Parsing VARCHAR to DATE where the VARCHAR column has multiple format

Question

Hello,
I'm a little stumped here so I figured I'd try posting here.
I have an application where I can not alter the code. This application has a form with a calender conrol on it that allows the user to select a date. This form is internationalized so date formats vary based on the country the user is coming form (mm/dd/yyyy, dd-Mon-yyyy, dd.mm.yyyy, etc). The application saved the string of the date as the calender control returns to the form, it does not save the value as a date value.
I need to preform a query that can do some cacluations off of this date. When there is only one date format returned in the query, there is no problem
CASE WHEN TO_DATE(d3.data,'Mon/dd/yyyy')-NVL(trunc(i3.end_date),trunc(SYSDATE)) < 0 THEN calc_business_days(TO_DATE(d3.data,'Mon/dd/yyyy'),NVL(trunc(i3.end_date),trunc(SYSDATE)))
WHEN TO_DATE(d3.data,'Mon/dd/yyyy')-NVL(trunc(i3.end_date),trunc(SYSDATE)) > 0 THEN 0-(calc_business_days( NVL(trunc(i3.end_date),trunc(SYSDATE)) ,TO_DATE(d3.data,'Mon/dd/yyyy')))
ELSE 0 END
However, this errors when there are multiple formats in the varchar column for the same query. Can anyone provide any guidance on how to handle this? I am using Oracle 10g.
Thanks

Answer 1

The simplest option is generally to create a new function that does the conversion and sets up a hierarchy of date formats. Obviously, there may be some data loss here-- '01/02/03' is a valid string in the format mm/dd/yy, dd/mm/yy, and yy/mm/dd but it means different things in each.
Something like
CREATE OR REPLACE FUNCTION my_to_date( p_in_string VARCHAR2 )
  RETURN date
IS
BEGIN
  RETURN to_date( p_in_string, 'MM/DD/YYYY' );
EXCEPTION
  WHEN others THEN
    BEGIN
      RETURN to_date( p_in_string, 'DD-MON-YYYY' );
    EXCEPTION
      WHEN others THEN
        BEGIN
          RETURN to_date( p_in_string, 'DD.MM.YYYY' );
        EXCEPTION
          WHEN others THEN
            RETURN NULL;
        END;
    END;
END;
/Justin