Math function ' ' on Date columns is showing incorrect results. Pls xplain!

Question

Hi guys,
I'm having a query wherein two date columns are used. One holds just the date part and the other date+time.
Columns are: - processing_date (holds on date)
authorised_ts (holds date+time)
The query is : -
SELECT a.processing_date,a.authorised_ts,
SUBSTR(a.processing_date,1,9),SUBSTR(a.authorised_ts,1,9)
FROM TEBINS01 a WHERE a.ref_instty_code = 'BULK'
AND a.status_code = 'PPAY'
AND SUBSTR(a.processing_date,1,9) < SUBSTR(a.authorised_ts,1,9)
I know that the substr function is creating the problem as it is converting the columns into characters.
*****The results show that 22-NOV-04 is greater than 12-JAN-05****
I've tried to use the below query to convert the above 2 dates into number(using to_number) via: -
SELECT TO_NUMBER(SUBSTR('22-NOV-04', LENGTH(RTRIM('22-NOV-04','1234567890'))-LENGTH('22-NOV-04'))) FROM dual
The result is 4
and when I use the above query for 12-JAN-05 the result is 5.
So even when I convert to number 5 is definitely greater than 4. But how and why is the substr function in the original query giving the wrong results?
Thanks in advance guys
B

Answer 1

As David points out, if the two columns are stored as dates, then you do not need to convert them at all to compare them. The only possible issue may be the fact that processing_date has no time component and authorised_ts has a time component.
Given the following:
SQL> DESC t;
Name                                      Null?    Type
ID                                                 NUMBER
PROCESSING_DATE                                    DATE
AUTHORISED_TS                                      DATE
SQL> SELECT * FROM t;
        ID PROCESSING_DATE      AUTHORISED_TS
         1 10-Aug-2005 00:00:00 09-Aug-2005 03:51:47
         2 06-Aug-2005 00:00:00 11-Aug-2005 07:44:42
         3 10-Aug-2005 00:00:00 08-Aug-2005 17:18:07
         4 08-Aug-2005 00:00:00 09-Aug-2005 11:45:59
         5 12-Aug-2005 00:00:00 08-Aug-2005 12:56:49
         6 06-Aug-2005 00:00:00 10-Aug-2005 01:43:02
         7 08-Aug-2005 00:00:00 07-Aug-2005 09:23:19
         8 10-Aug-2005 00:00:00 10-Aug-2005 11:04:21
         9 05-Aug-2005 00:00:00 08-Aug-2005 11:29:47
        10 11-Aug-2005 00:00:00 07-Aug-2005 14:51:53You obvioulsy would want id's 2, 4, 6, and 9, but what about 8? The two dates are on the same day. If you only want to consider cases where the day is less (i.e. exclude id 8) then you should query:
SQL> SELECT * FROM t
  2  WHERE processing_date < TRUNC(authorised_ts);
        ID PROCESSING_DATE      AUTHORISED_TS
         2 06-Aug-2005 00:00:00 11-Aug-2005 07:44:42
         4 08-Aug-2005 00:00:00 09-Aug-2005 11:45:59
         6 06-Aug-2005 00:00:00 10-Aug-2005 01:43:02
         9 05-Aug-2005 00:00:00 08-Aug-2005 11:29:47However, if you hink that id 8 should be included then it is simply:
SQL> SELECT * FROM t
  2  WHERE processing_date < authorised_ts;
        ID PROCESSING_DATE      AUTHORISED_TS
         2 06-Aug-2005 00:00:00 11-Aug-2005 07:44:42
         4 08-Aug-2005 00:00:00 09-Aug-2005 11:45:59
         6 06-Aug-2005 00:00:00 10-Aug-2005 01:43:02
         8 10-Aug-2005 00:00:00 10-Aug-2005 11:04:21
         9 05-Aug-2005 00:00:00 08-Aug-2005 11:29:47Note that the comparision does not depend on the display format of the date:
SQL> ALTER SESSION SET nls_date_format = 'mm/dd/yy';
Session altered.
SQL> SELECT * FROM t
  2  WHERE processing_date < TRUNC(authorised_ts);
        ID PROCESSI AUTHORIS
         2 08/06/05 08/11/05
         4 08/08/05 08/09/05
         6 08/06/05 08/10/05
         9 08/05/05 08/08/05HTH
John