mongoDB数据分组聚合问题

Question

MongoDB有类似这么一串数据

{
    id: '123',
    count: 10
},
{
    id: '124',
    count: 1
},
{
    id: '125',
    count: 2
},
{
    id: '126',
    count: 5
},
{
    id: '127',
    count: 8
}

怎么能够一次性聚合出count≤3的数量，3<count≤5的数量，5<count≤8的数量，8<count的数量?

Answer 1

$facet可以满足你的需求。注意boundaries是包含低边界，不包含高边界的，跟你的要求不太一样，所以我做了下调整:

db.bar.aggregate([{
    $facet: {
        countByCount: [{
            $bucket: {
                groupBy: "$count",
                boundaries: [0, 4, 6, 9],
                default: "Other",
                output: {
                    count: {$sum: 1}
                }
            }
        }]
    }
}])

结果：

{ "countByCount" : [ { "_id" : 0, "count" : 2 }, { "_id" : 4, "count" : 1 }, { "_id" : 6, "count" : 1 }, { "_id" : "Other", "count" : 1 } ] }