Q:

rownum confused result

when i code
select row_.*, rownum rownum_ from (select topic0_.id as id6_, topic0_.title as title6_, topic0_.publish_author_id as publish3_6_, topic0_.reply_number as reply4_6_, topic0_.read_number as read5_6_, topic0_.publish_author_name as publish6_6_, topic0_.publish_time as publish7_6_, topic0_.last_reply_time as last8_6_, topic0_.last_reply_name as last9_6_, topic0_.main_text as main10_6_, topic0_.FORUM_ID as FORUM11_6_ from topic topic0_ where topic0_.FORUM_ID='16F37E59FE494AC9B17A2340387335EF' order by case when topic0_.last_reply_time is not null then topic0_.last_reply_time else  topic0_.publish_time end desc) row_;

the result:
    ID6_    TITLE6_ PUBLISH3_6_ REPLY4_6_   READ5_6_    PUBLISH6_6_ PUBLISH7_6_ LAST8_6_    LAST9_6_    MAIN10_6_   FORUM11_6_  ROWNUM_
1   402840713e108ea4013e1090c95c0001    trytr   2   2   0   ss  2013-4-16 9:58:44   2013-4-16 9:59:21   ss  fdsfds  16F37E59FE494AC9B17A2340387335EF    1
2   402840713e108ea4013e1090127e0000    trytr   2   0   0   ss  2013-4-16 9:57:57           ytrytr  16F37E59FE494AC9B17A2340387335EF    2
3   402840713e0d3616013e0d3618c20000    cde0    2   0   0   ss  2013-4-15 18:20:49          abc0    16F37E59FE494AC9B17A2340387335EF    3
4   402840713e0d3616013e0d3618da0063    cde99   2   0   0   ss  2013-4-15 18:20:49          abc99   16F37E59FE494AC9B17A2340387335EF    4
5   402840713e0d3616013e0d3618da0062    cde98   2   0   0   ss  2013-4-15 18:20:49          abc98   16F37E59FE494AC9B17A2340387335EF    5
6   402840713e0d3616013e0d3618da0061    cde97   2   0   0   ss  2013-4-15 18:20:49          abc97   16F37E59FE494AC9B17A2340387335EF    6
7   402840713e0d3616013e0d3618da0060    cde96   2   0   0   ss  2013-4-15 18:20:49          abc96   16F37E59FE494AC9B17A2340387335EF    7
8   402840713e0d3616013e0d3618da005f    cde95   2   0   0   ss  2013-4-15 18:20:49          abc95   16F37E59FE494AC9B17A2340387335EF    8
9   402840713e0d3616013e0d3618da005e    cde94   2   0   0   ss  2013-4-15 18:20:49          abc94   16F37E59FE494AC9B17A2340387335EF    9
10  402840713e0d3616013e0d3618da005d    cde93   2   0   0   ss  2013-4-15 18:20:49          abc93   16F37E59FE494AC9B17A2340387335EF    10
11  402840713e0d3616013e0d3618da005c    cde92   2   0   0   ss  2013-4-15 18:20:49          abc92   16F37E59FE494AC9B17A2340387335EF    11
12  402840713e0d3616013e0d3618da005b    cde91   2   0   0   ss  2013-4-15 18:20:49          abc91   16F37E59FE494AC9B17A2340387335EF    12
13  402840713e0d3616013e0d3618d9005a    cde90   2   0   0   ss  2013-4-15 18:20:49          abc90   16F37E59FE494AC9B17A2340387335EF    13
14  402840713e0d3616013e0d3618d90059    cde89   2   0   0   ss  2013-4-15 18:20:49          abc89   16F37E59FE494AC9B17A2340387335EF    14
15  402840713e0d3616013e0d3618d90058    cde88   2   0   0   ss  2013-4-15 18:20:49          abc88   16F37E59FE494AC9B17A2340387335EF    15
16  402840713e0d3616013e0d3618d90057    cde87   2   0   0   ss  2013-4-15 18:20:49          abc87   16F37E59FE494AC9B17A2340387335EF    16
17  402840713e0d3616013e0d3618d90056    cde86   2   0   0   ss  2013-4-15 18:20:49          abc86   16F37E59FE494AC9B17A2340387335EF    17
18  402840713e0d3616013e0d3618d90055    cde85   2   0   0   ss  2013-4-15 18:20:49          abc85   16F37E59FE494AC9B17A2340387335EF    18
19  402840713e0d3616013e0d3618d90054    cde84   2   0   0   ss  2013-4-15 18:20:49          abc84   16F37E59FE494AC9B17A2340387335EF    19
20  402840713e0d3616013e0d3618d90053    cde83   2   0   0   ss  2013-4-15 18:20:49          abc83   16F37E59FE494AC9B17A2340387335EF    20
21  402840713e0d3616013e0d3618d90052    cde82   2   0   0   ss  2013-4-15 18:20:49          abc82   16F37E59FE494AC9B17A2340387335EF    21
22  402840713e0d3616013e0d3618d90051    cde81   2   0   0   ss  2013-4-15 18:20:49          abc81   16F37E59FE494AC9B17A2340387335EF    22
23  402840713e0d3616013e0d3618d80050    cde80   2   0   0   ss  2013-4-15 18:20:49          abc80   16F37E59FE494AC9B17A2340387335EF    23
24  402840713e0d3616013e0d3618d8004f    cde79   2   0   0   ss  2013-4-15 18:20:49          abc79   16F37E59FE494AC9B17A2340387335EF    24
25  402840713e0d3616013e0d3618d8004e    cde78   2   0   0   ss  2013-4-15 18:20:49          abc78   16F37E59FE494AC9B17A2340387335EF    25
26  402840713e0d3616013e0d3618d8004d    cde77   2   0   0   ss  2013-4-15 18:20:49          abc77   16F37E59FE494AC9B17A2340387335EF    26
27  402840713e0d3616013e0d3618d8004c    cde76   2   0   0   ss  2013-4-15 18:20:49          abc76   16F37E59FE494AC9B17A2340387335EF    27
28  402840713e0d3616013e0d3618d8004b    cde75   2   0   0   ss  2013-4-15 18:20:49          abc75   16F37E59FE494AC9B17A2340387335EF    28
29  402840713e0d3616013e0d3618d8004a    cde74   2   0   0   ss  2013-4-15 18:20:49          abc74   16F37E59FE494AC9B17A2340387335EF    29
30  402840713e0d3616013e0d3618d80049    cde73   2   0   0   ss  2013-4-15 18:20:49          abc73   16F37E59FE494AC9B17A2340387335EF    30
31  402840713e0d3616013e0d3618d80048    cde72   2   0   0   ss  2013-4-15 18:20:49          abc72   16F37E59FE494AC9B17A2340387335EF    31
32  402840713e0d3616013e0d3618d70047    cde71   2   0   0   ss  2013-4-15 18:20:49          abc71   16F37E59FE494AC9B17A2340387335EF    32
33  402840713e0d3616013e0d3618d70046    cde70   2   0   0   ss  2013-4-15 18:20:49          abc70   16F37E59FE494AC9B17A2340387335EF    33
34  402840713e0d3616013e0d3618d70045    cde69   2   0   0   ss  2013-4-15 18:20:49          abc69   16F37E59FE494AC9B17A2340387335EF    34
35  402840713e0d3616013e0d3618d70044    cde68   2   0   0   ss  2013-4-15 18:20:49          abc68   16F37E59FE494AC9B17A2340387335EF    35
36  402840713e0d3616013e0d3618d70043    cde67   2   0   0   ss  2013-4-15 18:20:49          abc67   16F37E59FE494AC9B17A2340387335EF    36
37  402840713e0d3616013e0d3618d70042    cde66   2   0   0   ss  2013-4-15 18:20:49          abc66   16F37E59FE494AC9B17A2340387335EF    37
38  402840713e0d3616013e0d3618d70041    cde65   2   0   0   ss  2013-4-15 18:20:49          abc65   16F37E59FE494AC9B17A2340387335EF    38
39  402840713e0d3616013e0d3618d70040    cde64   2   0   0   ss  2013-4-15 18:20:49          abc64   16F37E59FE494AC9B17A2340387335EF    39
40  402840713e0d3616013e0d3618d7003f    cde63   2   0   0   ss  2013-4-15 18:20:49          abc63   16F37E59FE494AC9B17A2340387335EF    40
41  402840713e0d3616013e0d3618d7003e    cde62   2   0   0   ss  2013-4-15 18:20:49          abc62   16F37E59FE494AC9B17A2340387335EF    41
42  402840713e0d3616013e0d3618d6003d    cde61   2   0   0   ss  2013-4-15 18:20:49          abc61   16F37E59FE494AC9B17A2340387335EF    42
43  402840713e0d3616013e0d3618d6003c    cde60   2   0   0   ss  2013-4-15 18:20:49          abc60   16F37E59FE494AC9B17A2340387335EF    43
44  402840713e0d3616013e0d3618d6003b    cde59   2   0   0   ss  2013-4-15 18:20:49          abc59   16F37E59FE494AC9B17A2340387335EF    44
45  402840713e0d3616013e0d3618d6003a    cde58   2   0   0   ss  2013-4-15 18:20:49          abc58   16F37E59FE494AC9B17A2340387335EF    45
46  402840713e0d3616013e0d3618d60039    cde57   2   0   0   ss  2013-4-15 18:20:49          abc57   16F37E59FE494AC9B17A2340387335EF    46
47  402840713e0d3616013e0d3618d60038    cde56   2   0   0   ss  2013-4-15 18:20:49          abc56   16F37E59FE494AC9B17A2340387335EF    47
48  402840713e0d3616013e0d3618d60037    cde55   2   0   0   ss  2013-4-15 18:20:49          abc55   16F37E59FE494AC9B17A2340387335EF    48
49  402840713e0d3616013e0d3618d60036    cde54   2   0   0   ss  2013-4-15 18:20:49          abc54   16F37E59FE494AC9B17A2340387335EF    49
50  402840713e0d3616013e0d3618d60035    cde53   2   0   0   ss  2013-4-15 18:20:49          abc53   16F37E59FE494AC9B17A2340387335EF    50
51  402840713e0d3616013e0d3618d50034    cde52   2   0   0   ss  2013-4-15 18:20:49          abc52   16F37E59FE494AC9B17A2340387335EF    51
52  402840713e0d3616013e0d3618d50033    cde51   2   0   0   ss  2013-4-15 18:20:49          abc51   16F37E59FE494AC9B17A2340387335EF    52
53  402840713e0d3616013e0d3618d50032    cde50   2   0   0   ss  2013-4-15 18:20:49          abc50   16F37E59FE494AC9B17A2340387335EF    53
54  402840713e0d3616013e0d3618d50031    cde49   2   0   0   ss  2013-4-15 18:20:49          abc49   16F37E59FE494AC9B17A2340387335EF    54
55  402840713e0d3616013e0d3618d50030    cde48   2   0   0   ss  2013-4-15 18:20:49          abc48   16F37E59FE494AC9B17A2340387335EF    55
56  402840713e0d3616013e0d3618d5002f    cde47   2   0   0   ss  2013-4-15 18:20:49          abc47   16F37E59FE494AC9B17A2340387335EF    56
57  402840713e0d3616013e0d3618d5002e    cde46   2   0   0   ss  2013-4-15 18:20:49          abc46   16F37E59FE494AC9B17A2340387335EF    57
58  402840713e0d3616013e0d3618d5002d    cde45   2   0   0   ss  2013-4-15 18:20:49          abc45   16F37E59FE494AC9B17A2340387335EF    58
59  402840713e0d3616013e0d3618d5002c    cde44   2   0   0   ss  2013-4-15 18:20:49          abc44   16F37E59FE494AC9B17A2340387335EF    59
60  402840713e0d3616013e0d3618d5002b    cde43   2   0   0   ss  2013-4-15 18:20:49          abc43   16F37E59FE494AC9B17A2340387335EF    60
61  402840713e0d3616013e0d3618d5002a    cde42   2   0   0   ss  2013-4-15 18:20:49          abc42   16F37E59FE494AC9B17A2340387335EF    61
62  402840713e0d3616013e0d3618d40029    cde41   2   0   0   ss  2013-4-15 18:20:49          abc41   16F37E59FE494AC9B17A2340387335EF    62
63  402840713e0d3616013e0d3618d40028    cde40   2   0   0   ss  2013-4-15 18:20:49          abc40   16F37E59FE494AC9B17A2340387335EF    63
64  402840713e0d3616013e0d3618d40027    cde39   2   0   0   ss  2013-4-15 18:20:49          abc39   16F37E59FE494AC9B17A2340387335EF    64
65  402840713e0d3616013e0d3618d40026    cde38   2   0   0   ss  2013-4-15 18:20:49          abc38   16F37E59FE494AC9B17A2340387335EF    65
66  402840713e0d3616013e0d3618d40025    cde37   2   0   0   ss  2013-4-15 18:20:49          abc37   16F37E59FE494AC9B17A2340387335EF    66
67  402840713e0d3616013e0d3618d40024    cde36   2   0   0   ss  2013-4-15 18:20:49          abc36   16F37E59FE494AC9B17A2340387335EF    67
68  402840713e0d3616013e0d3618d40023    cde35   2   0   0   ss  2013-4-15 18:20:49          abc35   16F37E59FE494AC9B17A2340387335EF    68
69  402840713e0d3616013e0d3618d40022    cde34   2   0   0   ss  2013-4-15 18:20:49          abc34   16F37E59FE494AC9B17A2340387335EF    69
70  402840713e0d3616013e0d3618d30021    cde33   2   0   0   ss  2013-4-15 18:20:49          abc33   16F37E59FE494AC9B17A2340387335EF    70
71  402840713e0d3616013e0d3618d30020    cde32   2   0   0   ss  2013-4-15 18:20:49          abc32   16F37E59FE494AC9B17A2340387335EF    71
72  402840713e0d3616013e0d3618d3001f    cde31   2   0   0   ss  2013-4-15 18:20:49          abc31   16F37E59FE494AC9B17A2340387335EF    72
73  402840713e0d3616013e0d3618d3001e    cde30   2   0   0   ss  2013-4-15 18:20:49          abc30   16F37E59FE494AC9B17A2340387335EF    73
74  402840713e0d3616013e0d3618d3001d    cde29   2   0   0   ss  2013-4-15 18:20:49          abc29   16F37E59FE494AC9B17A2340387335EF    74
75  402840713e0d3616013e0d3618d3001c    cde28   2   0   0   ss  2013-4-15 18:20:49          abc28   16F37E59FE494AC9B17A2340387335EF    75
76  402840713e0d3616013e0d3618d3001b    cde27   2   0   0   ss  2013-4-15 18:20:49          abc27   16F37E59FE494AC9B17A2340387335EF    76
77  402840713e0d3616013e0d3618d3001a    cde26   2   0   0   ss  2013-4-15 18:20:49          abc26   16F37E59FE494AC9B17A2340387335EF    77
78  402840713e0d3616013e0d3618d30019    cde25   2   0   0   ss  2013-4-15 18:20:49          abc25   16F37E59FE494AC9B17A2340387335EF    78
79  402840713e0d3616013e0d3618d30018    cde24   2   0   0   ss  2013-4-15 18:20:49          abc24   16F37E59FE494AC9B17A2340387335EF    79
80  402840713e0d3616013e0d3618d20017    cde23   2   0   0   ss  2013-4-15 18:20:49          abc23   16F37E59FE494AC9B17A2340387335EF    80
81  402840713e0d3616013e0d3618d20016    cde22   2   0   0   ss  2013-4-15 18:20:49          abc22   16F37E59FE494AC9B17A2340387335EF    81
82  402840713e0d3616013e0d3618d20015    cde21   2   0   0   ss  2013-4-15 18:20:49          abc21   16F37E59FE494AC9B17A2340387335EF    82
83  402840713e0d3616013e0d3618d20014    cde20   2   0   0   ss  2013-4-15 18:20:49          abc20   16F37E59FE494AC9B17A2340387335EF    83
84  402840713e0d3616013e0d3618d20013    cde19   2   0   0   ss  2013-4-15 18:20:49          abc19   16F37E59FE494AC9B17A2340387335EF    84
85  402840713e0d3616013e0d3618d20012    cde18   2   0   0   ss  2013-4-15 18:20:49          abc18   16F37E59FE494AC9B17A2340387335EF    85
86  402840713e0d3616013e0d3618d20011    cde17   2   0   0   ss  2013-4-15 18:20:49          abc17   16F37E59FE494AC9B17A2340387335EF    86
87  402840713e0d3616013e0d3618d20010    cde16   2   0   0   ss  2013-4-15 18:20:49          abc16   16F37E59FE494AC9B17A2340387335EF    87
88  402840713e0d3616013e0d3618d0000f    cde15   2   0   0   ss  2013-4-15 18:20:49          abc15   16F37E59FE494AC9B17A2340387335EF    88
89  402840713e0d3616013e0d3618cf000e    cde14   2   0   0   ss  2013-4-15 18:20:49          abc14   16F37E59FE494AC9B17A2340387335EF    89
90  402840713e0d3616013e0d3618cf000d    cde13   2   0   0   ss  2013-4-15 18:20:49          abc13   16F37E59FE494AC9B17A2340387335EF    90
91  402840713e0d3616013e0d3618cf000c    cde12   2   0   0   ss  2013-4-15 18:20:49          abc12   16F37E59FE494AC9B17A2340387335EF    91
92  402840713e0d3616013e0d3618cf000b    cde11   2   0   0   ss  2013-4-15 18:20:49          abc11   16F37E59FE494AC9B17A2340387335EF    92
93  402840713e0d3616013e0d3618cf000a    cde10   2   0   0   ss  2013-4-15 18:20:49          abc10   16F37E59FE494AC9B17A2340387335EF    93
94  402840713e0d3616013e0d3618ce0009    cde9    2   0   0   ss  2013-4-15 18:20:49          abc9    16F37E59FE494AC9B17A2340387335EF    94
95  402840713e0d3616013e0d3618ce0008    cde8    2   0   0   ss  2013-4-15 18:20:49          abc8    16F37E59FE494AC9B17A2340387335EF    95
96  402840713e0d3616013e0d3618ce0007    cde7    2   0   0   ss  2013-4-15 18:20:49          abc7    16F37E59FE494AC9B17A2340387335EF    96
97  402840713e0d3616013e0d3618ce0006    cde6    2   0   0   ss  2013-4-15 18:20:49          abc6    16F37E59FE494AC9B17A2340387335EF    97
98  402840713e0d3616013e0d3618ce0005    cde5    2   0   0   ss  2013-4-15 18:20:49          abc5    16F37E59FE494AC9B17A2340387335EF    98
99  402840713e0d3616013e0d3618ce0004    cde4    2   0   0   ss  2013-4-15 18:20:49          abc4    16F37E59FE494AC9B17A2340387335EF    99
100 402840713e0d3616013e0d3618ce0003    cde3    2   0   0   ss  2013-4-15 18:20:49          abc3    16F37E59FE494AC9B17A2340387335EF    100
101 402840713e0d3616013e0d3618ce0002    cde2    2   0   0   ss  2013-4-15 18:20:49          abc2    16F37E59FE494AC9B17A2340387335EF    101
102 402840713e0d3616013e0d3618cd0001    cde1    2   0   0   ss  2013-4-15 18:20:49          abc1    16F37E59FE494AC9B17A2340387335EF    102
103 402840713e0cb39c013e0cb41c040000    dfsdf   2   1   0   ss  2013-4-15 15:58:50          sdfsdfsdf   16F37E59FE494AC9B17A2340387335EF    103
104 402840713e0c9424013e0c94a8c30000    fffff   2   1   0   ss  2013-4-15 15:24:29          dgfdgfdg    16F37E59FE494AC9B17A2340387335EF    104
105 402840713e0c8e00013e0c8f1d190000    asf 2   2   0   ss  2013-4-15 15:18:25          dsfds   16F37E59FE494AC9B17A2340387335EF    105
106 402840713e0c80ce013e0c8158630000    ccc 2   1   0   ss  2013-4-15 15:03:23          dfsd    16F37E59FE494AC9B17A2340387335EF    106
107 402840713e0c5a97013e0c5b0c200000    hhhhh   2   2   0   ss  2013-4-15 14:21:33          fds 16F37E59FE494AC9B17A2340387335EF    107
108 402840713e0c58cd013e0c592b550000    hff 2   0   0   ss  2013-4-15 14:19:30          sdfs    16F37E59FE494AC9B17A2340387335EF    108
109 402840713e0c272d013e0c2e12c40000    gsfsdf  2   0   0   ss  2013-4-15 13:32:26          fdsfds  16F37E59FE494AC9B17A2340387335EF    109
110 402840713e0c25ef013e0c265f720000    fdsf    2   0   0   ss  2013-4-15 13:24:01          fsdfsdf 16F37E59FE494AC9B17A2340387335EF    110
111 402840713e0c2237013e0c228d4b0000    fdsf    2   0   0   ss  2013-4-15 13:19:51          sdfs    16F37E59FE494AC9B17A2340387335EF    111
112 402840713e0c1cc9013e0c1d0dc30000    fsdf    2   0   0   ss  2013-4-15 13:13:50          sdfdsfds    16F37E59FE494AC9B17A2340387335EF    112
113 402840713e0c1be8013e0c1c33bf0000    fsfds   2   0   0   ss  2013-4-15 13:12:54          fdsfsd  16F37E59FE494AC9B17A2340387335EF    113
114 402840713e0c1b0b013e0c1b72320000    fsfsd   2   0   0   ss  2013-4-15 13:12:05          fsdfsd  16F37E59FE494AC9B17A2340387335EF    114
115 402840713e0c19a9013e0c19f49b0000    GDFG    2   0   0   ss  2013-4-15 13:10:27          FDGDFGDF    16F37E59FE494AC9B17A2340387335EF    115
116 402840713e0c17c7013e0c184b200000    fdsfsd  2   0   0   ss  2013-4-15 13:08:38          fsdfsdfs    16F37E59FE494AC9B17A2340387335EF    116
117 402840713e0c0edc013e0c0f79d20000    fsdfds  2   0   0   ss  2013-4-15 12:59:00          fdsfdsfdsfd 16F37E59FE494AC9B17A2340387335EF    117
118 402840713e0bee4f013e0bef28150000    fdsfsd  2   0   0   ss  2013-4-15 12:23:42          fdsfsdf 16F37E59FE494AC9B17A2340387335EF    118
119 402840713e0b92fc013e0b9418cf0000    fdsfdsf 2   0   0   ss  2013-4-15 10:44:15          dsfdsfdsfds 16F37E59FE494AC9B17A2340387335EF    119

but when i code
       select row_.*, rownum rownum_ from (select topic0_.id as id6_, topic0_.title as title6_, topic0_.publish_author_id as publish3_6_, topic0_.reply_number as reply4_6_, topic0_.read_number as read5_6_, topic0_.publish_author_name as publish6_6_, topic0_.publish_time as publish7_6_, topic0_.last_reply_time as last8_6_, topic0_.last_reply_name as last9_6_, topic0_.main_text as main10_6_, topic0_.FORUM_ID as FORUM11_6_ from topic topic0_ where topic0_.FORUM_ID='16F37E59FE494AC9B17A2340387335EF' order by case when topic0_.last_reply_time is not null then topic0_.last_reply_time else  topic0_.publish_time end desc) row_ where rownum <=6;

the result is
    ID6_    TITLE6_ PUBLISH3_6_ REPLY4_6_   READ5_6_    PUBLISH6_6_ PUBLISH7_6_ LAST8_6_    LAST9_6_    MAIN10_6_   FORUM11_6_  ROWNUM_ 
1   402840713e108ea4013e1090c95c0001    trytr   2   2   0   ss  2013-4-16 9:58:44   2013-4-16 9:59:21   ss  fdsfds  16F37E59FE494AC9B17A2340387335EF    1   
2   402840713e108ea4013e1090127e0000    trytr   2   0   0   ss  2013-4-16 9:57:57           ytrytr  16F37E59FE494AC9B17A2340387335EF    2   
3   402840713e0d3616013e0d3618c20000    cde0    2   0   0   ss  2013-4-15 18:20:49          abc0    16F37E59FE494AC9B17A2340387335EF    3   
4   402840713e0d3616013e0d3618ce0003    cde3    2   0   0   ss  2013-4-15 18:20:49          abc3    16F37E59FE494AC9B17A2340387335EF    4   
5   402840713e0d3616013e0d3618ce0002    cde2    2   0   0   ss  2013-4-15 18:20:49          abc2    16F37E59FE494AC9B17A2340387335EF    5   
6   402840713e0d3616013e0d3618cd0001    cde1    2   0   0   ss  2013-4-15 18:20:49          abc1    16F37E59FE494AC9B17A2340387335EF    6   

why ?  i want the result is:
    ID6_    TITLE6_ PUBLISH3_6_ REPLY4_6_   READ5_6_    PUBLISH6_6_ PUBLISH7_6_ LAST8_6_    LAST9_6_    MAIN10_6_   FORUM11_6_  ROWNUM_
1   402840713e108ea4013e1090c95c0001    trytr   2   2   0   ss  2013-4-16 9:58:44   2013-4-16 9:59:21   ss  fdsfds  16F37E59FE494AC9B17A2340387335EF    1
2   402840713e108ea4013e1090127e0000    trytr   2   0   0   ss  2013-4-16 9:57:57           ytrytr  16F37E59FE494AC9B17A2340387335EF    2
3   402840713e0d3616013e0d3618c20000    cde0    2   0   0   ss  2013-4-15 18:20:49          abc0    16F37E59FE494AC9B17A2340387335EF    3
4   402840713e0d3616013e0d3618da0063    cde99   2   0   0   ss  2013-4-15 18:20:49          abc99   16F37E59FE494AC9B17A2340387335EF    4
5   402840713e0d3616013e0d3618da0062    cde98   2   0   0   ss  2013-4-15 18:20:49          abc98   16F37E59FE494AC9B17A2340387335EF    5
6   402840713e0d3616013e0d3618da0061    cde97   2   0   0   ss  2013-4-15 18:20:49          abc97   16F37E59FE494AC9B17A2340387335EF    6

If somebody could explain to me  I'd be most grateful.

A:

In your first query you have:
select row_.*, rownum rownum_ from (select ... order by ...) row_;

In the second you have:
select row_.*, rownum rownum_ from (select ... order by ...) row_
where rownum <= 6;

rownum is a pseudo-column that is generated as the last step in the query, over the result set that has already been produced. The where rownum <= 6 is being applied against the results of the inner select, but at that point in the outer select you have no explicit order by clause so the order the rownum will be assigned isn't guaranteed.
You could either repeat the order by clause in the outer query, which would make it even more unpleasant to deal with, or more simply move the rownum inside the inner select:
select * from (select ..., rownum rownum_ from ... order by ...)
where rownum_ <= 6;

It's unclear if the results form the your first query are in the correct order anyway; probably, but again probably not guaranteed, so you can also order that to make sure:
select * from (select ..., rownum rownum_ from ... order by ...)
where rownum_ <= 6
order by rownum_;

(Another approach would be to use an analytic function like dense_rank() against the case you're using in the order by, and then use that in the outer where to restrict the number of rows, but that's getting a bit away from the question...)

