查找共享在汇总步骤中找到的最大值(值)的所有文档 [英] Find all documents that share max(value) found in aggregate step

问题描述

我正在尝试查找共享 max(num_sold)的所有文档.我不能仅对降序进行排序并执行limit(1)，因为我会错过其他文档也具有相同max(num_sold)的情况.鉴于这个过于简化的数据集:

I'm trying to find all documents that share the max(num_sold). I cant just sort descending and do a limit(1) because I'd miss the case where other docs also have the same max(num_sold). Given this over-simplified dataset:

{"item":"Apple", "num_sold": 49}
{"item":"Orange", "num_sold": 55}
{"item":"Peach", "num_sold": 55}
{"item":"Grape", "num_sold": 20}
{"item":"Banana", "num_sold": 20}

我想回来

{"item":"Orange", "num_sold": 55}
{"item":"Peach", "num_sold": 55}    

使用SQL，这是一个简单的查询，然后联接回主数据集.我对如何在MongoDB中执行此操作感到有些困惑.

Using SQL, this is a simple query and then join back to the main dataset. I'm a bit stumped on how to do this in MongoDB.

我已经找到了最大值，但是如何获得具有该最大值的所有文档呢?

I've found the max, but how do I get all docs that have that max?

db.t.aggregate(
    {$group:{
          _id:null,
          num_sold:{$max:"$num_sold"}
          }
    }
);
{ "_id" : null, "num_sold" : 55 }

推荐答案

您可以通过在num_sold上进行分组，然后使用$sort和$limit管道阶段来仅获取具有最大值的文档来做到这一点:

You can do this by grouping on num_sold and then using $sort and $limit pipeline stages to get just the docs with the maximum value:

db.t.aggregate([
    // Group by num_sold, assembling an array of docs with each distinct value.
    {$group: {
        _id: '$num_sold',
        docs: {$push: '$$ROOT'}
    }},
    // Sort the groups by _id descending to put the max num_sold group first.
    {$sort: {_id: -1}},
    // Return just the first (max num_sold) group of docs.
    {$limit: 1}
])

输出:

{ 
    "_id" : 55.0, 
    "docs" : [
        {
            "_id" : ObjectId("5726a62879ce3350ff8d607e"), 
            "item" : "Orange", 
            "num_sold" : 55.0
        }, 
        {
            "_id" : ObjectId("5726a62879ce3350ff8d607f"), 
            "item" : "Peach", 
            "num_sold" : 55.0
        }
    ]
}

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