Free space in Tablespaces

SELECT a.tablespace_name TABLESPACE,
            a.total_space TOTAL_SPACE,
            b.free_space FREE_SPACE,
            a.total_space_mb TOTAL_SPACE_MB,
           (a.total_space_mb - b.free_space_mb) USED_SPACE_MB,
            b.free_space_mb FREE_SPACE_MB,
            ROUND(100 * (b.free_space / a.total_space),2) PCT_FREE
FROM
           (SELECT tablespace_name,
                        SUM(bytes) TOTAL_SPACE,
                        ROUND(SUM(bytes) / 1048576) TOTAL_SPACE_MB
            FROM    dba_data_files
             GROUP BY
             tablespace_name) a,
             (SELECT tablespace_name,
                          SUM(bytes) FREE_SPACE,
                          ROUND(SUM(bytes) / 1048576) FREE_SPACE_MB
               FROM
                          dba_free_space
              GROUP BY
                           tablespace_name) b
WHERE
a.tablespace_name = b.tablespace_name(+)
ORDER BY b.tablespace_name;