SQL & PL/SQL :: Find Total Space Occupied On Disk By Tablespaces Of Database

I am trying to find the space occupied on disk by the tablespaces of the database that contain tables, some (and not all) of whose columns are encrypted. My query is like this:

select distinct a.tablespace_name, file_name, bytes /(1024*1024*1024) File_Size_In_GB
from dba_data_files a, dba_tables b,
(select distinct owner, table_name from DBA_ENCRYPTED_COLUMNS) c
where
a.tablespace_name = b.tablespace_name and
b.owner = c.owner and
b.table_name = c.table_name
order by a.tablespace_name;

The output of the query is as shown in the attached file:

TABLESPACE_NAMEFILE_NAMEFILE_SIZE_IN_GB
DMS_DATAM:ORACLEORADATASPOPRODDMS_DATA_0044.DBF29.296875
DMS_DATAM:ORACLEORADATASPOPRODDMS_DATA_0045.DBF29.296875
DMS_DATAM:ORACLEORADATASPOPRODDMS_DATA_0051.DBF29.296875
DMS_DATAN:ORACLEORADATASPOPRODDMS_DATA_0012.DBF19.53125
[code]...

Since the output (under the heading Total Size of the tablespace) is probably the sum of all the datafiles returned by the query and is obviously incorrect, I have not given the rest of it. I also tried the following:

select distinct a.tablespace_name, file_name, bytes /(1024*1024*1024) File_Size_In_GB,
sum (bytes/(1024*1024*1024))over (partition by a.tablespace_name order by file_name) "Total Size of the tablespace"
from dba_data_files a, dba_tables b,
(select distinct owner, table_name from DBA_ENCRYPTED_COLUMNS) c
where
a.tablespace_name = b.tablespace_name and
b.owner = c.owner and
b.table_name = c.table_name
order by a.tablespace_name ;
[code]...

Here, the fig. under the heading "Total Size of the tablespace" are probably the sum of all the records returned by the query if distinct is not used i.e all the data file sizes returned by the query.

tune my query and get the desired results? I think this can be achieved by group by with rollup, cube, order by and grouping functions, but am not sure how to proceed. I know that I can get the results by using Enterprise Mgr. Console in 2 mins., but would still like to get the results with the queries.