Ans:
ALTER SESSION SET NLS_DATE_FORMAT= 'MONTH-DD-YYYY';
SELECT HireDate
FROM Emp
WHERE HireDate BETWEEN '01-JAN-81' AND '31-dec-82';
WHERE HireDate BETWEEN '01-JAN-81' AND '31-dec-82'
*
ERROR at line 3:
ORA-01843: not a valid month
Since default date format is changed, you must provide the dates in Oracle accepted valid format.
SELECT HireDate
FROM Emp
WHERE HireDate BETWEEN 'January-01-1981' AND 'december-31-1982';