SQL - GROUP BY 最大值

Question

Note: I'm not sure if I gave this question the most leading title since I'm not sure on the correct approach towards this, but I couldn't find other examples anywhere since it's quite a specific query.

So, I have a table "votes", which is filled with votes created by users (uniquely identified as a number in the user_id column) which correspond to relevent posts in another table (vote records "upvote" each relevent post within the user interface).

I intend to sort these votes (by datetime) in order of latest vote created for each post ( post_id column), and as such, avoiding duplicate returned values of each post_id .

I input the following query:

SELECT id, user_id, post_id, created, MAX(created)
FROM votes
GROUP BY post_id, user_id
ORDER BY max(created) DESC

And get returned:

Table: votes

 id  |  user_id  |  post_id  |        created        |    MAX(created)
 ----+-----------+-----------+-----------------------+--------------------
 115 | 1         | 42        | 2014-07-03 23:08:31   | 2016-03-07 12:08:31
 ----+-----------+-----------+-----------------------+--------------------        
 237 | 2         | 101       | 2014-02-13 23:05:14   | 2016-03-05 23:05:14         
 ----+-----------+-----------+-----------------------+--------------------  
 431 | 7         | 944       | 2014-10-22 22:58:37   | 2016-03-03 19:58:37
 ----+-----------+-----------+-----------------------+--------------------
 255 | 15        | 101       | 2014-02-15 14:02:01   | 2016-02-01 23:05:14
 ----+-----------+-----------+-----------------------+--------------------
 ... | ...       | ...       | ...                   | ...

As you can see, there is a duplicate of the post_id "101". The result of this query seems to sort by maximum created time for each user_id , showing duplicated post_id 's, eg there are two post_id column rows of "101", when I would only like to diplay the only post_id column value of "101" which has the maximum created time (MAX( created )).

The post_id and user_id columns seemingly must be grouped together, else if I just group by post_id I'm unable to sort by MAX( created ) since it won't return the max( created ) for each post_id.

How do I remove these duplicated post_id values that don't return the maximum created time?

What I'm after:

Table: votes

 id  |  user_id  |  post_id  |        created        |    MAX(created)
 ----+-----------+-----------+-----------------------+--------------------
 115 | 1         | 42        | 2014-07-03 23:08:31   | 2016-03-07 12:08:31
 ----+-----------+-----------+-----------------------+--------------------        
 237 | 2         | 101       | 2014-02-13 23:05:14   | 2016-03-05 23:05:14         
 ----+-----------+-----------+-----------------------+--------------------  
 431 | 7         | 944       | 2014-10-22 22:58:37   | 2016-03-03 19:58:37
 ----+-----------+-----------+-----------------------+--------------------
 ... | ...       | ...       | ...                   | ...

Answer 1

Assuming you only want the last vote for each post:

SELECT  v.*
FROM    posts p
JOIN    votes v
ON      v.id =
        (
        SELECT  id
        FROM    votes vi
        WHERE   post_id = p.id
        ORDER BY
                created DESC
        LIMIT 1
        )

Answer 2

If you are looking for getting last user_id whom edited post_id , try group by post_id and ordering by time desc (or id if it is auto increment).

 SELECT tbl.* , GROUP_CONCAT('(',tbl.user_id,',',tbl.created,')') as myhistory FROM
   (SELECT id, user_id, post_id, created, MAX(created)
   FROM votes
   ORDER BY max(created) DESC
   ) as tbl
 GROUP BY tbl.post_id

If you need history for (user_id,time) you can use group_concat function as mentioned in code for myhistory column.

Answer 3

SELECT maintable.*
FROM TABLE_NAME maintable
LEFT OUTER JOIN TABLE_NAME temporarytable
 ON maintable.GROUPING_BY_COLUMN = temporarytable.GROUPING_BY_COLUMN
 AND maintable.COLUMN_WHERE_THE_MAXIMUM_IS_NEEDED < temporarytable.COLUMN_WHERE_THE_MAXIMUM_IS_NEEDED
WHERE temporarytable.COLUMN_WHERE_THE_MAXIMUM_IS_NEEDED IS NULL
ORDER BY PRIMARY_KEY_COLUMN DESC
LIMIT 50;

An alternative way to get the maximum value from a group. This query does not require aggregation, as is the case with “GROUP BY”.

In addition, when grouping using “GROUP BY”, each of the groups is sorted by primary key, which also takes a lot of time.

My query compares the values of one table with another. Until he can find nothing more. If nothing else is found, then this is the maximum.

This query can help you save time getting the maximum value from the group.