alter nls_date_format

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16-03-2025

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Mastering Date Formats in Oracle: A Guide to ALTER SESSION SET NLS_DATE_FORMAT

Oracle's date handling can sometimes feel like navigating a labyrinth. One of the most common points of confusion involves date formatting, and specifically, how to control how dates are displayed using the ALTER SESSION SET NLS_DATE_FORMAT command. This article will guide you through understanding and effectively using this powerful command.

Understanding NLS_DATE_FORMAT

NLS_DATE_FORMAT is a parameter within Oracle's National Language Support (NLS) settings. It dictates the format in which Oracle displays dates. Without specifying a format, Oracle uses a default setting that might not be suitable for your needs or application's desired output. The default often varies depending on your Oracle installation and system locale.

The ALTER SESSION SET NLS_DATE_FORMAT command allows you to temporarily change the date format for the current session. This is particularly useful for debugging, testing, or specific reports where you need a consistent display. Crucially, this change only affects the current session and is not persistent across sessions or database restarts.

How to Use ALTER SESSION SET NLS_DATE_FORMAT

The command's syntax is straightforward:

ALTER SESSION SET NLS_DATE_FORMAT = 'format_string';

Replace format_string with the desired date format. Oracle uses a specific set of format masks to define the display. Here are some common examples:

• 'DD-MON-YYYY': Displays the date as 24-SEP-2023 (day, month abbreviation, four-digit year).
• 'MM/DD/YYYY': Displays the date as 09/24/2023 (month, day, four-digit year).
• 'YYYY-MM-DD': Displays the date as 2023-09-24 (year, month, day) - often preferred for data consistency and sorting.
• 'Month DD, YYYY': Displays the date as September 24, 2023 (full month name, day, four-digit year).
• 'DD-MON-RR': Displays the date as 24-SEP-23 (day, month abbreviation, two-digit year). Caution: Two-digit year formats can lead to ambiguity.

Example:

To change the date format to YYYY-MM-DD for your current session, execute the following SQL statement:

ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD';

Now, any subsequent SELECT statements that retrieve dates will show them in the new format.

SELECT SYSDATE FROM dual;

Important Considerations:

• Temporary Change: Remember, this change is only for the current session. Closing the connection or restarting the database will revert to the default or previously set NLS_DATE_FORMAT.
• Permanent Changes (NLS_DATE_FORMAT at the Instance/Database Level): For a persistent change across all sessions, you would need to modify the NLS_DATE_FORMAT parameter at the database or instance level. This requires higher privileges and is generally handled by database administrators.
• Format Models: Oracle's date format models provide a wide range of options for customizing the display. Refer to the Oracle documentation for a complete list of format elements.

Conclusion:

The ALTER SESSION SET NLS_DATE_FORMAT command offers a simple yet powerful way to control date display within your Oracle sessions. By understanding the format masks and their applications, you can ensure your date output is consistent, readable, and meets the specific needs of your applications and reports. Remember to consider the temporary nature of this command and choose the appropriate level of modification (session vs. database) based on your requirements.