Common pitfalls and best practices in Oracle database date addition and subtraction operations

This article aims to explore in-depth common problems caused by implicit type conversion and improper NLS date formatting when performing date addition and subtraction operations in Oracle databases, especially year calculation errors across centuries. We will parse the root of the problem in detail and show through sample code how to adopt direct date arithmetic and appropriate functions (such as TRUNC) to avoid unnecessary type conversions and ensure the accuracy and reliability of date calculations, especially when performing SQL updates in Java code.

Implicit conversion traps for Oracle date addition and subtraction

In Oracle database, when mathematical operations on values of type DATE or TIMESTAMP (such as adding or subtracting a number), Oracle treats it as days to add or subtract. However, if the TO_DATE function is introduced in the process, and its input parameters are implicitly converted strings, or the format model does not match the actual data, it can lead to unexpected results, especially when processing the year.

In the original question, the SQL statement is as follows:

 UPDATE CUS_LOGS SET START_DATE=to_date(systimestamp 3,'DD-MON-RRRRR'), END_DATE=to_date(systimestamp 21921,'DD-MON-RRRR')  
WHERE CUS_ID IN ('9b90cb8175ba0ca60175ba12d8711006');

The core problem here is to_date(systimestamp N,'DD-MON-RRRR'). Although our intention is to add the number of days directly to systimestamp, the TO_DATE function forces Oracle to implicitly convert the result of systimestamp N (a TIMESTAMP type) to a string after performing addition, and then try to convert it back to the DATE type using the 'DD-MON-RRRRR' format model.

This implicit conversion process is affected by the NLS_DATE_FORMAT parameter of the current session. If NLS_DATE_FORMAT is set to DD-MON-RR or DD-MON-YY, then the year part may contain only two digits when implicitly converted to a string. For example, 2082-11-08 may be implicitly converted to '08-NOV-82'. When the TO_DATE function attempts to convert '08-NOV-82' back to date with the 'DD-MON-RRRR' format model, the RRR format model interprets the two-bit year 82 as 1982 (because the RR format usually interprets 00-49 as 20xx, 50-99 as 19xx, and RRRR will use this explanation in this context), rather than the expected 2082, resulting in a year error.

The following example demonstrates the effect of NLS_DATE_FORMAT on implicit conversion: Assume the current date is 2022-11-02.

 -- Set the date format of the session to DD-MON-RR, simulate the environment that may cause problems ALTER SESSION SET NLS_DATE_FORMAT = 'DD-MON-RR';

SELECT
  TO_DATE(SYSDATE 3,'DD-MON-RRRR') AS "A (2022 3 days)",
  TO_CHAR(TO_DATE(SYSDATE 3,'DD-MON-RRRR'), 'YYYY-MM-DD') AS "B (YYYY format of A),"
  TO_DATE(SYSDATE 21921,'DD-MON-RRRR') AS "C (2022 21921 days)",
  TO_CHAR(TO_DATE(SYSDATE 21921,'DD-MON-RRRR'), 'YYYY-MM-DD') AS "D (YYYY format of C)",
  TO_DATE(SYSDATE 3,'DD-MON-YYYY') AS "E (2022 3 days, YYYY)",
  TO_CHAR(TO_DATE(SYSDATE 3,'DD-MON-YYYY'), 'YYYY-MM-DD') AS "F (YYYY format of E)",
  TO_DATE(SYSDATE 21921,'DD-MON-YYYY') AS "G (2022 21921 Days, YYYY)",
  TO_CHAR(TO_DATE(SYSDATE 21921,'DD-MON-YYYY'), 'YYYY-MM-DD') AS "H (YYYY format of G)"
FROM DUAL;

Execute the above query and you will observe results similar to the following (the specific date depends on the execution date):

A (2022 3 days)	B (YYYY format of A)	C (2022 21921 days)	D (YYYY format of C)	E (2022 3 days, YYYY)	F (YYYY format of E)	G (2022 21921 days, YYYY)	H (YYYY format of G)
05-NOV-22	2022-11-05	08-NOV-82	1982-11-08	05-NOV-22	0022-11-05	08-NOV-82	0082-11-08

From the results, we can see that when the calculation result is 2082-11-08, 82 is misunderstood as 1982 because it is implicitly converted to the two-bit year string '08-NOV-82' and then parsed by TO_DATE(..., 'DD-MON-RRRRR'). If DD-MON-YYYY is used, the two-digit year 82 will be interpreted as 0082, which is even more deviant.

Correct date addition and subtraction operation

Oracle database itself supports adding and subtracting numbers to adjust dates directly by adding and subtracting numbers to DATE and TIMESTAMP types. A number represents a day. Therefore, to increase the date or timestamp to a specified number of days, the most direct and safe way is to avoid any unnecessary TO_DATE or TO_CHAR conversions.

Use SYSTIMESTAMP or SYSDATE to add or subtract the number of days directly

SYSTIMESTAMP returns the current system date and time (including time zone), and SYSDATE returns the current system date and time (excluding time zone, accuracy to seconds). Both can be added and subtracted directly with numbers.

 -- Example: Directly add and subtract SYSTIMESTAMP-- It is recommended to set NLS_TIMESTAMP_TZ_FORMAT first to display the TIMESTAMP result clearly ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT = 'YYYY-MM-DD HH24:MI:SS TZR';
ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD';

SELECT
  SYSTIMESTAMP AS "current timestamp",
  SYSTIMESTAMP 3 AS "Timestamp after 3 days",
  SYSTIMESTAMP 21921 AS "21921 Days Time Stamp"
FROM DUAL;

The results will clearly show the correct future date:

Current timestamp	Time stamp after 3 days	Time stamp after 21921 days
2022-11-02 10:42:24 00:00	2022-11-05 10:42:24 00:00	2082-11-08 10:42:24 00:00

If only the date part is needed, or the target column is of DATE type, it is more concise to use SYSDATE:

 -- Example: Directly add and subtract SELECT to SYSDATE
  SYSDATE AS "Current Date",
  SYSDATE 3 AS "Date after 3 days",
  SYSDATE 21921 AS "Date after 21921 Days"
FROM DUAL;

The result is also correct:

Current date	3 days later date	21921 days later date
2022-11-02	2022-11-05	2082-11-08

Remove the time part: Use TRUNC()

If you want the date part of the calculation to start from midnight of the day (00:00:00) the time part can be truncated using the TRUNC() function. TRUNC (sysdate) will set the time part of sysdate to midnight.

 -- Example: Use TRUNC() to ensure that SELECT is calculated from midnight of the day
  TRUNC(SYSDATE) AS "Midnight of the day",
  TRUNC(SYSDATE) 3 AS "Midnight in 3 days",
  TRUNC(SYSDATE) 21921 AS "21921 Days After Midnight"
FROM DUAL;

This is useful for ensuring date consistency, for example, when you only care about dates and not about specific points in time.

Final solution

According to the above analysis, the original UPDATE statement should be modified to perform date arithmetic directly, and the option to use TRUNC() to ensure that the time part starts from midnight.

 UPDATE CUS_LOGS SET
    START_DATE = TRUNC(SYSDATE) 3,
    END_DATE = TRUNC(SYSDATE) 21921
WHERE CUS_ID IN ('9b90cb8175ba0ca60175ba12d8711006');

This SQL statement avoids any TO_DATE call that may cause implicit conversion problems, and directly utilizes Oracle's built-in support for adding and subtracting numbers of date types, ensuring the accuracy of the calculation.

Further date operation considerations

Although direct addition and subtraction of numbers are suitable for days, Oracle provides special functions for adding and subtracting months or years:

• ADD_MONTHS(date, integer) : Used to increase or decrease a month on a specified date. For example, ADD_MONTHS(TRUNC(SYSDATE), 60*12) can increase the date by 60 years. It should be noted that ADD_MONTHS will handle the end of the month date, for example, adding one month to January 31 will get February 28 (or 29).
• INTERVAL Literal : Time intervals can be more explicitly represented, such as SYSDATE INTERVAL '3' DAY or SYSTIMESTAMP INTERVAL '1' YEAR. However, the INTERVAL YEAR TO MONTH type may cause an error when handling February 29 spanning a leap year, for example, DATE '2020-02-29' INTERVAL '1' YEAR throws an invalid date error because there is no February 29 in 2021. For adding and subtracting days, adding numbers directly is usually easier and safer.

When choosing a date operation method, the most direct and implicit conversions should be given priority. For adding or subtracting days, it is best practice to add or subtract numbers directly to values of DATE or TIMESTAMP types.

Summarize

When doing date addition and subtraction operations in Oracle databases, be sure to be alert to the pitfalls that implicit type conversion and NLS_DATE_FORMAT parameters may bring, especially when it comes to TO_DATE functions and two-bit year format models (such as RR). Best practices are:

1. Avoid unnecessary TO_DATE or TO_CHAR conversions : When you need to increase or decrease the number of days on the values of DATE or TIMESTAMP types, just add or subtract them directly.
2. Use TRUNC() function : If you need to reset the time part of the date to midnight (00:00:00), use TRUNC(date).
3. Understanding the impact of NLS parameters : Understand how the NLS_DATE_FORMAT settings of a session affect implicit string conversion of date and timestamps, which helps diagnose potential problems.
4. Select the appropriate function : For date operations other than days (such as months or years), use special functions such as ADD_MONTHS and pay attention to their behavioral characteristics.

Following these principles can ensure that your Oracle date operations are accurate and avoid business problems caused by date calculation errors.

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