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Oracle的分析函数over()

他说Python 2022-03-11 阅读 88
oraclesql

问题:各市随即抽样5000条数据

聚合函数用group by分组,每个分组返回一个统计值。但不能对每行进行分析,得不到问题的结果。而分析函数采用partition by分组,并且每组每行都可以返回一个统计值。辅助row_number()函数和生成随机数函数,取rownum<=5000的数据。

select *
from (select ROW_NUMBER() over(partition by 市代码 order by RAND()) row_num 
			 ,t1.*
       fromas t1  ) t2
where row_num <= 5000

分析函数带有一个开窗函数over(),包含三个分析子句:
分组(partition by)
排序(order by)
窗口(rows)

前两个较为常见,第三个表示分析函数的作用范围,默认范围是组内所有记录。
相当于

SUM(SAL) OVER(PARTITION BY E.DEPTNO 
              ORDER BY E.SAL
              ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING)

其它表示范围的写法还有:
第一行至当前行:

ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW

当前行至最后一行:

ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING

当前行的上一行(rownum-1)到当前行:

ROWS BETWEEN 1 PRECEDING AND CURRENT ROW

当前行的上一行(rownum-1)到当前行的下辆行(rownum+2):

ROWS BETWEEN 1 PRECEDING AND 2 FOLLOWING

———————————————————————————————————————
rows 和 range的区别:
以计算工资总和为例

rows

sum(salary)over(partition by dept order by user_id rows between current row and 2 following ) as sum_salary

sum_salary值则为当前行到下面两行的salary累加值

range

sum(salary)over(partition by dept order by user_id range between current row and 500 following ) as sum_salary

sum_salary值则为当前行到之后比当前行salary值最大大500以内的累加值

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