Aggregate Functions Oracle PLSQL Tutorial

SQL>
SQL> select         owner,
  2                 tablespace_name,
  3                 sum(bytes)/(1024*1024) total_meg
  4      from dba_extents
  5      where rownum < 50
  6      group by cube (owner,tablespace_name)
  7      /
OWNER                          TABLESPACE_NAME                 TOTAL_MEG
------------------------------ ------------------------------ ----------
                                                                       4
                               SYSTEM                                  4
SYS                                                                    4
SYS                            SYSTEM                                  4