Query to Get the Size of Oracle DB

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select
"Reserved_Space(MB)", "Reserved_Space(MB)" - "Free_Space(MB)" "Used_Space(MB)","Free_Space(MB)"
from(
select 
(select sum(bytes/(1014*1024)) from dba_data_files) "Reserved_Space(MB)",
(select sum(bytes/(1024*1024)) from dba_free_space) "Free_Space(MB)"
from dual
);