问题描述
我正在尝试为表中行的子集的字段计算聚合函数.问题是我想一次找到k个行的每个组合的均值-因此，对于所有行，我想找到(说)10行的每个组合的均值.所以:
I am trying to calculate an aggregate function for a field for a subset of rows in a table. The problem is that I'd like to find the mean of every combination of rows taken k at a time --- so for all the rows, I'd like to find (say) the mean of every combination of 10 rows. So:
id | count
----|------
1 | 5
2 | 3
3 | 6
...
30 | 16
应该给我
ids 1..10的平均值；ID 1，3..11;ID 1、4..12，以此类推.我知道这会产生很多行.
mean of ids 1..10; ids 1, 3..11; ids 1, 4..12, and so so. I know this will yield a lot of rows.
对于从数组中找到组合，有一些答案.我可以通过编程来做到这一点，一次获取30个ID 10，然后 SELECT 对其进行编码.是否可以使用 PARTITION BY ， TABLESAMPLE 或其他功能(例如python的 itertools.combinations())?(据我所知， TABLESAMPLE 本身不能保证我选择的是哪行的子集.)
There are SO answers for finding combinations from arrays. I could do this programmatically by taking 30 ids 10 at a time and then SELECTing them. Is there a way to do this with PARTITION BY, TABLESAMPLE, or another function (something like python's itertools.combinations())? (TABLESAMPLE by itself won't guarantee which subset of rows I am selecting as far as I can tell.)
推荐答案
引用的答案中描述的方法是静态的.一个更方便的解决方案可能是使用递归.
The method described in the cited answer is static. A more convenient solution may be to use recursion.
示例数据:
drop table if exists my_table;
create table my_table(id int primary key, number int);
insert into my_table values
(1, 5),
(2, 3),
(3, 6),
(4, 9),
(5, 2);
查询可在5个元素集中找到2个元素子集(k组合，k = 2):
Query which finds 2 element subsets in 5 element set (k-combination with k = 2):
with recursive recur as (
select
id,
array[id] as combination,
array[number] as numbers,
number as sum
from my_table
union all
select
t.id,
combination || t.id,
numbers || t.number,
sum+ number
from my_table t
join recur r on r.id < t.id
and cardinality(combination) < 2 -- param k
)
select combination, numbers, sum/2.0 as average -- param k
from recur
where cardinality(combination) = 2 -- param k
combination | numbers | average
-------------+---------+--------------------
{1,2} | {5,3} | 4.0000000000000000
{1,3} | {5,6} | 5.5000000000000000
{1,4} | {5,9} | 7.0000000000000000
{1,5} | {5,2} | 3.5000000000000000
{2,3} | {3,6} | 4.5000000000000000
{2,4} | {3,9} | 6.0000000000000000
{2,5} | {3,2} | 2.5000000000000000
{3,4} | {6,9} | 7.5000000000000000
{3,5} | {6,2} | 4.0000000000000000
{4,5} | {9,2} | 5.5000000000000000
(10 rows)
对于k = 3的相同查询给出:
The same query for k = 3 gives:
combination | numbers | average
-------------+---------+--------------------
{1,2,3} | {5,3,6} | 4.6666666666666667
{1,2,4} | {5,3,9} | 5.6666666666666667
{1,2,5} | {5,3,2} | 3.3333333333333333
{1,3,4} | {5,6,9} | 6.6666666666666667
{1,3,5} | {5,6,2} | 4.3333333333333333
{1,4,5} | {5,9,2} | 5.3333333333333333
{2,3,4} | {3,6,9} | 6.0000000000000000
{2,3,5} | {3,6,2} | 3.6666666666666667
{2,4,5} | {3,9,2} | 4.6666666666666667
{3,4,5} | {6,9,2} | 5.6666666666666667
(10 rows)
当然，如果不需要，您可以从查询中删除数字.
Of course, you can remove numbers from the query if you do not need them.
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