How to select row with the latest timestamp from duplicated rows in a database table?

Question

I have a table with duplicate & triplicate rows - how do I select the rows that are duplicated but have the latest timestamp as well as the un-duped rows?

 -------------------------------------
| pk_id | user_id | some_timestamp    |
|-------------------------------------|
| 1     | 123     | 10-Jun-12 14.30   |
| 2     | 123     | 19-Jun-12 21.50   |
| 3     | 567     | 10-Jun-12 09.23   |
| 4     | 567     | 12-Jun-12 09.45   |
| 5     | 567     | 13-Jun-12 08.40   |
| 6     | 890     | 13-Jun-12 08.44   |
 -------------------------------------

So that I end up with:

 -------------------------------------
| pk_id | user_id | some_timestamp    |
|-------------------------------------|
| 2     | 123     | 19-Jun-12 21.50   |
| 5     | 567     | 13-Jun-12 08.40   |
| 6     | 890     | 13-Jun-12 08.44   |
 -------------------------------------

Answer 1

SELECT * FROM ( 
SELECT pk_id, 
       user_id, 
       some_timestamp, 
       ROW_NUMBER() OVER (PARTITION BY user_id ORDER BY some_timestamp DESC) col 
FROM table) x  
WHERE x.col = 1

Answer 2

try this

select * from table 
where some_timestamp 
   in (select max(some_timestamp)
       from table group by user_id)

Answer 3

Try this, I made a SQLFIDDLE which returns the correct set of data

SELECT * FROM YourTable AS T1
INNER JOIN
( SELECT user_id , MAX(some_timestamp) AS some_timestamp FROM YourTable 
GROUP BY user_id
) AS T2
ON T1.User_Id = T2.User_Id AND T1.some_timestamp = T2.some_timestamp
ORDER BY 1

http://sqlfiddle.com/#!6/f7bba/6