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$group (aggregation)

Definition

$group

Groups documents by some specified expression and outputs to the next stage a document for each distinct grouping. The output documents contain an _id field which contains the distinct group by key. The output documents can also contain computed fields that hold the values of some accumulator expression grouped by the $group’s _id field. $group does not order its output documents.

The $group stage has the following prototype form:

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{ $group: { _id: <expression>, <field1>: { <accumulator1> : <expression1> }, ... } }

The _id field is mandatory; however, you can specify an _id value of null, or any other constant value, to calculate accumulated values for all the input documents as a whole.

The remaining computed fields are optional and computed using the <accumulator> operators.

The _id and the <accumulator> expressions can accept any valid expression. For more information on expressions, see Expressions.

Considerations

Accumulator Operator

The <accumulator> operator must be one of the following accumulator operators:

Name	Description
$sum	Returns a sum of numerical values. Ignores non-numeric values. Changed in version 3.2: Available in both $group and $project stages.
$avg	Returns an average of numerical values. Ignores non-numeric values. Changed in version 3.2: Available in both $group and $project stages.
$first	Returns a value from the first document for each group. Order is only defined if the documents are in a defined order. Available in $group stage only.
$last	Returns a value from the last document for each group. Order is only defined if the documents are in a defined order. Available in $group stage only.
$max	Returns the highest expression value for each group. Changed in version 3.2: Available in both $group and $project stages.
$min	Returns the lowest expression value for each group. Changed in version 3.2: Available in both $group and $project stages.
$push	Returns an array of expression values for each group. Available in $group stage only.
$addToSet	Returns an array of unique expression values for each group. Order of the array elements is undefined. Available in $group stage only.
$stdDevPop	Returns the population standard deviation of the input values. Changed in version 3.2: Available in both $group and $project stages.
$stdDevSamp	Returns the sample standard deviation of the input values. Changed in version 3.2: Available in both $group and $project stages.

$group Operator and Memory

The $group stage has a limit of 100 megabytes of RAM. By default, if the stage exceeds this limit, $group will produce an error. However, to allow for the handling of large datasets, set the allowDiskUse option to true to enable $group operations to write to temporary files. See db.collection.aggregate() method and the aggregate command for details.

Changed in version 2.6: MongoDB introduces a limit of 100 megabytes of RAM for the $group stage as well as the allowDiskUse option to handle operations for large datasets.

Examples

Calculate Count, Sum, and Average

Given a collection sales with the following documents:

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{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-03-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-03-01T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-03-15T09:00:00Z") }
{ "_id" : 4, "item" : "xyz", "price" : 5, "quantity" : 20, "date" : ISODate("2014-04-04T11:21:39.736Z") }
{ "_id" : 5, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-04-04T21:23:13.331Z") }

Group by Month, Day, and Year

The following aggregation operation uses the $group stage to group the documents by the month, day, and year and calculates the total price and the average quantity as well as counts the documents per each group:

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db.sales.aggregate(
   [
      {
        $group : {
           _id : { month: { $month: "$date" }, day: { $dayOfMonth: "$date" }, year: { $year: "$date" } },
           totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } },
           averageQuantity: { $avg: "$quantity" },
           count: { $sum: 1 }
        }
      }
   ]
)

The operation returns the following results:

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{ "_id" : { "month" : 3, "day" : 15, "year" : 2014 }, "totalPrice" : 50, "averageQuantity" : 10, "count" : 1 }
{ "_id" : { "month" : 4, "day" : 4, "year" : 2014 }, "totalPrice" : 200, "averageQuantity" : 15, "count" : 2 }
{ "_id" : { "month" : 3, "day" : 1, "year" : 2014 }, "totalPrice" : 40, "averageQuantity" : 1.5, "count" : 2 }

Group by null

The following aggregation operation specifies a group _id of null, calculating the total price and the average quantity as well as counts for all documents in the collection:

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db.sales.aggregate(
   [
      {
        $group : {
           _id : null,
           totalPrice: { $sum: { $multiply: [ "$price", "$quantity" ] } },
           averageQuantity: { $avg: "$quantity" },
           count: { $sum: 1 }
        }
      }
   ]
)

The operation returns the following result:

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{ "_id" : null, "totalPrice" : 290, "averageQuantity" : 8.6, "count" : 5 }

Retrieve Distinct Values

Given a collection sales with the following documents:

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{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-03-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-03-01T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-03-15T09:00:00Z") }
{ "_id" : 4, "item" : "xyz", "price" : 5, "quantity" : 20, "date" : ISODate("2014-04-04T11:21:39.736Z") }
{ "_id" : 5, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-04-04T21:23:13.331Z") }

The following aggregation operation uses the $group stage to group the documents by the item to retrieve the distinct item values:

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db.sales.aggregate( [ { $group : { _id : "$item" } } ] )

The operation returns the following result:

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{ "_id" : "xyz" }
{ "_id" : "jkl" }
{ "_id" : "abc" }

Pivot Data

A collection books contains the following documents:

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{ "_id" : 8751, "title" : "The Banquet", "author" : "Dante", "copies" : 2 }
{ "_id" : 8752, "title" : "Divine Comedy", "author" : "Dante", "copies" : 1 }
{ "_id" : 8645, "title" : "Eclogues", "author" : "Dante", "copies" : 2 }
{ "_id" : 7000, "title" : "The Odyssey", "author" : "Homer", "copies" : 10 }
{ "_id" : 7020, "title" : "Iliad", "author" : "Homer", "copies" : 10 }

Group title by author

The following aggregation operation pivots the data in the books collection to have titles grouped by authors.

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db.books.aggregate(
   [
     { $group : { _id : "$author", books: { $push: "$title" } } }
   ]
)

The operation returns the following documents:

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{ "_id" : "Homer", "books" : [ "The Odyssey", "Iliad" ] }
{ "_id" : "Dante", "books" : [ "The Banquet", "Divine Comedy", "Eclogues" ] }

Group Documents by author

The following aggregation operation uses the $$ROOT system variable to group the documents by authors. The resulting documents must not exceed the BSON Document Size limit.

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db.books.aggregate(
   [
     { $group : { _id : "$author", books: { $push: "$$ROOT" } } }
   ]
)

The operation returns the following documents:

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{
  "_id" : "Homer",
  "books" :
     [
       { "_id" : 7000, "title" : "The Odyssey", "author" : "Homer", "copies" : 10 },
       { "_id" : 7020, "title" : "Iliad", "author" : "Homer", "copies" : 10 }
     ]
}

{
  "_id" : "Dante",
  "books" :
     [
       { "_id" : 8751, "title" : "The Banquet", "author" : "Dante", "copies" : 2 },
       { "_id" : 8752, "title" : "Divine Comedy", "author" : "Dante", "copies" : 1 },
       { "_id" : 8645, "title" : "Eclogues", "author" : "Dante", "copies" : 2 }
     ]
}

See also

The Aggregation with the Zip Code Data Set tutorial provides an extensive example of the $group operator in a common use case.

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