select a.tablespace_name,
       totalspace,
       nvl(freespace, 0) freespace,
       (totalspace - nvl(freespace, 0)) used,
       ((totalspace - nvl(freespace, 0)) / totalspace) * 100 "%USED"
  from (select tablespace_name, sum(bytes) / 1048576 totalspace
          from dba_data_files
         group by tablespace_name) a,
       (select tablespace_name, sum(Bytes) / 1048576 freespace
          from dba_free_space
         group by tablespace_name) b
 where a.tablespace_name = b.tablespace_name(+)
   and ((totalspace - nvl(freespace, 0)) / totalspace) * 100 > 90
--and nvl(freespace,0) < 1000  -- only list TSs < 1GB free
 order by 5 desc;