postgres聚合函数调用可能未嵌套 [英] postgres aggregate function calls may not be nested

问题描述

我有一个查询：

    select sum(
        sum((Impressions / Count) * Volume) / sum(Volume)
    ) as frequency 
    from datatable;

但是我无法在postgres中执行此操作，因为它使用了嵌套聚合。还有另一种不用嵌套聚合写此方法的方法吗？

however I cannot execute this in postgres because it uses nested aggregations. Is there another way to write this without using nested aggregations?

推荐答案

我假设您需要首先计算某些项目组的内部公式，而不是对结果求和。我将 product 列用作分组列的任意选择。我还将 Count 重命名为 dcount 。

I will assume that you need to calculate the inner formula for some groups of items first and than sum results. I am using product column as an arbitrary choice for grouping column. I also renamed Count to dcount.

SQLFiddle

示例数据：

create table sample (
  product varchar,
  dcount int,
  impressions int,
  volume int
);

insert into sample values ('a', 100, 10, 50);
insert into sample values ('a', 100, 20, 40);
insert into sample values ('b', 100, 30, 30);
insert into sample values ('b', 100, 40, 30);
insert into sample values ('c', 100, 50, 10);
insert into sample values ('c', 100, 60, 100);

查询：

select
  sum(frequency) as frequency
from 
  (
  select
    product,
    sum((impressions / dcount::numeric) * volume) / sum(volume) as frequency
  from 
    sample
  group by
    product
  ) x;

重点是您不能嵌套聚合函数。如果需要汇总聚合，则需要使用子查询。

The point is that you cannot nest aggregate functions. If you need to aggregate aggregates then you need to use subquery.

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