从网上借用的sql语句，稍做修改，验证可行。

select
df.tablespace_name "TABLESPACE_NAME",totalspace "TOTALSPACE/M",freespace "FREESPACE/M",round((1-freespace/totalspace)*100,2) "USED%"
from 
         (select tablespace_name,round(sum(bytes)/1024/1024) totalspace from    dba_data_files group by tablespace_name) df, 
         (select tablespace_name,round(sum(bytes)/1024/1024) freespace from dba_free_space group by tablespace_name) fs
where df.tablespace_name=fs.tablespace_name;

附上结果：
TABLESPACE_NAME      TOTALSPACE/M      FREESPACE/M      USED%
------------------------------            ------------ -                ----------               ----------
UNDOTBS1                                  25                                21                        16
SYSAUX                                       240                                 3                    98.75
USERS                                            5                                  2                          60
SYSTEM                                      480                                  4                    99.17
EXAMPLE                                    100                               32                          68