关于Oracle中排序后取前几行的问题

 查询语句一、

    SELECT V.STAMP,TO_CHAR(V.GPSTIME,'YYYY-MM-DD HH24:MI:SS'),V.PHOTOURL,V.SMALLPHOTOURL,V.TOUCH_OFF,V.VIDICON_NO,V.SPEED,V.COURSE,V.STATE
       FROM T_VEHICLE_PHOTO V
      WHERE V.ISMANAGE = 0
        AND V.TOUCH_OFF = 27
        AND V.NUMBER_PLATE = '苏F14899'
        AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS')
        AND ROWNUM <= 6
      ORDER BY V.STAMP desc

 

查询语句二、

     SELECT V.STAMP,TO_CHAR(V.GPSTIME,'YYYY-MM-DD HH24:MI:SS'),V.PHOTOURL,V.SMALLPHOTOURL,V.TOUCH_OFF,V.VIDICON_NO,V.SPEED,V.COURSE,V.STATE
       FROM T_VEHICLE_PHOTO V
      WHERE V.ISMANAGE = 0
        AND V.TOUCH_OFF = 27
        AND V.NUMBER_PLATE = '苏F14899'
        AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS')
        AND ROWNUM <6
      ORDER BY V.STAMP desc

 

查询语句一的结果:

STAMP

2008-11-2 8:11:52
2008-11-1 8:02:09
2008-10-31 17:16:02
2008-10-31 13:41:58
2008-10-31 10:53:00
2008-10-31 7:24:57

 

查询语句二的结果：

STAMP

2008-11-1 8:02:09
2008-10-31 17:16:02
2008-10-31 13:41:58
2008-10-31 10:53:00
2008-10-31 7:24:57

 

不加行数限制的结果：

2008-11-2 8:11:52
2008-11-2 7:27:57
2008-11-1 17:44:29
2008-11-1 14:32:07
2008-11-1 11:44:59
2008-11-1 8:02:09
2008-10-31 17:16:02
2008-10-31 13:41:58
2008-10-31 10:53:00
2008-10-31 7:24:57
2008-10-30 17:36:48

 

由此可以看出，如果不使用<=你拿到的数据会少一行，并且很奇怪是第一行，正常要想法应是最后一行才对，那么现在在比较以下两种写法:

 

select 标题,发布时间 from (select 标题,发布时间 from hhsys.tongzhigonggao order by 发布时间 desc) where rownum<11 

 

这样是拿排序后的前10

 

select 标题,发布时间 from hhsys.tongzhigonggao where rownum<=10 order by 发布时间 desc

 

这样的话应是拿到前10行再进行排序，但他实际上又是拿到排序后的前10行，真搞不明白。

 

 

噢，弄明白，了上面的语句写法是错误的。

 

 

正确的语句如下：

     SELECT * FROM (
     SELECT V.STAMP,TO_CHAR(V.GPSTIME,'YYYY-MM-DD HH24:MI:SS'),V.PHOTOURL,V.SMALLPHOTOURL,V.TOUCH_OFF,V.VIDICON_NO,V.SPEED,V.COURSE,V.STATE
       FROM T_VEHICLE_PHOTO V
      WHERE V.ISMANAGE = 0
        AND V.TOUCH_OFF = 27
        AND V.NUMBER_PLATE = '苏F14899'
        AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS') 
      ORDER BY V.STAMP DESC
      ) WHERE ROWNUM <= 6

 

也就是，如果你要取排序后的结果，不能将ROWNUM <= 6放在order by前面。

=======================================================================

     SELECT V.STAMP,TO_CHAR(V.GPSTIME,'YYYY-MM-DD HH24:MI:SS'),V.PHOTOURL,V.SMALLPHOTOURL,V.TOUCH_OFF,V.VIDICON_NO,V.SPEED,V.COURSE,V.STATE
       FROM T_VEHICLE_PHOTO V
      WHERE V.ISMANAGE = 0
        AND V.TOUCH_OFF = 27
        AND V.NUMBER_PLATE = '苏F14899'
        AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS')

正常数据：

1 2008-10-31 7:24:57
2 2008-10-31 10:53:00
3 2008-10-31 13:41:58
4 2008-10-31 17:16:02
5 2008-11-1 8:02:09
6 2008-11-2 8:11:52
7 2008-11-1 11:44:59
8 2008-11-1 14:32:07
9 2008-11-1 17:44:29
10 2008-11-2 7:27:57
11 2008-10-30 17:36:48

 

===================================================

     SELECT V.STAMP,TO_CHAR(V.GPSTIME,'YYYY-MM-DD HH24:MI:SS'),V.PHOTOURL,V.SMALLPHOTOURL,V.TOUCH_OFF,V.VIDICON_NO,V.SPEED,V.COURSE,V.STATE
       FROM T_VEHICLE_PHOTO V
      WHERE V.ISMANAGE = 0
        AND V.TOUCH_OFF = 27
        AND V.NUMBER_PLATE = '苏F14899'
        AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS')
        AND ROWNUM <= 6

取前6行数据：
1 2008-10-31 7:24:57
2 2008-10-31 10:53:00
3 2008-10-31 13:41:58
4 2008-10-31 17:16:02
5 2008-11-1 8:02:09
6 2008-11-2 8:11:52

 

=========================================================

     SELECT V.STAMP,TO_CHAR(V.GPSTIME,'YYYY-MM-DD HH24:MI:SS'),V.PHOTOURL,V.SMALLPHOTOURL,V.TOUCH_OFF,V.VIDICON_NO,V.SPEED,V.COURSE,V.STATE
       FROM T_VEHICLE_PHOTO V
      WHERE V.ISMANAGE = 0
        AND V.TOUCH_OFF = 27
        AND V.NUMBER_PLATE = '苏F14899'
        AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS')
        AND ROWNUM <= 6
      ORDER BY V.STAMP DESC
取前6行然后排序的数据：

1 2008-11-2 8:11:52
2 2008-11-1 8:02:09
3 2008-10-31 17:16:02
4 2008-10-31 13:41:58
5 2008-10-31 10:53:00
6 2008-10-31 7:24:57

 

总结：

取排序后的前几行，应该用：

select * from(select * from test order by stamp desc) where rownum<= 6  (表示排序后取前几行)

 

而不应该使用：

select * from test where rownum<= 6  order by stamp desc (表示取前几行后再排序)