sum(x) over( partition by y ORDER BY z ) 分析

之前用過row_number()，rank()等排序與over( partition by ... ORDER BY ...)，這兩個比較好理解: 先分組，然後在組內排名。

今天突然碰到sum(...) over( partition by ... ORDER BY ... )，居然搞不清除怎麼執行的，所以查了些資料，做了下實操。

1. 從最簡單的開始

　　sum(...) over( )，對所有行求和

　　sum(...) over( order by ... )，和 =  第一行 到 與當前行同序號行的最後一行的所有值求和，文字不太好理解，請看下圖的算法解析。

with aa as
( 
SELECT 1 a,1 b, 3 c FROM dual union
SELECT 2 a,2 b, 3 c FROM dual union
SELECT 3 a,3 b, 3 c FROM dual union
SELECT 4 a,4 b, 3 c FROM dual union
SELECT 5 a,5 b, 3 c FROM dual union
SELECT 6 a,5 b, 3 c FROM dual union
SELECT 7 a,2 b, 3 c FROM dual union
SELECT 8 a,2 b, 8 c FROM dual union
SELECT 9 a,3 b, 3 c FROM dual
)
SELECT a,b,c,
sum(c) over(order by b) sum1,--有排序，求和當前行所在順序號的C列所有值
sum(c) over() sum2--無排序，求和 C列所有值

2. 與 partition by 結合

　　sum(...) over( partition by... )，同組內所行求和

　　sum(...) over( partition by... order by ... )，同第1點中的排序求和原理，只是範圍限制在組內

with aa as
( 
SELECT 1 a,1 b, 3 c FROM dual union
SELECT 2 a,2 b, 3 c FROM dual union
SELECT 3 a,3 b, 3 c FROM dual union
SELECT 4 a,4 b, 3 c FROM dual union
SELECT 5 a,5 b, 3 c FROM dual union
SELECT 6 a,5 b, 3 c FROM dual union
SELECT 7 a,2 b, 3 c FROM dual union
SELECT 7 a,2 b, 8 c FROM dual union
SELECT 9 a,3 b, 3 c FROM dual
)
SELECT a,b,c,sum(c) over( partition by b ) partition_sum,
sum(c) over( partition by b order by a desc) partition_order_sum
  FROM aa;