Oracle tablespace usage script

SELECT /* + RULE */  
df.tablespace_name AS "Tablespace",
df.bytes / (1024 * 1024 * 1024) AS "Size (GB)",
Trunc(fs.bytes / (1024 * 1024 * 1024)) AS "Free (GB)"       
FROM 
(
SELECT 
tablespace_name,
Sum(bytes) AS bytes 
FROM 
dba_free_space
GROUP BY
tablespace_name
) fs, 
( 
SELECT 
tablespace_name,
SUM(bytes) AS bytes
FROM 
dba_data_files
GROUP BY 
tablespace_name

) df
WHERE 
fs.tablespace_name = df.tablespace_name
ORDER BY 3 desc