[Oracle 8i] Query for N rows by column value?

Question

I was just wondering if what I want to do is possible within my query (rather than programmatically)...
I want to return the N most recent records for each unique value in a particular column.
Here's a sample table:
CREATE TABLE     orders
(     order_no     numeric(10)
     part_no          varchar(5)
     close_date     date
     order_qty     numeric(10)
     scrap_qty     numeric(10)
     CONSTRAINT order_pk PRIMARY KEY (order_no)
);And some sample data....
INSERT INTO     orders     VALUES
(0000012345,'ABC-1',TO_DATE('01-01-2010','mm-dd-yyyy'),10,1);
INSERT INTO     orders     VALUES
(0000013498,'ABC-1',TO_DATE('01-05-2010','mm-dd-yyyy'),12,2);
INSERT INTO     orders     VALUES
(0000033452,'ABC-1',TO_DATE('01-10-2010','mm-dd-yyyy'),5,0);
INSERT INTO     orders     VALUES
(0000001468,'ABC-1',TO_DATE('01-15-2010','mm-dd-yyyy'),15,1);
INSERT INTO     orders     VALUES
(0000022349,'BR723',TO_DATE('01-03-2010','mm-dd-yyyy'),8,1);
INSERT INTO     orders     VALUES
(0000069581,'BR723',TO_DATE('01-05-2010','mm-dd-yyyy'),5,0);
INSERT INTO     orders     VALUES
(0000436721,'BR723',TO_DATE('01-10-2010','mm-dd-yyyy'),14,1);
INSERT INTO     orders     VALUES
(0000213446,'A5001',TO_DATE('01-06-2010','mm-dd-yyyy'),5,1);
INSERT INTO     orders     VALUES
(0000327987,'A5001',TO_DATE('01-08-2010','mm-dd-yyyy'),5,0);
INSERT INTO     orders     VALUES
(0000041353,'A5001',TO_DATE('01-14-2010','mm-dd-yyyy'),12,1);
INSERT INTO     orders     VALUES
(0000011241,'A5001',TO_DATE('01-15-2010','mm-dd-yyyy'),5,1);In this example, what I want to return are the 2 most recent orders (by close_date) for each part number.
Here is a table with the results I want to get, based on the scenario above:
order_no     part_no          close_date     order_qty     scrap_qty
0000001468     'ABC-1'          '01-15-2010'     15          1
0000033452     'ABC-1'          '01-10-2010'     5          0
0000436721     'BR723'          '01-10-2010'     14          1
0000069581     'BR723'          '01-05-2010'     5          0
0000011241     'A5001'          '01-15-2010'     5          1
0000041353     'A5001'          '01-14-2010'     12          1Is it possible to write a query to get these results, or am I going to have to query for all available data, and find the 2 most recent rows programmatically?
Thanks in advance!

Answer 1

Hi,
user11033437 wrote:
I'm going to test that out right now. I think if it works, I may need to use dense_rank() rather than rank(), because it is possible that two orders for the same part number could have the same close date, and according to what I've looked up on the rank() and dense_rank() functions, rank() can give non-consecutive results if the values are the same.What's wrong with non-consecutive values?
Use RANK, DENSE_RANK or ROW_NUMBER depending on what you want.
For example; say a certain part has been ordered 8 times:
3 times with close_date January 29, 2010 (all at exactly the same time),
4 times with close_date January 28, 2010 (all at exactly the same time), and
1 time with close_date January 27, 2010.
If you ask for the last 2 rows:
RANK will give you the 3 rows from January 29. (All 3 have an equal claim to being in the top 2.)
DENSE_RANK will give you the 7 rows from January 28-29 (the last two values , regardless of how many rows have them).
ROW_NUMBER will give you 2 rows from January 29. (Which 2? It's arbitrary unless you add a tie-breaker to the ORDER BY clause.)
All these functions are available in Oracle 8.1.