Receiving error when calculating two fields with different date format

Question

I am more familiar with SQL Server than Oracle, so after searching online without success, I am asking here.
I'm using PL/SQL Developer with Oracle DB 11g Enterprise Release 11.2.0.2.0 64 Bit
MyTable:
ID_Number     VarChar2
Date_Received     Date
Select ID_Number,
     Date_Received,
     To_Date(substr(ID_Number, 1,6), 'YYMMDD') SentDate,    
     Date_Received - To_Date(substr(ID_Number, 1,6), 'YYMMDD') NumDays
From MyTable
Where substr(ID_Number, 7,3) in ('ABC', 'ABD')
and Length(Trim(Translate((substr(ID_Number, 1,6)), '0123456789', ' ' ))) is null
ID_Number                    Date_Received          SentDate          NumDays
131002ABC1654106     10/16/2013               10/2/2013          14
131004ABD8813899     4/12/2013                 4/8/2013            4
131014ABD1844832     10/16/2013               10/14/2013          2
Sometimes the first 6 characters in the ID_Number are not numbers, and the Length(Trim(Translate removes those records
I just want records where NumDays > 2
I've tried putting the query in a subquery and using Where NumDays > 2 outside.  I've also tried using the computation directly in the Where clause.  Without this in the Where clause it runs fine, with it in either spot I get the following error:
ORA-01931: Date format picture ends before converting entire input string
I'm not sure how to put both dates in the same format.  I've tried declaring the format to no avail.  I do not understand how I can calculate within the select but not use the same calculation within the Where clause.
Thank you for your help.

Answer 1

Hi,
SQL is a language for describing the results you want.  How the system gets those results is up to it; you don't have much say regarding which conditions will be applied when.
One place where you can control the order of things is in a CASE expression.  When you say
CASE
    WHEN  condition_1
    THEN  expression_1
END
you can be sure that expression_1 will only be evaluated when cond_1 is TRUE.
Try something like this:
WITH  got_sent_date AS
    SELECT  id_number, date_received
    ,       CASE
                WHEN  TRANSLATE ( SUBSTR (id_number, 1, 6)
                                , 'x 0123456789'
                                , 'x'
                                )  IS NULL
                THEN  TO_DATE ( SUBSTR (id_number, 1 6)
                              , 'YYMMDD'
            END     AS sent_date
    FROM    MyTable
    WHERE   SUBSTR (id_number, 7, 3) IN ('ABC', 'ABD')
SELECT  id_number
,       date_received
,       sent_date
,       date_received - sent_date    AS num_days
FROM    got_sent_date
WHERE   date_received  > sent_date + 2
If you'd care to post some sample data (CREATE TABLE and INSERT statements) and the results you want from that data, then I could test this.
Of course, you'll still have run-time errors if id_number starts with 6 digits, but they don't happen to be valid, e.g. '131100' or '130229'.  That's one of the many reasons why storing date information in VARCHAR2 columns is such a bad idea.  To get around that problem, see
https://forums.oracle.com/message/4255051