MongoDB查找并返回所有最大值

问题描述

我正在尝试获取具有最大值的文档列表.如果我知道具有最大值的文档数，我可以指定它(就像在另一个 stackoverflow 解决方案mongodb 如何从集合中获取最大值"中的解决方案中一样)，但如果我不知道该怎么做，我可以指定它不知道文档的数量是多少.

例如使用以下文件:

{name:"a", age:10}{姓名:b"，年龄:11}{姓名:d"，年龄:12}{名称:c"，年龄:12}

所以我知道有 2 个文档的最大年龄为 12.因此我可以编写以下查询

db.collection.find().sort({age: -1).limit(2)

我使用 limit(2) 是因为我知道有 2 个文档具有最大值，但是我该如何自动化呢?我可以计算具有最大值的记录，将其存储在一个变量中并像 limit(n) 一样使用它吗?还有其他方法吗?

解决方案

可以使用聚合框架获取结果

var group = {$group:{_id:"$age", names:{$push:"$name"}, count:{$sum:1}}}var sort = {$sort:{"_id":-1}}var limit= {$limit:1}db.selrac.aggregate([组、排序、限制])

输出如下所示:

{_id":12.0，姓名":["d",C"],计数":2.0}

或者如果需要有完整的文档参考替换组:

var group = {$group:{_id:"$age", fullDocument:{$push:"$$ROOT"}, count:{$sum:1}}}

输出:

{_id":12.0，完整文档":[{"_id" : ObjectId("57509a890d4ae20f6de06657"),名称":d"，年龄":12.0},{"_id" : ObjectId("57509a890d4ae20f6de06658"),"名称" : "c",年龄":12.0}],计数":2.0}

欢迎任何评论！

I'm trying to get a list of documents that have the max value. I can specify it if I know the number of documents with max value (like in the solution in this another stackoverflow solution 'mongodb how to get max value from collections'), but I don't know how to do it if I don't know what the number of documents is.

For example, using the following documents:

{name:"a", age:10}
{name:"b", age:11}
{name:"d", age:12}
{name:"c", age:12}

So I know that there are 2 documents with max age of 12. Therefore I can write the following query

db.collection.find().sort({age: -1).limit(2)

I use limit(2) because I know that there are 2 documents with a max value, but how can I automate that? Can I count the records with max value, store it in a variable and use it like limit(n)? Is there any other way to do it?

解决方案

you can use aggregation framework to get results

var group = {$group:{_id:"$age", names:{$push:"$name"}, count:{$sum:1}}}
var sort = {$sort:{"_id":-1}}
var limit= {$limit:1}
db.selrac.aggregate([group, sort, limit])

and output looks like this:

{
    "_id" : 12.0,
    "names" : [
        "d",
        "c"
    ],
    "count" : 2.0
}

or if there is a need to have full document reference replace group by this:

var group = {$group:{_id:"$age", fullDocument:{$push:"$$ROOT"}, count:{$sum:1}}}

{
    "_id" : 12.0,
    "fullDocument" : [
        {
            "_id" : ObjectId("57509a890d4ae20f6de06657"),
            "name" : "d",
            "age" : 12.0
        },
        {
            "_id" : ObjectId("57509a890d4ae20f6de06658"),
            "name" : "c",
            "age" : 12.0
        }
    ],
    "count" : 2.0
}

Any comments welcome!

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