Which aggregation function to use in a GROUP BY query?

Question

PostgreSQL

There is a data sample that I received by SQL query

SELECT ref_id, id, created_at FROM "images"
GROUP BY ref_id, id
ORDER BY created_at desc
LIMIT 20

id	ref_id	created_at
25553	1	2014-06-07 13:38:46
25408	1	2014-06-07 13:36:42
25105	1	2014-06-07 13:32:43
25068	1	2014-06-07 13:32:16
24986	670	2014-06-07 01:53:49
24978	670	2014-06-07 01:53:44
24977	670	2014-06-07 01:53:43
24862	666	2014-06-07 01:52:15
24050	602	2014-06-07 01:33:43
24051	602	2014-06-07 01:33:43
20963	1818	2014-06-06 03:16:09
19681	1723	2013-06-07 01:37:50
19680	1723	2013-06-07 01:37:49
18452	236	2013-06-07 01:14:36
16913	517	2012-10-05 01:37:09
16750	658	2012-10-05 01:36:34
15229	696	2012-10-05 01:18:52
14416	1192	2012-10-04 21:41:37
14417	1192	2012-10-04 21:41:37
13107	749	2012-03-24 20:08:59

I expected to get no more than one row with each ref_id in the selection, choosing from images with the given ref_id only the latest date.
Like this:

id	ref_id	created_at
25553	1	2014-06-07 13:38:46
24986	670	2014-06-07 01:53:49
24862	666	2014-06-07 01:52:15
24050	602	2014-06-07 01:33:43
20963	1818	2014-06-06 03:16:09
19681	1723	2013-06-07 01:37:50
18452	236	2013-06-07 01:14:36
16913	517	2012-10-05 01:37:09
16750	658	2012-10-05 01:36:34
15229	696	2012-10-05 01:18:52
14416	1192	2012-10-04 21:41:37
13107	749	2012-03-24 20:08:59

Which aggregation function to use in a GROUP BY query to achieve what you want? Help make a request.

Answer 1

I expected to get no more than one row in the sample with each ref_id

Why would you expect to see only one ref_id if you explicitly said then group by id?
Given the name and the fact that postgresql basically agreed to execute the query, id is the primary key. And grouping by primary key is expected to not change the result.

select  distinct on (ref_id)  ref_id,  id,  created_at from tablename  order by ref_id, created_at desc;

Answer 2

I do not know what about group by
But usually it is done by joining itself and choosing the desired (maximum) date.
Or through max() try to pull out the desired date.
Either select * from (subquery where grouping and sorting by max date)
Or just sort by date the query that is.
In general, a lot of options, which one to choose for yourself.