Oracle passing a variable to a function - sql

Question

I have some SQL code, which uses a hard coded date. My goal is to remove the hard coded dates and replace them with variables to make the code generic.
My test CASE is the following;
CREATE OR REPLACE FUNCTION IsDate( p_str IN VARCHAR2, p_format IN VARCHAR2 ) RETURN NUMBER AS
V_date DATE;
BEGIN
V_Date := TO_DATE( p_str, p_format );
RETURN 1;
EXCEPTION
WHEN OTHERS THEN
RETURN 0;
END;
This works fine
select IsDate(DATE '2021-07-31','YYYY-MM-DD') from dual;
This causes an error (see below). Can someone please explain the issue and how can it be fixed.
I'm testing on live SQL. Thanks in advance to all who answer.
exec :my_date := DATE '2021-07-31';
select IsDate( DATE my_date,'YYYY-MM-DD') from dual;
i am getting below error
ORA-01008: not all variables bound ORA-06512: at "SYS.DBMS_SQL", line 1721
ORA-00936: missing expression

Answer 1

There a special function VALIDATE_CONVERSION since 12.2 that does exactly what you want to achieve:
VALIDATE_CONVERSION
Example:
SQL> select VALIDATE_CONVERSION('01-01-2000' as date,'dd-mm-yyyy') chk1 from dual;
CHK1
----------
1
SQL> select VALIDATE_CONVERSION('01-01-2000' as date,'yyyy-mm-dd') chk2 from dual;
CHK2
----------
0
Also livesql.oracle.com doesn't support exec command which is an SQL*Plus command. So if you want to test your functions with bind variables, you can use simple PL/SQL Variables:
declare
string_date varchar2(100);
FUNCTION IsDate( p_str IN VARCHAR2, p_format IN VARCHAR2 ) RETURN NUMBER AS
V_date DATE;
BEGIN
V_Date := TO_DATE( p_str, p_format );
RETURN 1;
EXCEPTION
WHEN OTHERS THEN
RETURN 0;
END;
begin
string_date:='31-12-2000';
dbms_output.put_line(IsDate(string_date,'dd-mm-yyyy'));
end;
/

Answer 2

The DATE keyword expects a literal, you cannot use a string variable.
Regarding your function:
FUNCTION IsDate( p_str IN VARCHAR2, p_format IN VARCHAR2 ) RETURN NUMBER
It expects two VARCHAR2, i.e. string values. However when you run IsDate(DATE '2021-07-31','YYYY-MM-DD') the you pass a DATE value into the function, not a string.
Checking the format of a DATE value is pointless, because a DATE values itself has no format, it is stored as an internal binary value. Never use TO_DATE on a value which is already a DATE. Function TO_DATE expects a string which shall be converted to a DATE.
What is displayed when you select a DATE is defined (by default) with your current user session NLS_DATE_FORMAT setting. You can change the default output format for example with ALTER SESSION SET NLS_DATE_FORMAT = 'DD.MM.YYYY'; or you set it explicitly by using the TO_CHAR() function.
When you call your function IsDate(DATE '2021-07-31','YYYY-MM-DD') then Oracle makes an implicit cast to a string. i.e. it runs
V_Date := TO_DATE( TO_CHAR(p_str, SYS_CONTEXT('USERENV', 'NLS_DATE_FORMAT')), p_format );
When you pass a DATE value, then your function actually acts like this:
CREATE OR REPLACE FUNCTION IsDate( p_str IN VARCHAR2, p_format IN VARCHAR2 ) RETURN NUMBER AS
ret NUMBER;
BEGIN
SELECT COUNT(*)
INTO ret
FROM NLS_SESSION_PARAMETERS
WHERE PARAMETER = 'NLS_DATE_FORMAT'
AND VALUE = p_format;
RETURN ret;
-- p_str is ignored completely
END;
Note, in Oracle 18 the TO_DATE function provides the DEFAULT ... ON CONVERSION ERROR clause, so it may be an overkill to write the extra function.