Convert String date format in Jalali to Gregorian

As a_horse indicated, select min(date)... is sufficient. The DECODE and multiple conversions just slow things down without accomplishing anything. Even as parts of queries, you should eliminate this needless processing.

As Phil indicated, the displaying of dates is controlled by the NLS_DATE_FORMAT parameter and can be altered to display dates as you wish. This does change how the queries run (unless there are missing format specifications or SQL injection is being done). Change the display format like this:

 ALTER SESSION SET NLS_DATE_FORMAT='YYYYMMDD';

When doing a to_date on a two digit year, use YY when you want the first two digits of the year to match the current year, but use RR when you want the first two digits to be 20 for years ending in 0-49 and 19 for years ending in 50-99. See the SQL Language Reference for more information:

The RR datetime format element is similar to the YY datetime format element, but it provides additional flexibility for storing date values in other centuries. The RR datetime format element lets you store 20th century dates in the 21st century by specifying only the last two digits of the year.

If you use the TO_DATE function with the YY datetime format element, then the year returned always has the same first 2 digits as the current year. If you use the RR datetime format element instead, then the century of the return value varies according to the specified two-digit year and the last two digits of the current year.

That is:

• If the specified two-digit year is 00 to 49, then

  • If the last two digits of the current year are 00 to 49, then the returned year has the same first two digits as the current year.

  • If the last two digits of the current year are 50 to 99, then the first 2 digits of the returned year are 1 greater than the first 2 digits of the current year.

• If the specified two-digit year is 50 to 99, then

  • If the last two digits of the current year are 00 to 49, then the first 2 digits of the returned year are 1 less than the first 2 digits of the current year.

  • If the last two digits of the current year are 50 to 99, then the returned year has the same first two digits as the current year.

The following examples demonstrate the behavior of the RR datetime format element.

SELECT TO_CHAR(TO_DATE('27-OCT-98', 'DD-MON-RR'), 'YYYY') "Year" FROM DUAL; 
Year
---- 
1998 

SELECT TO_CHAR(TO_DATE('27-OCT-17', 'DD-MON-RR'), 'YYYY') "Year" FROM DUAL; 
Year
----
2017

Your belief that this should not be necessary is correct based on your assumptions. Normally this means the assumptions should be rechecked (unless there is data corruption or a bug). If you run the following you should get the same results from the first query as the second:

DROP TABLE t1;
CREATE TABLE t1 AS (
   SELECT to_date('01-FEB-2011','DD-MON-YYYY')+level myDateColumn 
   FROM dual CONNECT BY level <=120);

select * from t1 
where myDateColumn 
between to_date('13-FEB-11', 'DD-MON-YY') AND TO_DATE('15-FEB-11', 'DD-MON-YY');

select myDateColumn from t1 
where to_date(myDateColumn) 
between to_date('13-FEB-11', 'DD-MON-YY') AND TO_DATE('15-FEB-11', 'DD-MON-YY');

To check some of the assumptions, can you show the results of the following query?

SELECT myDateColumn, to_date(myDateColumn)
   , to_char(myDateColumn,'DD-MON-YY HH.MI.SS PM'), to_char(myDateColumn,'YYYY') 
FROM myTable
WHERE to_date(myDateColumn)
BETWEEN to_date('13-FEB-11', 'DD-MON-YY') AND TO_DATE('15-FEB-11', 'DD-MON-YY');

The first three columns should all show identical information and the last should verify that the year is correct.