GROUP and MAX Query

Question

Hi
I have two tables that store following information
CREATE TABLE T_FEED (FEED_ID NUMBER, GRP_NUM NUMBER);
CREATE TABLE T_FEED_RCV (FEED_ID NUMBER, RCV_DT DATE);
INSERT INTO T_FEED VALUES (1, 1);
INSERT INTO T_FEED VALUES (2, 1);
INSERT INTO T_FEED VALUES (3, 2);
INSERT INTO T_FEED VALUES (4, NULL);
INSERT INTO T_FEED VALUES (5, NULL);
INSERT INTO T_FEED_RCV VALUES (2, '1-MAY-2009');
INSERT INTO T_FEED_RCV VALUES (3, '1-FEB-2009');
INSERT INTO T_FEED_RCV VALUES (4, '12-MAY-2009');
COMMIT;
I join these tables using the following query to return all the feeds and check when each feed was received:
SELECT
F.FEED_ID,
F.GRP_NUM,
FR.RCV_DT
FROM T_FEED F
LEFT OUTER JOIN T_FEED_RCV FR
ON F.FEED_ID = FR.FEED_ID
ORDER BY GRP_NUM, RCV_DT DESC;
Output
Line: ----------
FEED_ID     GRP_NUM     RCV_DT
1     1     
2     1     5/1/2009
3     2     2/1/2009
5          
4          5/12/2009
Actually I want the maximum date of when we received the feed. Grp_Num tells which feeds are grouped together. NULL grp_num means they are not grouped so treat them as individual group. In the example - Feeds 1 and 2 are in one group and any one of the feed is required. Feed 3, 4 and 5 are individual groups and all the three are required.
I need a single query that should return the maximum date for the feeds. For the example the result should be NULL because.. out of feed 1 and 2 the max date is 5/1/2009. For feed 3 the max date is 2/1/2009, for feed 4 it is 5/12/2009 and for feed 4 it is NULL. Since one of the required feed is null so the results should be null.
DELETE FROM T_FEED;
DELETE FROM T_FEED_RCV;
COMMIT;
INSERT INTO T_FEED VALUES (1, 1);
INSERT INTO T_FEED VALUES (2, 1);
INSERT INTO T_FEED VALUES (3, NULL);
INSERT INTO T_FEED VALUES (4, NULL);
INSERT INTO T_FEED_RCV VALUES (2, '1-MAY-2009');
INSERT INTO T_FEED_RCV VALUES (3, '1-FEB-2009');
INSERT INTO T_FEED_RCV VALUES (4, '12-MAY-2009');
COMMIT;
For above inserts, the result should be for feed 1 and 2 - 5/1/2009, feed 3 - 2/1/2009 and feed 4 - 5/12/2009. So the max of these dates is 5/12/2009.
I tried using MAX function grouped by GRP_NUM and also tried using DENSE_RANK but unable to resolve the issue. I am not sure how can I use the same query to return - not null value for same group and null (if any) for those that doesn't belong to any group. Appreciate if anyone can help me..

Answer 1

Hi,
Kuul13 wrote:
Thanks Frank!
Appreciate your time and solution. I tweaked your earlier solution which was more cleaner and simple and built the following query to resolve the prblem.
SELECT * FROM (
SELECT NVL (F.GRP_NUM, F.CARR_ID || F.FEED_ID || TO_CHAR(EFF_DT, 'MMDDYYYY')) AS GRP_ID
,MAX (FR.RCV_DT) AS MAX_DT
FROM T_FEED F
LEFT OUTER JOIN T_FEED_RCV FR ON F.FEED_ID = FR.FEED_ID
GROUP BY NVL (F.GRP_NUM, F.CARR_ID || F.FEED_ID || TO_CHAR(EFF_DT, 'MMDDYYYY'))
ORDER BY MAX_DT DESC NULLS FIRST)
WHERE ROWNUM=1;
I hope there are no hidden issues with this query than the later one provided by you.Actually, I can see 4 issues with this. I admit that some of them are unlikely, but why take any chances?
(1) The first argument to NVL is a NUMBER, the second (being the result of ||) is a VARCHAR2. That means one of them will be implicitly converted to the type of the other. This is just the kind of thing that behaves differently in different versions or Oracle, so it may work fine for a year or two, and then, when you change to another version, mysteriously quit wiorking. When you have to convert from one type of data to another, always do an explicit conversion, using TO_CHAR (for example).
(2)
F.CARR_ID || F.FEED_ID || TO_CHAR(EFF_DT, 'MMDDYYYY)'will produce a key like '123405202009'. grp_num is a NUMBER with no restriction on the number of digits, so it could conceivably be 123405202009. The made-up grp_ids must never be the same any real grp_num.
(3) The combination (carr_id, feed_id, eff_dt) is unique, but using TO_CHAR(EFF_DT, 'MMDDYYYY) assumes that the combination (carr_id, feed_id, TRUNC (eff_dt)) is unique. Even if eff_dt is always entered as (say) midnight (00:00:00) now, you may decide to start using the time of day sometime in the future. What are the chances that you'll remember to change this query when you do? Not very likely. If multiple rows from the same day are relatively rare, this is the kind of error that could go on for months before you even realize that there is an error.
(4) Say you have this data in t_feed:
carr_id      feed_id  eff_dt       grp_num
1        234      20-May-2009  NULL
12       34       20-May-2009  NULL
123      4        20-May-2009  NULLAll of these rows will produce the same grp_id: 123405202009.
Using NVL, as you are doing, allows you to get by with just one sub-query, which is nice.
You can do that and still address all the problems above:
SELECT  *
FROM     (
     SELECT  NVL2 ( F.GRP_NUM
               , 'A' || TO_CHAR (f.grp_num)
               , 'B' || TO_CHAR (f.carr_id) || ':' ||
                            TO_CHAR (f.feed_id) || ':' ||
                            TO_CHAR ( f.eff_dt
                                        , 'MMDDYYYYHH24MISS'
               ) AS grp_id
     ,     MAX (FR.RCV_DT) AS MAX_DT
     FROM               T_FEED      F
     LEFT OUTER JOIN  T_FEED_RCV  FR        ON  F.FEED_ID = FR.FEED_ID
     GROUP BY  NVL2 ( F.GRP_NUM
                 , 'A' || TO_CHAR (f.grp_num)
                 , 'B' || TO_CHAR (f.carr_id) || ':' ||
                                 TO_CHAR (f.feed_id) || ':' ||
                                 TO_CHAR ( f.eff_dt
                                             , 'MMDDYYYYHH24MISS'
     ORDER BY  MAX_DT   DESC   NULLS FIRST
WHERE      ROWNUM = 1;I would still use two sub-queries, adding one to compute grp_id, so we don't have to repeat the NVL2 expression. I would also use a WITH clause rather than in-line views.
Do you find it easier to read the query above, or the simpler query you posted in your last message?
Please make things easy on yourself and the people who want to help you. Always format your code so that the way the code looks on the screen makes it clear what the code is doing.
In particular, the formatting should make clear
(a) where each clause (SELECT, FROM, WHERE, ...) of each query begins
(b) where sub-queries begin and end
(c) what each argument to functions is
(d) the scope of parentheses
When you post formatted text on this site, type these 6 characters:
before and after the formatted text, to preserve spacing.
The way you post the DDL (CREATE TABLE ...)  and DML (INSERT ...) statements is great: I wish more people were as helpful as you.
There's no need to format the DDL and DML.  (If you want to, then go ahead: it does help a little.)