php mysql Group By to get latest record, not first record

Question

The Table:

(`post_id`, `forum_id`, `topic_id`, `post_time`) 
(79, 8, 4, \'2012-11-19 06:58:08\');
(80, 3, 3, \'2012-11-19 06:58:42\'),
(81, 9, 9, \'2012-11-1         


        

Answer 1

try something like

SELECT post_id, forum_id, topic_id 
FROM   (SELECT post_id, forum_id, topic_id
        FROM posts
        ORDER BY post_time DESC) 
GROUP BY topic_id 
ORDER BY topic_id desc
LIMIT 0,5

change the order by and limit as needed.

Answer 2

This will also work fine for you.

SELECT *
FROM (
  SELECT post_id, forum_id, topic_id FROM posts
  ORDER BY post_time DESC
  LIMIT 5
) customeTable
GROUP BY topic_id

Answer 3

If you select attributes that are not used in the group clause, and are not aggregates, the result is unspecified. I.e you don't know which rows the other attributes are selected from. (The sql standard does not allow such queries, but MySQL is more relaxed).

The query should then be written e.g. as

SELECT post_id, forum_id, topic_id
FROM posts p
WHERE post_time =
  (SELECT max(post_time) FROM posts p2
   WHERE p2.topic_id = p.topic_id
   AND p2.forum_id = p.forum_id)
GROUP BY forum_id, topic_id, post_id
ORDER BY post_time DESC
LIMIT 5;

or

SELECT post_id, forum_id, topic_id FROM posts
NATURAL JOIN
(SELECT forum_id, topic_id, max(post_time) AS post_time
 FROM posts
 GROUP BY forum_id, topic_id) p
ORDER BY post_time
LIMIT 5;

Answer 4

Maybe not the best way of doing it but sometimes the function group_concat() can be userfull, it will return a string of all aggregated values sorted like you want and seperated by comma (coupled value are separated by space). I then use the function SPLIT_STRING() to cut the first id in the string.

SELECT  
post_id, 
SPLIT_STRING( group_concat( forum_id, post_time ORDER BY post_time DESC ) ,' ',1 )as forum_id, 
SPLIT_STRING( group_concat( topic_id, post_time ORDER BY post_time DESC ) ,' ',1 )as topic_id ,
FROM posts 
GROUP BY topic_id 
ORDER BY post_time DESC
LIMIT 5

So the aggregated forum_id, post_time will be like this :

81 2012-11-19 06:59:04,83 2012-11-19 16:07:46

So you need to work with a string representation of integers and datetime couples, each couples separated by comma so I used this function to get the first INT :

CREATE FUNCTION SPLIT_STRING(str VARCHAR(255), delim VARCHAR(12), pos INT)
RETURNS VARCHAR(255)
RETURN REPLACE(SUBSTRING(SUBSTRING_INDEX(str, delim, pos),
       LENGTH(SUBSTRING_INDEX(str, delim, pos-1)) + 1),
       delim, '');

Note : function SPLIT_STRING(str, delim, pos) was found here : Equivalent of explode() to work with strings in MySQL

Answer 5

It's not very beautiful , but it works:

SELECT * FROM (SELECT  post_id, forum_id, topic_id FROM posts
ORDER BY post_time DESC) as temp
GROUP BY topic_id