在oracle中，null與數字相加減，結果為null，

所以要用nvl()函式進行處理

declare 
V_Num varchar2(20);
V_Num2 varchar2(20);
V_Num3 varchar2(20);
V_Num4 varchar(20);
begin
        select nvl(null,0) + 100 into V_Num from dual;
        select null - 200 into V_Num2 from dual;
        select 300 + null into V_Num3 from dual;
        select 400 - null into V_Num4 from dual;
        dbms_output.put_line(V_Num);
        dbms_output.put_line(V_Num2);
        dbms_output.put_line(V_Num3);
        dbms_output.put_line(V_Num4);
end;