If we write a query in the from clause then this is called inline view or online query.

The inline view is a construct in Oracle SQL where you can place a query in the SQL FROM clause just as if the query was a table name.

select
fs.tablespace_name Tablespace
(df.totalspace - fs.freespace) Used MB
fs.freespace Free MB
df.totalspace Total MB
round(100 * (fs.freespace / df.totalspace)) Pct. Free
from
(select
tablespace_name
round(sum(bytes) / 1048576) TotalSpace
from
dba_data_files
group by
tablespace_name
) df
(select
tablespace_name
round(sum(bytes) / 1048576) FreeSpace
from
dba_free_space
group by
tablespace_name
) fs
where
df.tablespace_name fs.tablespace_name;