do you mean "select file_name from dba_data_files"?

there are no files in root directory. i wouldn't fail, anyway.

SQL> select basename ('nopath.txt') from dual;

BASENAME('NOPATH.TXT')
--------------------------------------------------------------------------------
nopath.txt

1 row selected.

here are the begining of all file names.
SQL> r
1 select substr(file_name, 1, 7), count(*)
2 from dba_data_files
3* group by substr(file_name, 1, 7)

SUBSTR( COUNT(*)
------- ----------
/oracle 319

1 row selected.