SELECT
ts.tablespace_name,
ts.size_m as Size_MB,
round(s.used_m) as Used_MB
FROM
(
select f.tablespace_name, sum(f.bytes)/(1024*1024) size_m
from dba_data_files f
group by f.tablespace_name
) ts,
(
select s.tablespace_name, sum(s.bytes)/(1024*1024) used_m
from dba_segments s
group by s.tablespace_name
) s
WHERE
s.tablespace_name(+) = ts.tablespace_name
ORDER BY
ts.tablespace_name
;