рез-тат разбит на группы

 select sum(bytes)/1024/1024 "SUM in Mb" , owner from dba_segments group by owner

 SUM in Mb OWNER
---------- ------------------------------
   32,4375 MDSYS
       ,25 TSMSYS
       ,25 DMSYS
      1,75 MILLER
        ,5 OUTLN
     4,625 CTXSYS
   15,5625 OLAPSYS
    1,5625 HR
   22,1875 SYSTEM
     3,625 EXFSYS
      ,375 SCOTT

 SUM in Mb OWNER
---------- ------------------------------
        56 SH
    9,0625 OE
    1,5625 DBSNMP
        ,5 ORDSYS
        53 SYSMAN
    11,875 PM
      48,5 XDB
     1,625 IX
    586,75 SYS
     6,875 WMSYS

SQL> set pagesize 20
SQL>  select sum(bytes)/1024/1024 "SUM in Mb" , owner from dba_segments group by
 owner;

 SUM in Mb OWNER
---------- ------------------------------
   32.4375 MDSYS
       .25 TSMSYS
       .25 DMSYS
        .5 OUTLN
     4.625 CTXSYS
   15.5625 OLAPSYS
   22.3125 SYSTEM
     3.625 EXFSYS
      .625 SCOTT
    1.5625 DBSNMP
        .5 ORDSYS
    66.875 SYSMAN
   7939.75 IFSAPP
   48.0625 XDB
        31 ZBDEV
  1064.375 SYS
    5.9375 WMSYS

17 rows selected.

SQL>