Question

I have a table with this data:

ID   voting_ID   username   timestamp              XMLBallot
1    9           voter01    23. 4. 2012 8:54:45    xmldata
2    9           voter01    21. 4. 2012 14:00:34   xmldata
3    9           voter02    20. 4. 2012 16:01:10   xmldata
4    11          voter01    23. 4. 2012 8:40:45    xmldata
5    9           voter03    19. 4. 2012 21:18:49   xmldata

I need to get only one newest ballot for each voter (username) in specific voting_ID.

For example data I need to be returned for @voting_ID=9

ID   voting_ID   username   timestamp              XMLBallot
1    9           voter01    23. 4. 2012 8:54:45    xmldata
3    9           voter02    20. 4. 2012 16:01:10   xmldata
5    9           voter03    19. 4. 2012 21:18:49   xmldata

Please help me build that SQL Server 2008 query.. thank you
PS: table name is ballots

Answer 1

You have several options here but using adding a ROW_NUMBER grouped by user and sorted (descending) on your timestamp allows you to easily select the latest records.

Using ROW_NUMBER

SELECT *
FROM   (
         SELECT ID, voting_ID, username, timestamp, XMLBallot
                , rn = ROW_NUMBER() OVER (PARTITION BY voting_ID, username ORDER BY timestamp DESC)
         FROM   Ballots
       ) bt 
WHERE  rn = 1

Alternatively, you can select the maximum timestamp per user and join on that.

Using MAX

SELECT bt.ID, bt.voting_ID, bt.username, bt.timestamp, bt.XMLBallot
FROM   Ballots bt
       INNER JOIN (
          SELECT username, voting_ID, timestamp = MAX(timestamp)
          FROM   Ballots
          GROUP BY
                 username, voting_ID
        ) btm ON btm.username = bt.Username
                 AND btm.voting_ID = bt.voting_ID
                 AND btm.timestamp = bt.timestamp