Why dateformat matters?

Question

I am running into a problem with casting on Date, it firstly seems a bug in Oracle DB, but I tried both 9i and 10g and got the same result. Could someone please explain why dateformat matters in this case?
Thanks in avdance.
1.
select BirthDate,
cast(cast(BirthDate as CHARACTER (60)) as DATE)
from Employees
1937/09/19 00:00:00 2037/09/19 00:00:00 <----- ???
1948/12/08 00:00:00 2048/12/08 00:00:00 <----- ???
1952/02/19 00:00:00 1952/02/19 00:00:00
1955/03/04 00:00:00 1955/03/04 00:00:00
2. use default dateformat
select BirthDate, to_date(to_char(BirthDate))from Employees
1937/09/19 00:00:00 2037/09/19 00:00:00 <----- ???
1948/12/08 00:00:00 2048/12/08 00:00:00 <----- ???
1952/02/19 00:00:00 1952/02/19 00:00:00
1955/03/04 00:00:00 1955/03/04 00:00:00
3. Specify dateformat
select BirthDate,
to_date(to_char(BirthDate, 'YYYY-MM-DD' ), 'YYYY-MM-DD')
from Employees
1937/09/19 00:00:00 1937/09/19 00:00:00
1948/12/08 00:00:00 1948/12/08 00:00:00
1952/02/19 00:00:00 1952/02/19 00:00:00
1955/03/04 00:00:00 1955/03/04 00:00:00

Answer 1

The RR format mask exists to help people who still have two digit years. It defines a window for when Oracle assumes a 19xx year and a window for when Oracle assumes a 20xx year. If you are told that something happened in "78", Oracle assumes that you mean 1978 when you use the RR format mask. If you are told that something happend in "04", Oracle assumes that you mean 2004.
Just to clarify what is happening here, Oracle is evaluating things as follows
CAST( BirthDate AS CHARACTER(60) )uses the session's NLS_DATE_FORMAT to create a string, i.e.
"19-Sep-37"Now, it evaluates
CAST( "19-Sep-37" AS DATE )again using the default NLS_DATE_FORMAT. However, "RR" is not completely reversable, and assumes that "37" here refers to a date in the future, 2047.
You can modify your NLS_DATE_FORMAT at a session level. Doing so would change the behavior of your CAST statements, which may be desirable if you have existing code that you need to hack to work. Obviously, though, since this is a session-level setting, it can cause maintenance nightmares if different sessions have different settings
SCOTT @ HP92 Local> create table t1( col1 DATE );
Table created.
SCOTT @ HP92 Local> insert into t1 values( to_date('09/19/1937', 'MM/DD/YYYY' ) );
1 row created.
SCOTT @ HP92 Local> ed
Wrote file afiedt.buf
  1* insert into t1 values( to_date('09/19/2037', 'MM/DD/YYYY' ) )
SCOTT @ HP92 Local> /
1 row created.
SCOTT @ HP92 Local> select cast( cast( col1 AS CHARACTER(60) ) AS DATE ) from t1;
CAST(CAST
19-SEP-37
19-SEP-37
SCOTT @ HP92 Local> alter session set nls_date_format = 'DD-MON-YYYY HH24:MI:SS';
Session altered.
SCOTT @ HP92 Local> select cast( cast( col1 AS CHARACTER(60) ) AS DATE ) from t1;
CAST(CAST(COL1ASCHAR
19-SEP-1937 00:00:00
19-SEP-2037 00:00:00Justin
Distributed Database Consulting, Inc.
http://www.ddbcinc.com/askDDBC