How to append '0' to single Digit Numbers

DROP TABLE AX;

CREATE TABLE AX ( COL NUMBER );

INSERT INTO AX ( SELECT ROWNUM FROM ALL_OBJECTS WHERE ROWNUM < 11 );

SELECT TO_CHAR(COL,'00') FROM AX;

TO_
---
01
02
03
04
05
06
07
08
09
10

9 rows selected.

SQL> SELECT TO_NUMBER(TO_CHAR(COL,'00')) NUM FROM AX;

NUM
----------
1
2
3
4
5
6
7
8
9

9 rows selected.