6 rows selected.

SQL> drop tablespace users including contents and datafiles;
drop tablespace users including contents and datafiles
*
ERROR at line 1:
ORA-12919: Can not drop the default permanent tablespace

SQL> drop tablespace rhys including contents and datafiles;

Tablespace dropped.

SQL>
SQL> alter database default tablespace test;

Database altered.

SQL> drop tablespace users including contents and datafiles;

Tablespace dropped.

SQL> select file#,name,status from v$datafile;

FILE# NAME STATUS
---------- -------------------------------------------------- ------------------------------
1 +DATAGROUP1/rhys/datafile/system.311.836735137 SYSTEM
2 +DATAGROUP1/rhys/datafile/sysaux.279.836735137 ONLINE
3 +DATAGROUP1/rhys/datafile/undotbs1.257.836735137 ONLINE
5 +DATAGROUP1/rhys/datafile/test.dbf ONLINE

SQL> conn amy/rhys
Connected.
SQL> select count(*) from t;

COUNT(*)
----------
227000