1 + NULL is NULL

If you try to add a null to a number, the result is a null. You can see what I mean in the example below, which I tested in Oracle 10:

SQL> select 1 from dual
  2  /

         1
----------
         1

SQL> select nvl(null,'null') from dual
  2  /

NVL(NULL,'NULL')
----------------
null

SQL> select nvl(to_char(1+null),'null') from dual
  2  /

NVL(TO_CHAR(1+NULL),'NULL')
---------------------------
null

SQL>