Need help with RANK() on NULL data

Question

Hi All
I am using Oracle 10g and running a query with RANK(), but it is not returning a desired output. Pleas HELP!!
I have a STATUS table that shows the history of order status.. I have a requirement to display the order and the last status date (max). If there is any NULL date for an order then show NULL.
STATUS
ORD_NO | STAT | DT
1 | Open |
1 | Pending |
2 | Open |
2 | Pending |
3 | Open |1/1/2009
3 | Pending |1/6/2009
3 | Close |
4 | Open |3/2/2009
4 | Close |3/4/2009
Result should be (max date for each ORD_NO otherwise NULL):
ORD_NO |DT
1 |
2 |
3 |
4 |3/4/2009
CREATE TABLE Status (ORD_NO NUMBER, STAT VARCHAR2(10), DT DATE);
INSERT INTO Status VALUES(1, 'Open', NULL);
INSERT INTO Status VALUES(1, 'Pending', NULL);
INSERT INTO Status VALUES(2, 'Open', NULL);
INSERT INTO Status VALUES(2, 'Pending',NULL);
INSERT INTO Status VALUES(3, 'Open', '1 JAN 2009');
INSERT INTO Status VALUES(3,'Pending', '6 JAN 2009');
INSERT INTO Status VALUES(3, 'Close', NULL);
INSERT INTO Status VALUES(4, 'Open', '2 MAR 2009');
INSERT INTO Status VALUES(4, 'Close', '4 MAR 2009');
COMMIT;
I tried using RANK function to rank all the orders by date. So used ORDER BY cluse on date in descending order thinking that the null dates would be on top and will be grouped together by each ORD_NO.
SELECT ORD_NO, DT, RANK() OVER (PARTITION BY ORD_NO ORDER BY DT DESC)
FROM Status;
...but the result was something..
ORD_NO |DT |RANKING
*1 | | 1*
*1 | | 1*
*2 | | 1*
*2 | | 1*3 | | 1
3 |1/6/2009 | 2
3 |1/1/2009 | 3
4 |3/4/2009 | 1
4 |3/2/2009 | 2
I am not sure why didn't the first two ORD_NOs didn't group together and why ranking of 1 was assigned to them. I was assuming something like:
ORD_NO |DT |RANKING
*1 | | 1*
*1 | | 2*
*2 | | 1*
*2 | | 1*
3 | | 1
3 |1/6/2009 | 2
3 |1/1/2009 | 3
4 |3/4/2009 | 1
4 |3/2/2009 | 2
Please guide me if I am missing something here?
Regards
Sri

Answer 1

Hi,
If i well understood, you don't need rank
SELECT   ord_no, MAX (dt)KEEP (DENSE_RANK LAST ORDER BY dt) dt
    FROM status
GROUP BY ord_no
SQL> select * from status;
    ORD_NO STAT       DT
         1 Open
         1 Pending
         2 Open
         2 Pending
         3 Open       2009-01-01
         3 Pending    2009-01-06
         3 Close
         4 Open       2009-03-02
         4 Close      2009-03-04
9 ligne(s) sélectionnée(s).
SQL> SELECT   ord_no, MAX (dt)KEEP (DENSE_RANK LAST ORDER BY dt) dt
  2      FROM status
  3  GROUP BY ord_no;
    ORD_NO DT
         1
         2
         3
         4 2009-03-04
SQL>