Hướng dẫn mongodb count distinct aggregation

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$count

Passes a document to the next stage that contains a count of the number of documents input to the stage.

Note

$count has the following prototype form:

<string> is the name of the output field which has the count as its value. <string> must be a non-empty string, must not start with $ and must not contain the . character.

The $count stage is equivalent to the following $group + $project sequence:

db.collection.aggregate( [
   { $group: { _id: null, myCount: { $sum: 1 } } },
   { $project: { _id: 0 } }
] )

where myCount would be the output field that contains the count. You can specify another name for the output field.

Tip

See also:

A collection named scores has the following documents:

{ "_id" : 1, "subject" : "History", "score" : 88 }
{ "_id" : 2, "subject" : "History", "score" : 92 }
{ "_id" : 3, "subject" : "History", "score" : 97 }
{ "_id" : 4, "subject" : "History", "score" : 71 }
{ "_id" : 5, "subject" : "History", "score" : 79 }
{ "_id" : 6, "subject" : "History", "score" : 83 }

The following aggregation operation has two stages:

1. The $match stage excludes documents that have a score value of less than or equal to 80 to pass along the documents with score greater than 80 to the next stage.

2. The $count stage returns a count of the remaining documents in the aggregation pipeline and assigns the value to a field called passing_scores.

db.scores.aggregate(
  [
    {
      $match: {
        score: {
          $gt: 80
        }
      }
    },
    {
      $count: "passing_scores"
    }
  ]
)

The operation returns the following results:

I try to count distinct values in MongoDB, but I'm not getting the results I expect.

Here is a sample of the data:

{
        "_id" : ObjectId("55d354b0d5cec8aad571b8fe"),
        "version" : "0.4",
        "scan-time" : {
                "start" : 1439913109,
                "end" : 1439913136
        },
        "services" : [
                {
                        "service-type" : "SV1",
                        "service-version" : "V1",
                        "location" : {
                                "ipv4-address" : "192.168.1.1",
                                "protocol" : "tcp"
                        },
                        "classification-method" : "probed"
                },
                {
                        "service-type" : "SV1",
                        "service-version" : "V2",
                        "location" : {
                                "ipv4-address" : "192.168.1.2",
                                "protocol" : "tcp"
                        },
                        "classification-method" : "probed"
                },
                {
                        "location" : {
                                "ipv4-address" : "192.168.1.3",
                                "protocol" : "tcp"
                        },
                        "classification-method" : "probed",
                        "service-type" : "SV1",
                        "service-version" : "V3"
                },
                {
                        "service-type" : "SV2",
                        "service-version" : null,
                        "location" : {
                                "ipv4-address" : "192.168.1.4",
                                "protocol" : "tcp"
                        },
                        "classification-method" : "probed"
                }
        ]
}

I can list all the distinct values using this query:

db.collection.distinct("services.service-type")

However, I also need a count. I tried this, but it doesn't give the results I want:

db.collection.aggregate(
    [
       {
         $group : {
            _id : "$services.service-type",
            count: { $sum: 1 }
         }
       }
    ]
)

I need to see this:

SV1: 3
SV2: 1

MongoDB version:

> db.version()
3.0.5

Thanks for any help!