SQL

-- The space each data file uses.
select e.file_id,f.file_name,sum(e.blocks) allocated,f.blocks "FILE SIZE" from dba_extents e join dba_data_files f on e.file_id=f.file_id where f.status='AVAILABLE' group by e.file_id,f.file_name,f.blocks order by e.file_id;

-- The space each data file uses.
select e.file_id, f.file_name, sum(e.blocks) allocated, f.blocks "FILE SIZE" from dba_extents e join dba_data_files f on e.file_id = f.file_id where f.status = 'AVAILABLE' group by e.file_id, f.file_name, f.blocks order by e.file_id;

-- The space each data file uses.
  select e.file_id,
         f.file_name,
         sum(e.blocks) allocated,
         f.blocks "FILE SIZE"
    from dba_extents e
         join dba_data_files f on e.file_id = f.file_id
   where f.status = 'AVAILABLE'
group by e.file_id, f.file_name, f.blocks
order by e.file_id;