SQL & PL/SQL :: Fetch Tablespace And Datafile Count With Used / Freespace?

I'm trying to fetch tablespace and its datafile count along with used space,freespace and total size of a tablespace. I've written a query like this, but the logic is wrong it is giving me incorrect rows. It is running in loops and shows me incorrect number of datafiles.

Quote: select DISTINCT b.tablespace_name "Tablespace Name",count(b.file_ID) "Num of Datafiles",
TO_CHAR( b.bytes/(1024*1024*1024),'990.00') "Total SIZE(GB)" ,
TO_CHAR((b.bytes - sum(nvl(a.bytes,0)))/(1024*1024*1024),'990.00') "Used Space(GB)" ,
TO_CHAR(sum(nvl(a.bytes,0))/(1024*1024*1024),'990.00') "Free Space(GB)"
from sys.dba_free_space a, sys.dba_data_files b
where a.file_id = b.file_id
group by b.tablespace_name,b.file_name, b.file_id, b.bytes,online_status
order by b.tablespace_name;