SQL query to get most recent row for each instance of a given key

I'm trying to get the ip, user, and most recent timestamp from a table which may contain both the current ip for a user and one or more prior ips. I'd like one row for each user containing the most recent ip and the associated timestamp. So if a table looks like this:

username      |  ip      |  time_stamp  
--------------|----------|--------------  
ted           | 1.2.3.4  | 10  
jerry         | 5.6.6.7  | 12  
ted           | 8.8.8.8  | 30  

I'd expect the output of the query to be:

jerry    |  5.6.6.7   |  12
ted      |  8.8.8.8   |  30  

Can I do this in a single sql query? In case it matters, the DBMS is Postgresql.

Try this:

Select u.[username]
      ,u.[ip]
      ,q.[time_stamp]
From [users] As u
Inner Join (
    Select [username]
          ,max(time_stamp) as [time_stamp]
    From [users]
    Group By [username]) As [q]
On u.username = q.username
And u.time_stamp = q.time_stamp

Nice elegant solution with ROW_NUMBER window function (supported by PostgreSQL - see in SQL Fiddle):

SELECT username, ip, time_stamp FROM (
 SELECT username, ip, time_stamp, 
  ROW_NUMBER() OVER (PARTITION BY username ORDER BY time_stamp DESC) rn
 FROM Users
) tmp WHERE rn = 1;

Something like this:

select * 
from User U1
where time_stamp = (
  select max(time_stamp) 
  from User 
  where username = U1.username)

should do it.

Both of the above answers assume that you only have one row for each user and time_stamp. Depending on the application and the granularity of your time_stamp this may not be a valid assumption. If you need to deal with ties of time_stamp for a given user, you'd need to extend one of the answers given above.

To write this in one query would require another nested sub-query - things will start getting more messy and performance may suffer.

I would have loved to have added this as a comment but I don't yet have 50 reputation so sorry for posting as a new answer!

Can't post comments yet, but @Cristi S's answer works a treat for me.

In my scenario, I needed to keep only the most recent 3 records in Lowest_Offers for all product_ids.

Need to rework his SQL to delete - thought that this would be ok, but syntax is wrong.

DELETE from (
SELECT product_id, id, date_checked,
  ROW_NUMBER() OVER (PARTITION BY product_id ORDER BY date_checked DESC) rn
FROM lowest_offers
) tmp WHERE > 3;

I've been using this because I'm returning results from another table. Though I'm trying to avoid the nested join if it helps w/ one less step. Oh well. It returns the same thing.

select
users.userid
, lastIP.IP
, lastIP.maxdate

from users

inner join (
    select userid, IP, datetime
    from IPAddresses
    inner join (
	    select userid, max(datetime) as maxdate
	    from IPAddresses
	    group by userid
	    ) maxIP on IPAddresses.datetime = maxIP.maxdate and IPAddresses.userid = maxIP.userid
	) as lastIP on users.userid = lastIP.userid