oracle View statement sharing of allocated and unallocated tablespace space

selectb.tablespace_name,mbytes_alloc,mbytes_free
from(select round(sum(bytes)/1024/1024) mbytes_free,tablespace_name
 from dba_free_space
 group by tablespace_name) a,
(select round(sum(bytes)/1024/1024) mbytes_alloc,tablespace_name
 from dba_data_files
 group by tablespace_name) b
 where a.tablespace_name(+)=b.tablespace_name
and b.tablespace_name='USERS'
 /