What is Oracle's Default Date Format?

Question

I have an Oracle DB, and I don't control the date format. I want to know what the date format is to ensure that searches like

select * from search where search_date>='03/16/2016 00:00:00'

work as expected.

Answer 1

Don't do that - you are relying on implicit data type conversion which is going to fail at some point.

You have two options:

1) Use a proper ANSI SQL date literal:

select * 
from search 
where search_date >= timestamp '2016-03-16 00:00:00';

2) use to_date() (or to_timestamp()) and use a custom format.

select * 
from search 
where search_date >= to_date('03/16/2016 00:00:00', 'mm/dd/yyyy hh24:mi:ss');

With to_date() you should avoid any format that is language dependent. Use numbers for the month, not abbreviations (e.g. 'Mar' or 'Apr') because they again rely on the client language.

More details can be found in the manual:
https://docs.oracle.com/cd/E11882_01/server.112/e41084/sql_elements003.htm#SQLRF51062

---

Never rely on implicit data type conversion.

Answer 2

You can get all the NLS session parameters with the query:

SELECT * FROM NLS_SESSION_PARAMETERS;

or, if you have the permissions GRANT SELECT ON V_$PARAMETER TO YOUR_USERNAME;, you can use the command:

SHOW PARAMETER NLS;

If you just want the date format then you can do either:

SELECT * FROM NLS_SESSION_PARAMETERS WHERE PARAMETER = 'NLS_DATE_FORMAT';

or

SHOW PARAMETER NLS_DATE_FORMAT;

However, you could also use ANSI date (or timestamp) literals which are format agnostic. An ANSI date literal has the format DATE 'YYYY-MM-DD' and a timestamp literal has the format TIMESTAMP 'YYYY-MM-DD HH24:MI:SS.FF9'. So your query would be:

select * from search where search_date>= DATE '2016-03-16'

or

select * from search where search_date>= TIMESTAMP '2016-03-16 00:00:00'

Answer 3

What is Oracle's Default Date Format?

A DATE doesn't have any format. Oracle does not store dates in the format you see. It stores it internally in 7 bytes with each byte storing different components of the datetime value.

Byte    Description
----    -------------------------------------------------
1       Century value but before storing it add 100 to it
2       Year and 100 is added to it before storing
3       Month
4       Day of the month
5       Hours but add 1 before storing it
6       Minutes but add 1 before storing it
7       Seconds but add 1 before storing it

To display, use TO_CHAR with proper FORMAT MODEL.

For comparing, use TO_DATE with proper FORMAT MODEL.

What you see as a format by default, is your locale specific NLS settings.

SQL> select parameter, value from v$nls_parameters where parameter='NLS_DATE_FORMAT';

PARAMETER       VALUE
--------------- ----------------------------------------------------------------
NLS_DATE_FORMAT DD-MON-RR

SQL> select sysdate from dual;

SYSDATE
---------
17-MAR-16

SQL> select to_char(sysdate, 'mm/dd/yyyy hh24:mi:ss') from dual;

TO_CHAR(SYSDATE,'MM
-------------------
03/17/2016 12:48:41

SQL>

search_date>='03/16/2016 00:00:00'

You are comparing a DATE with a string literal. Always, explicitly convert the string into date using TO_DATE and proper format mask.

TO_DATE('03/16/2016', 'MM/DD/YYYY')

Or, if you dealing only with the date part and not concerned with the time portion, then use the ANSI date literal which uses a fixed format DATE 'YYYY-MM-DD'

DATE '2016-03-16'

You might just be lucky to get an output due to an implicit datatype conversion based on your locale specific NLS settings. Never ever rely on implicit datatype conversion, it might work for you, might fail for others where the nls settings are different.