DATE format - while defining the table column

Question

Hi Friends,
I came across a piece of code which was applying a TO_DATE() function on a date type column.
I have two tables t1 and t2 and they have col1 and col2 columns respectively of type DATE
t1
col1 DATE
t2
col2 DATEalso there is a variable var1 and is defined as below:
var1 DATE;now I have conditions as below in the code:
AND to_date(t1.col1,'dd/mm/yyyy') = to_date(t2.col2,'dd/mm/yyyy')
AND to_date(t1.col1,'dd/mm/yyyy') = to_date(var1,'dd/mm/yyyy')This code failed on Production with an error: ORA-01861 literal does not match format string
I changed the code to remove TO_DATE() and it worked fine. Dev clarified that they used this function so that if the two columns or variable are in diff date format.,..they can be put in sync.
e.g. if col1 is dd/mm/yyyy and col2 is mm/dd/yyyy then there will be an issue.
My doubt is.. in a given environment if we are creating a table and defining a column of type DATE , or defining a variable of type DATE, can their format be different at any time? I think No.?
In what scenario can the date format be different?
Thanks,

Answer 1

Having said that date format is only for display purposes, if you tend to avoid the error without changing the code then you can modify the NLS_DATE_FORMAT parameter at session level.
SQL> alter session set nls_date_format = 'dd-mm-yyyy'
  2  /
Session altered.
SQL> select to_date(sysdate,'dd/mm/yyyy') from dual
  2  /
TO_DATE(SY
29-12-2008
SQL> alter session set nls_date_format = 'mm-dd-yyyy'
  2  /
Session altered.
SQL> select to_date(sysdate,'dd/mm/yyyy') from dual
  2  /
select to_date(sysdate,'dd/mm/yyyy') from dual
ERROR at line 1:
ORA-01843: not a valid month
SQL> select to_date(sysdate,'mm/dd/yyyy') from dual
  2  /
TO_DATE(SY
12-29-2008Thanks,
Karthick.