_created        |
+------+--------+-----------------+------------+---------------------+
|    3 |      2 |               2 |         95 | 2012-05-05 00:00:00 |
|    2 |      2 |               1 |         87 | 2012-03-03 00:00:00 |
+------+--------+-----------------+------------+---------------------+