Problem: Incorrect displaying of time difference (in months) between Python and Oracle

The background of my problem is:

• there's SQL query in Python that extracts particular data from Oracle database
• one column/information that is extracted from Oracle is information about how long a customer has open account by the company (in years)

The particular part of the query:

ROUND(MONTHS_BETWEEN(sysdate, TO_DATE(c.registration_date, 'dd.mm.yy')) / 12, 1) AS Years_Client

The problem:

• when I run the query in Oracle (SQL Developer), I receive correct values
• BUT when I run the query within Python code, it returns a column that contains both negative values or really high values (e.g. 21), which in the first place is a nonsense and in the second as well (as the company even doesn't exist that long)

<class 'pandas.core.series.Series'>

• checked Oracle documentation
• tired to run the whole query both in Oracle SQL Developer and Python
• it works perfectly fine in Oracle, but not in Python

NEVER use TO_DATE on a column that is already a DATE.

TO_DATE takes a string as the first argument and with you pass it a DATE then Oracle needs to convert it to a string so that TO_DATE can convert it back to a date - which either:

1. is pointless (as it was already a date); or

2. will cause errors (as is your case) because Oracle will convert your query to:

   ROUND(
     MONTHS_BETWEEN(
       sysdate,
       TO_DATE(
         TO_CHAR(
           c.registration_date,
           (SELECT value FROM NLS_SESSION_PARAMETERS WHERE parameter = 'NLS_DATE_FORMAT')
         ),
         'dd.mm.yy'
       )
     ) / 12,
     1
   )
   

   and if the NLS_DATE_FORMAT does not match dd.mm.yy then it will either raise an exception or you will get an erroneous value.

In your case:

• in SQL Developer, the NLS_DATE_FORMAT matches dd.mm.yy and the query works but the TO_DATE call is pointless.
• in Python, the NLS_DATE_FORMAT is different and does not match dd.mm.yy so you are getting erroneous values.

To solve it, remove the TO_DATE:

ROUND(MONTHS_BETWEEN(sysdate, c.registration_date) / 12, 1) AS Years_Client

For example, if you have the data:

CREATE TABLE table_name (registration_date) AS
SELECT SYSDATE FROM DUAL UNION ALL
SELECT DATE '2024-04-01' FROM DUAL UNION ALL
SELECT DATE '1901-04-24' FROM DUAL;

and the NLS_DATE_FORMAT is YY-MM-DD then:

SELECT ROUND(MONTHS_BETWEEN(sysdate, TO_DATE(c.registration_date, 'dd.mm.yy')) / 12, 1)
        AS Years_Client,
       ROUND(MONTHS_BETWEEN(sysdate, c.registration_date) / 12, 1)
        AS actual_Years_Client
FROM   table_name c

Outputs

fiddle