SQL> conn scott/tiger
Connected.
SQL> variable a number;
SQL> variable b number;
SQL> variable x number;
SQL> exec :a :=0; :b := 0;  := 7369;
PL/SQL procedure successfully completed.

SQL> select job, nvl(empno, :a), nvl(comm, :b) from emp where empno = :x;

JOB       NVL(EMPNO,:A) NVL(COMM,:B)
--------------------------------------------------------------------------------
---------------------------------------------------------------------------------------------
CLERK              7369            0

SQL> select sql_id, sql_text from v$sql where sql_text like 'select job, nvl(empno,%';

SQL_ID
--------------------------------------------------------------------------------
SQL_TEXT
--------------------------------------------------------------------------------
3d1079n1gakc1
select job, nvl(empno, :a), nvl(comm, :b) from emp where empno = 

SQL> column name format a10
SQL> column VALUE_STRING format a20
SQL> select name, position, datatype_string, last_captured, value_string from v$sql_bind_capture
  2  where sql_id = '3d1079n1gakc1' order by position;

NAME         POSITION DATATYPE_STRING LAST_CAP VALUE_STRING
--------------------------------------------------------------------------------
:A                  1 NUMBER
:B                  2 NUMBER
:x                  3 NUMBER          03/06/08 7369