The to_date() error in where clause of proc code

Question

In a proc code, firstly I use "SELECT to_char(f_date_1, 'DDMONYY') into :char_date FROM TBL_1;" to get the value of f_date_1.
Then I use "SELECT ID FROM TBL_2 WHERE f_date_2 = :char_date;" to get the value of ID according char_date.
In table, the type of f_date_1 and f_date_2 is DATE.
char_date is "char char_date[10]".
My question is:
1. If I use WHERE f_date_2 = to_date(:char_date, 'DDMONYY'), it will prompt: "-1861, ORA-01861: literal does not match format string".
2. If I use WHERE f_date_2 = :char_date, it will work well.
Since the type of f_date_2 is DATE and the type of char_date is CHAR, why does to_date(:char_date, 'DDMONYY') have problems?
Thanks.

Answer 1

I was not able to duplicate the error you got, but I did spot something that may be affecting your results:
*Set up test table using HR.EMPLOYEES*
create table TEST_DATE as
select employee_id, last_name, hire_date
from employees
var char_date char(10)
begin
select to_char(hire_date,'DDMONYY') into :char_date
from test_date
where rownum = 1;
end;
*case 1*
select *
from test_date
where hire_date = to_date(:char_date,'DDMONYY')
*case 2*
select *
from test_date
where hire_date = :char_date --> this assumes a implicit char to date conversionWhile I got case 2 to work, it assumes that the database is configured to recognize the character mask as a valid string format. If you tried running that query on a different database, this query may fail as well.
To verify why case 1 may not have worked, do a quick check:
select :char_date from dualIt is possible that you may have applied a different character mask between your first query:
>
SELECT to_char(f_date_1, 'DDMONYY') into :char_date FROM TBL_1;
>
and the WHERE clause of your second query:
>
WHERE f_date_2 = to_date(:char_date, 'DDMONYY')
>
RP