MAX for each subset

Question

I have a table that looks like this:

+---------------------------------------------+
| EVENT_ID | ITEM_ID | PERSON_ID | EVENT_DATE |
+---------------------------------------------+
| 123      | 1       | 2         | 21-AUG-03  |
| 124      | 1       | 3         | 22-AUG-03  |
| 125      | 1       | 45        | 23-AUG-03  |
| 126      | 1       | (null)    | 24-AUG-03  |
| 127      | 2       | 2         | 25-AUG-03  |
| 128      | 2       | 6         | 26-AUG-03  |
| 129      | 2       | 1         | 27-AUG-03  |
+---------------------------------------------+

So, I need to get the latest (MAX(event_date)), non-null person_id per item_id. We have come up with a fairly simple PL/SQL approach to this, but were trying to get the job done with straight SQL. Anyone have an idea?

Just as a side-note, the event_id will not always be sequential like this. We have two redundant DB servers.

Thanks ahead of time.

Answer 1

Try this

create table event_table (
    event_id int,
    item_id int,
    person_id int,
    event_date date
);

insert into event_table values (123, 1, 2,    to_date('21-aug-03','DD-MON-YY'));
insert into event_table values (124, 1, 3,    to_date('22-aug-03','DD-MON-YY')); 
insert into event_table values (125, 1, 45,   to_date('23-aug-03','DD-MON-YY'));  
insert into event_table values (126, 1, null, to_date('24-aug-03','DD-MON-YY'));  
insert into event_table values (127, 2, 2,    to_date('25-aug-03','DD-MON-YY'));  
insert into event_table values (128, 2, 6,    to_date('26-aug-03','DD-MON-YY'));  
insert into event_table values (129, 2, 1,    to_date('27-aug-03','DD-MON-YY'));  


select * from event_table;

select t1.item_id, t1.Person_id, t1.event_date
from event_table t1
join 
(select item_id , max(event_date) max_event_date 
   from event_table where Person_id is not null group by item_id ) t2 
on t1.item_id = t2.item_id and t1.event_date = t2.max_event_date;

Answer 2

The following requires only one full table scan.

SELECT Item_ID, Person_ID, Event_Date FROM
(
   SELECT Event_ID, Person_ID, Item_ID, Event_Date
      , MAX(EVENT_DATE) OVER (PARTITION BY Item_ID) HighEventDate 
   FROM Event_Table WHERE Person_ID IS NOT NULL
)
WHERE Event_Date = HighEventDate;