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SELECT df.tablespace_name "Tablespace"
     , totalusedspace "Used MB"
     , (df.totalspace - tu.totalusedspace) "Free MB"
     , df.totalspace "Total MB"
     , ROUND(100 * ((df.totalspace - tu.totalusedspace) / df.totalspace)) "Pct. Free"
  FROM (SELECT   tablespace_name, ROUND(SUM(bytes) / 1048576) TotalSpace
            FROM dba_data_files
        GROUP BY tablespace_name) df
     , (SELECT   ROUND(SUM(bytes) / (1024 * 1024)) totalusedspace, tablespace_name
            FROM dba_segments
        GROUP BY tablespace_name) tu
 WHERE df.tablespace_name = tu.tablespace_name;