first_value（）over（由asc命令）是否与last_value（）over（由desc命令）相同？

Question

不应该first_value（）有序降序给出与last_value（）有序升序相同的结果吗？我运行了以下查询，并在查询下方附加了结果。

select random_date, trunc(random_date,'MM') random_month, random_value,
first_value(random_value) over (partition by trunc(random_date,'MM') order by random_date desc) first_,
last_value(random_value) over (partition by trunc(random_date,'MM') order by random_date) last_
from 
(select to_date(round (dbms_random.value (1, 28))
        || '-'
        || round (dbms_random.value (02, 03))
        || '-'
        || round (dbms_random.value (2014, 2014)),
        'DD-MM-YYYY') + level - 1 random_date,
        round(100*(dbms_random.value)) random_value
from dual
connect by level <= 10) order by 2, 1;

Answer 1

您忘记了用于分析功能的移动窗口。查看差异（无界前后和无界后续之间使用的行）：

SQL> select random_date, trunc(random_date,'MM') random_month, random_value,
  2  first_value(random_value) over (partition by trunc(random_date,'MM') order by random_date desc rows between unbounded preceding and unbounded following) first_,
  3  last_value(random_value) over (partition by trunc(random_date,'MM') order by random_date rows between unbounded preceding and unbounded following) last_
  4  from
  5  (select to_date(round (dbms_random.value (1, 28))
  6          || '-'
  7          || round (dbms_random.value (02, 03))
  8          || '-'
  9          || round (dbms_random.value (2014, 2014)),
 10          'DD-MM-YYYY') + level - 1 random_date,
 11          round(100*(dbms_random.value)) random_value
 12  from dual
 13  connect by level <= 10) order by 2, 1;

RANDOM_DATE RANDOM_MONTH RANDOM_VALUE     FIRST_      LAST_
----------- ------------ ------------ ---------- ----------
02.02.2014  01.02.2014             93         75         75
09.02.2014  01.02.2014             78         75         75
11.02.2014  01.02.2014             69         75         75
12.02.2014  01.02.2014             13         75         75
21.02.2014  01.02.2014             91         75         75
25.02.2014  01.02.2014             75         75         75
01.03.2014  01.03.2014             54         80         80
15.03.2014  01.03.2014             37         80         80
16.03.2014  01.03.2014             92         80         80
17.03.2014  01.03.2014             80         80         80

10 rows selected

Answer 2

尝试此分析函数。要实现您的目标，请使用rows between unbounded preceding and unbounded。

first_value(random_value) over (partition by trunc(random_date,'MM') order by random_date desc ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) first_,
last_value(random_value) over (partition by trunc(random_date,'MM') order by random_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) last_

完整代码：

select random_date, trunc(random_date,'MM') random_month, random_value,
first_value(random_value) over (partition by trunc(random_date,'MM') order by random_date desc ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) first_,
last_value(random_value) over (partition by trunc(random_date,'MM') order by random_date ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING) last_

from 
(select to_date(round (dbms_random.value (1, 28))
        || '-'
        || round (dbms_random.value (02, 03))
        || '-'
        || round (dbms_random.value (2014, 2014)),
        'DD-MM-YYYY') + level - 1 random_date,
        round(100*(dbms_random.value)) random_value
from dual
connect by level <= 10) order by 2, 1;

Answer 3

正如我看到你做的first_value这只用于字符串，但是你需要一个数字，所以把它改成min和max 在字符串中，10将在2之前，但在数字2之前是