When implicit conversion takes place

Question

hi all,
I have two queries say
sql>SELECT * FROM EMP WHERE HIREDATE BETWEEN '20-FEB-81' AND '20-FEB-82';
EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
7499 ALLEN SALESMAN 7698 20-FEB-81 1600 300 30
7521 WARD SALESMAN 7698 22-FEB-81 1250 500 30
7566 JONES MANAGER 7839 02-APR-81 2975 20
7654 MARTIN SALESMAN 7698 28-SEP-81 1250 1400 30
7698 BLAKE MANAGER 7839 01-MAY-81 2850 30
7782 10 deleted MANAGER 7839 09-JUN-81 2450 10
7839 10 deleted PRESIDENT 17-NOV-81 5000 10
7844 TURNER SALESMAN 7698 08-SEP-81 1500 0 30
7900 JAMES CLERK 7698 03-DEC-81 950 30
7902 FORD ANALYST 7566 03-DEC-81 3000 20
7934 10 deleted CLERK 7782 23-JAN-82 1300 10
AND
sql>select '17-SEP-2010'+1 from dual;
ERROR at line 1:
ORA-01722: invalid number
now in the first case i have passed a string and that got converted into a date
but in the second case i am trying to add one day to the entered date it is not getting converted in the second case.
when actually is implicit conversion done.
thanks and regards,
sri ram.

Answer 1

I'd like to recommand you NEVER to rely on IMPLICIT conversion. It depends on the NLS_DATE_FORMAT parameter. If you supply a VARCHAR that can be converted to a date, it will use this default format to try and convert the date. But this can result of weird result if you don't know what is the NLS_DATE_FORMAT ;-)
So, as an advice, always pass DATE to function that accept dates by converting them yourself (TO_DATE(a_date, 'the format you want')...
Cheers,