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$count (aggregation accumulator operator)

Definition

New in version 5.0.

$count

Returns the number of documents in a group.

$count is available in these stages:

  • $bucket

  • $bucketAuto

  • $group

  • $setWindowFields (Available starting in MongoDB 5.0)

Note

Disambiguation

This page describes the $count aggregation accumulator. For the $count aggregation pipeline stage, see $count (aggregation pipeline).

Syntax

$count syntax:

{ $count: { } }

$count does not accept any parameters.

Behavior

$count is functionally equivalent to using { $sum : 1 } within the $group stage.

Tip

$sum

Examples

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

The cakeSales collection is used in the following examples.

Use in $group Stage

This example uses $count in the $group stage to count the number of documents in the cakeSales collection for each state:

db.cakeSales.aggregate( [
   {
      $group: {
         _id: "$state",
         countNumberOfDocumentsForState: {
            $count: {}
         }
      }
   }
] )

In the example:

  • _id: "$state" groups the documents by the state field value. There are groups for CA and WA.

  • $count: {} sets the countNumberOfDocumentsForState field to the number of documents that share the same state field value.

In this output, the number of documents for CA and WA is shown in the countNumberOfDocumentsForState field:

{ "_id" : "CA", "countNumberOfDocumentsForState" : 3 }
{ "_id" : "WA", "countNumberOfDocumentsForState" : 3 }

Use in $setWindowFields Stage

This example uses $count in the $setWindowFields stage to count the number of documents in the cakeSales collection for each state defined in the window:

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            countNumberOfDocumentsForState: {
               $count: {},
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:

  • partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.

  • sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.

  • output sets the countNumberOfDocumentsForState field to the number of documents using $count that is run in a documents window.

    The window contains documents between an unbounded lower limit and the current document in the output. This means $count returns the number of documents between the beginning of the partition and the current document.

In this output, the number of documents for CA and WA is shown in the countNumberOfDocumentsForState field:

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "countNumberOfDocumentsForState" : 1 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "countNumberOfDocumentsForState" : 2 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "countNumberOfDocumentsForState" : 3 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "countNumberOfDocumentsForState" : 1 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "countNumberOfDocumentsForState" : 2 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "countNumberOfDocumentsForState" : 3 }

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