Return boolean value from oracle function
Boolean values can only be used in other PL/SQL code, not in Oracle SQL. If you want a function whose return value is available in a select ... from dual then you will need to define the function to return varchar2 with the return values 'true' and 'false' respectively (or 'T' and 'F', or return number, with the values 1 and 0).
As sad as this is, Oracle SQL does not support the Boolean data type (although the programming language PL/SQL does).