關於Oracle中排序後取前幾行的問題
查詢語句一、
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 = '蘇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
查詢語句二、
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 = '蘇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
查詢語句一的結果:
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
查詢語句二的結果：
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
不加行數限制的結果：
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<=你拿到的資料會少一行，並且很奇怪是第一行，正常要想法應是最後一行才對，那麼現在在比較以下兩種寫法:
select 標題,釋出時間 from (select 標題,釋出時間 from hhsys.tongzhigonggao order by 釋出時間 desc) where rownum<11
這樣是拿排序後的前10
select 標題,釋出時間 from hhsys.tongzhigonggao where rownum<=10order by 釋出時間 desc
這樣的話應是拿到前10行再進行排序，但他實際上又是拿到排序後的前10行，真搞不明白。
噢，弄明白，了上面的語句寫法是錯誤的。
正確的語句如下：
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 = '蘇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，如果你要取排序後的結果，不能將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 = '蘇F14899'
AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS')
正常資料：
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 = '蘇F14899'
AND V.STAMP < TO_DATE('2008-11-02 10:00:00', 'YYYY-MM-DD HH24:MI:SS')
AND ROWNUM <= 6
取前6行資料：
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 = '蘇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行然後排序的資料：
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
總結：
取排序後的前幾行，應該用：
select * from(select * from test order by stamp desc) where rownum<= 6 (表示排序後取前幾行)
而不應該使用：
select * from test where rownum<= 6 order by stamp desc (表示取前幾行後再排序)