nls_date_format tips

Oracle nls_date_format tips

Question:  In understand that the internal storage of a DATE or TIMESTAMP column is independent of the display value.  Can you please explain how nls_date_format is used to display date values?

Answer:  The Oracle nls_date_format environmental variable does much more than control the date display within Oracle.  Changing the value of nls_date_format will change the way that the data is displayed in Oracle SQL*Plus:

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY MM DD';
select sysdate from dual;

ALTER SESSION SET NLS_DATE_FORMAT = 'HH24:MI:SS';
select sysdate from dual;

You can change the way that a date or timestamp column is display at any time by altering your session to re-set nls_date_format.  You can use this alter session set nls_date_format in SQL*Plus, PL/SQL:

alter session set nls_date_format = 'YYYY MM DD';

alter session set nls_date_format = 'HH24:MI:SS';

You can also change the nls_date_format in many procedural languages such as C, C++ COBOL, Java, JavaScript, PHP and many others.  In any language with an Oracle pre-compiler (Pro*C, Pro*COBOL, Pro*Fortran, etc.), you can enter the alter session set nls_date_format directly within your program. 

Oracle will accept the command and comment out the Oracle directives before passing the program to the native language complier.  In other languages that don;t support direct "alter session set nls_date_format", there is separate syntax::

• Oracle Pro*C: 
  
  exec sql alter session
  
• Perl DBI: 

  $db=DBI->connect(“dbi:Oracle:Local”,”scott”,”tiger”);

  $db->do(“ALTER SESSION SET NLS_DATE_FORMAT='DD-MON-RR'”);

You can also use the use the to_char format mask to change the display value of a date or timestamp column:

select to_char(sysdate,'HH24:MI:SS') from dual;
select to_char(sysdate,'YYY MM DD') from dual;

The nls_date_format also allows you to display a date up to 1/100 of a second, quite precise date display:

alter session set nls_date_format = 'DD-Mon-YYYY HH24:MI:SS.FF'

Beware, the system wide default for nls_date_format requires a database bounce and you cannot change nls_date_format system wide with an alter system set nls_date_format statement:

SQL> alter system set nls_date_format='Month DD, yyyy';
alter system set nls_date_format='Month DD, yyyy'
*
ERROR at line 1:
ORA-02096: specified initialization parameter is not modifiable with this
option

For more details of using nls_date format to change the data display format, see our related notes on using nls_date_format in specific conditions.

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