SQL> declare
   2   
   3     procedure print (caption varchar2, value boolean) is
   4     begin
   5       if value then
   6         dbms_output.put_line (caption || ' = true');
   7       elsif not value then
   8         dbms_output.put_line (caption || ' = false');
   9       elsif value is null then
  10         dbms_output.put_line (caption || ' = null');
  11       else
  12         dbms_output.put_line (caption || ' = ????');
  13       end if;
  14     end;
  15   
  16   begin
  17     print ('true and null', true and null);
  18     print ('false and null', false and null);
  19     print ('true or null', true or null);
  20     print ('false or null', false or null);
  21   end;
  22   /

true and null = null
false and null = false
true or null = true
false or null = null

PL/SQL procedure successfully completed