ROWNUM and Aggregate Functions

When working on aggregate functions, you must be aware of rownum. Resultset firstly taken with rownum and then aggregate functions are processed.
Look at example below:

Connected to Oracle Database 10g Enterprise Edition Release 10.2.0.1.0
Connected as SYS
SQL>
SQL> drop table testtab;
Table dropped
SQL> CREATE TABLE testtab AS  SELECT o.object_name n, mod(rownum, 5) i, rownum r FROM all_objects o WHERE rownum < 11;
Table created
SQL> SELECT * FROM testtab;
N                                       I          R
------------------------------ ---------- ----------
ICOL$                                   1          1
I_USER1                                 2          2
CON$                                    3          3
UNDO$                                   4          4
C_COBJ#                                 0          5
I_OBJ#                                  1          6
PROXY_ROLE_DATA$                        2          7
I_IND1                                  3          8
I_CDEF2                                 4          9
I_PROXY_ROLE_DATA$_1                    0         10
10 rows selected
SQL> SELECT * FROM testtab WHERE i = 0;
N                                       I          R
------------------------------ ---------- ----------
C_COBJ#                                 0          5
I_PROXY_ROLE_DATA$_1                    0         10
SQL> SELECT max(r) FROM testtab WHERE i = 0 and rownum = 1;
    MAX(R)
----------
         5
SQL> SELECT max(r) FROM testtab WHERE i = 0;
    MAX(R)
----------
        10
SQL> SELECT * FROM (SELECT max(r) FROM testtab WHERE i = 0)  WHERE rownum = 1;
    MAX(R)
----------
        10
SQL>