计算MongoDB聚合框架中的中位数 [英] Calculate the median in MongoDB aggregation framework

问题描述

是否有一种使用MongoDB聚合框架计算中位数的方法?

Is there a way to calculate the median using the MongoDB aggregation framework?

推荐答案

在一般情况下，中位数在计算时有些棘手，因为它涉及对整个数据集进行排序，或者使用深度也与数据集大小.这也许就是为什么许多数据库没有开箱即用的中间值运算符(MySQL也没有)的原因.

The median is somewhat tricky to compute in the general case, because it involves sorting the whole data set, or using a recursion with a depth that is also proportional to the data set size. That's maybe the reason why many databases don't have a median operator out of the box (MySQL doesn't have one, either).

计算中位数的最简单方法是使用以下两个语句(假设我们要在其上计算中位数的属性称为a，并且希望在集合中的所有文档coll上使用它):

The simplest way to compute the median would be with these two statements (assuming the attribute on which we want to compute the median is called a and we want it over all documents in the collection, coll):

count = db.coll.count();
db.coll.find().sort( {"a":1} ).skip(count / 2 - 1).limit(1);

这等同于人们建议使用MySQL .

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