用下面的逻辑判断一下：
SQL> CREATE OR REPLACE FUNCTION F_IS_NUM(P_NUM IN VARCHAR2) RETURN VARCHAR2 AS
2 V_TMP NUMBER;
3 BEGIN
4 IF P_NUM IS NULL THEN
5 RETURN NULL;
6 END IF;
7 V_TMP := TO_NUMBER(P_NUM);
8 RETURN 'T';
9 EXCEPTION
10 WHEN OTHERS THEN
11 RETURN 'N';
12 END;
13 /
