Oracle Sql: Select only the latest records by an id and a date

Question

My table looks like this:

ID       FROM     WHEN
1        mario    24.10.19
1        robin    23.10.19
2        mario    24.10.19    
3        robin    23.10.19
3        mario    22.10.19   

I just want the newest records from an ID. So the result should look like this:

ID       FROM     WHEN
1        mario    24.10.19
2        mario    24.10.19    
3        robin    23.10.19

I dont know how to get this result

Answer 1

There are multiple methods. For just three columns in Oracle, I have had good luck with group by:

select id,
       max("from") keep (dense_rank first order by "when" desc) as "from",
       max("when") as when
from t
group by id;

Often a correlated subquery performs well, in this case, with an index on (id, when):

select t.*
from t
where t."when" = (select max(t2."when") from t t2 where t2.id = t.id);

And the canonical solution is to use window functions:

select t.*
from (select t.*,
             row_number() over (partition by id order by "when" desc) as seqnum
      from t
     ) t
where seqnum = 1;

Oracle has a smart optimizer but this has to do a bit more work, because row numbers are assigned to all rows before the filtering. That can make this a wee bit slower (in some databases) than alternative, but it is still a very viable solution.