Why oracle "to_date" accepts Date-parameters

Question

I tested out function "to_date" as you see below and I see that Oracle allows me to give to that function parameter with textual data type and also allows to parameter's datatype to be Date.
Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.4.0
SQL>
SQL> select to_date(sysdate) as s from dual;--21.10.2010
S
21.10.2010
SQL> select to_date(sysdate, 'DD.MM.YYYY') as s from dual;--21.10.2010
S
21.10.2010
SQL> select to_date('21.10.2010', 'DD.MM.YYYY') as s from dual;--21.10.2010
S
21.10.2010
SQL> select to_date(to_date('21.10.2010', 'DD.MM.YYYY')) as s from dual;--21.10.2010
S
21.10.2010
SQL> I entered into Search engine "http://www.lycos.com/" phrase "oracle to_date function" and it gave me link to documentation of that function:
http://download.oracle.com/docs/cd/B19306_01/server.102/b14200/functions183.htm
and the documentation says:
>
Purpose
TO_DATE converts char of CHAR, VARCHAR2, NCHAR, or NVARCHAR2 datatype to a value of DATE datatype.
>
My question is: why the function accepts Date data types? what occures behind curtains when Date is passed in? Or what explanation there is, that documentation seems to lie?
Also, have you too noticed that the forum has been oftenly down near past days?

Answer 1

Any function that accepts a VARCHAR2 can be passed a DATE. Oracle will simply implicitly cast the DATE to a VARCHAR2 (using the session's NLS_DATE_FORMAT). The same way that it would implicitly cast a NUMBER to a VARCHAR2.
Of course, if the implicit conversion of the date to a string doesn't match the explicit format mask passed to TO_DATE, you'll have a problem. If your session's NLS_DATE_FORMAT was 'MM.DD.YYYY', for example, the TO_DATE would fail because Oracle would implicitly cast the date to a string in the format MM.DD.YYYY and then try to convert that to a date using the format mast DD.MM.YYYY.
Justin