refactoring PL/SQL

I encountered the following PL/SQL code(variables modified) at work:

PROCEDURE test(i_w IN a.w%type,o_result IN OUT resu_cur_type) IS
BEGIN
    IF i_w IS NULL THEN
      open o_result for SELECT a.x, b.y FROM a,b WHERE a.z=b.z;
    ELSE
      open o_result for SELECT a.x, b.y FRO开发者_JAVA百科M a,b WHERE a.z=b.z AND a.w=i_w;
    END IF;
END test;

I think the above is equivalent to the following:

PROCEDURE test(i_w IN a.w%type,o_result IN OUT resu_cur_type) IS
BEGIN
      open o_result for SELECT a.x, b.y FROM a,b WHERE a.z=b.z AND NVL(a.w,1)=NVL(NVL(i_w,a.w),1);
END test;

Won't it be the same?. Could somebody please explain is there any reason to use the original version?. Please note that the original cursor is a 50 line query, so refactoring might improve its readability and keep it simple.

Edit:

To address the problem in Kevin Burton's answer, I've updated the WHERE condition.

---

As Kevin Burton said, your new query is not equivalent. This is though:

PROCEDURE test(i_w IN a.w%type,o_result IN OUT resu_cur_type) IS
BEGIN
      open o_result for SELECT a.x, b.y FROM a,b
                        WHERE a.z=b.z 
                        AND (i_w IS NULL OR a.w=i_w);
END test;

However, the original code may be more efficient as can use an index on a.w when i_w is not null but do something different when it is null - i.e. it tailors the query to the parameters used.

---

if a.w is null you will not select the records

as

a.w=NVL(i_w,a.w); 

will be

a.w=NULL