Boolean Wrapper Function used in DML where clause

Hi,

I am looking for an explanation of the following behaviour.

I have a pl/sql function that checks to see if a record has expired and returns a boolean:

FUNCTION has_expired(p_record_ref IN NUMBER) RETURN BOOLEAN IS

BEGIN

  -- Processing

  RETURN (boolean result of processing)

END;

I know that I cannot use this function directly in sql as it does not understand boolean data types.

I have created a wrapper function that will return Y where the boolean function returns TRUE and N where FALSE.

I can now use this wrapper function in selecting from a table to return any records that have expired - this is working correctly: