$group (aggregation)
Definition
$group
The
$group
stage separates documents into groups according to a "group key". The output is one document for each unique group key.A group key is often a field, or group of fields. The group key can also be the result of an expression. Use the
_id
field in the$group
pipeline stage to set the group key. See below for usage examples.In the
$group
stage output, the_id
field is set to the group key for that document.The output documents can also contain additional fields that are set using accumulator expressions.
Note
$group
does not order its output documents.
Compatibility
You can use $group
for deployments hosted in the following
environments:
MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud
MongoDB Enterprise: The subscription-based, self-managed version of MongoDB
MongoDB Community: The source-available, free-to-use, and self-managed version of MongoDB
Syntax
The $group
stage has the following prototype form:
{ $group: { _id: <expression>, // Group key <field1>: { <accumulator1> : <expression1> }, ... } }
Field | Description |
---|---|
_id | Required. The _id expression specifies the group key.
If you specify an _id value of null, or any other
constant value, the $group stage returns a single
document that aggregates values across all of the input
documents. See the Group by Null example. |
field | Optional. Computed using the
accumulator operators. |
The _id
and the accumulator operators
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:
Changed in version 5.0.
Name | Description |
---|---|
Returns the result of a user-defined accumulator function. | |
Returns an array of unique expression values for each group. Order of the array elements is undefined. Changed in version 5.0: Available in the | |
Returns an average of numerical values. Ignores non-numeric values. Changed in version 5.0: Available in the | |
Returns the bottom element within a group according to the specified sort order. New in version 5.2. Available in the | |
Returns an aggregation of the bottom New in version 5.2. Available in the | |
Returns the number of documents in a group. Distinct from the New in version 5.0: Available in the | |
Returns a value from the first document for each group. Order is only defined if the documents are sorted. Distinct from the Changed in version 5.0: Available in the | |
Returns an aggregation of the first New in version 5.2: Available in the | |
Returns a value from the last document for each group. Order is only defined if the documents are sorted. Distinct from the Changed in version 5.0: Available in the | |
Returns an aggregation of the last New in version 5.2: Available in the | |
Returns the highest expression value for each group. Changed in version 5.0: Available in the | |
Returns an aggregation of the New in version 5.2. Available in | |
Returns a document created by combining the input documents
for each group. | |
Returns the lowest expression value for each group. Changed in version 5.0: Available in the | |
Returns an aggregation of the New in version 5.2. Available in | |
Returns an array of expression values for documents in each group. Changed in version 5.0: Available in the | |
Returns the population standard deviation of the input values. Changed in version 5.0: Available in the | |
Returns the sample standard deviation of the input values. Changed in version 5.0: Available in the | |
Returns a sum of numerical values. Ignores non-numeric values. Changed in version 5.0: Available in the | |
Returns the top element within a group according to the specified sort order. New in version 5.2. Available in the | |
Returns an aggregation of the top New in version 5.2. Available in the |
$group
and Memory Restrictions
If the $group
stage exceeds 100 megabytes of RAM, MongoDB writes
data to temporary files. However, if the
allowDiskUse option is set to false
,
$group
returns an error. For more information, refer to
Aggregation Pipeline Limits.
$group
Performance Optimizations
This section describes optimizations to improve the performance of
$group
. There are optimizations that you can make manually
and optimizations MongoDB makes internally.
Optimization to Return the First Document of Each Group
If a pipeline sorts
and groups
by the same field and the $group
stage only uses the
$first
accumulator operator, consider adding an index on the grouped field which matches the sort order. In some
cases, the $group
stage can use the index to quickly find
the first document of each group.
Example
If a collection named foo
contains an index { x: 1, y: 1 }
,
the following pipeline can use that index to find the first document
of each group:
db.foo.aggregate([ { $sort:{ x : 1, y : 1 } }, { $group: { _id: { x : "$x" }, y: { $first : "$y" } } } ])
Slot-Based Query Execution Engine
Starting in version 5.2, MongoDB uses the slot-based execution
query engine to execute $group
stages
if either:
$group
is the first stage in the pipeline.All preceding stages in the pipeline can also be executed by the slot-based engine.
For more information, see $group
Optimization.
Examples
Count the Number of Documents in a Collection
In mongosh
, create a sample collection named
sales
with the following documents:
db.sales.insertMany([ { "_id" : 1, "item" : "abc", "price" : Decimal128("10"), "quantity" : Int32("2"), "date" : ISODate("2014-03-01T08:00:00Z") }, { "_id" : 2, "item" : "jkl", "price" : Decimal128("20"), "quantity" : Int32("1"), "date" : ISODate("2014-03-01T09:00:00Z") }, { "_id" : 3, "item" : "xyz", "price" : Decimal128("5"), "quantity" : Int32( "10"), "date" : ISODate("2014-03-15T09:00:00Z") }, { "_id" : 4, "item" : "xyz", "price" : Decimal128("5"), "quantity" : Int32("20") , "date" : ISODate("2014-04-04T11:21:39.736Z") }, { "_id" : 5, "item" : "abc", "price" : Decimal128("10"), "quantity" : Int32("10") , "date" : ISODate("2014-04-04T21:23:13.331Z") }, { "_id" : 6, "item" : "def", "price" : Decimal128("7.5"), "quantity": Int32("5" ) , "date" : ISODate("2015-06-04T05:08:13Z") }, { "_id" : 7, "item" : "def", "price" : Decimal128("7.5"), "quantity": Int32("10") , "date" : ISODate("2015-09-10T08:43:00Z") }, { "_id" : 8, "item" : "abc", "price" : Decimal128("10"), "quantity" : Int32("5" ) , "date" : ISODate("2016-02-06T20:20:13Z") }, ])
The following aggregation operation uses the $group
stage
to count the number of documents in the sales
collection:
db.sales.aggregate( [ { $group: { _id: null, count: { $count: { } } } } ] )
The operation returns the following result:
{ "_id" : null, "count" : 8 }
This aggregation operation is equivalent to the following SQL statement:
SELECT COUNT(*) AS count FROM sales
Retrieve Distinct Values
The following aggregation operation uses the $group
stage
to retrieve the distinct item values from the sales
collection:
db.sales.aggregate( [ { $group : { _id : "$item" } } ] )
The operation returns the following result:
{ "_id" : "abc" } { "_id" : "jkl" } { "_id" : "def" } { "_id" : "xyz" }
Group by Item Having
The following aggregation operation groups documents by the item
field, calculating the total sale amount per item and returning only
the items with total sale amount greater than or equal to 100:
db.sales.aggregate( [ // First Stage { $group : { _id : "$item", totalSaleAmount: { $sum: { $multiply: [ "$price", "$quantity" ] } } } }, // Second Stage { $match: { "totalSaleAmount": { $gte: 100 } } } ] )
- First Stage:
- The
$group
stage groups the documents byitem
to retrieve the distinct item values. This stage returns thetotalSaleAmount
for each item. - Second Stage:
- The
$match
stage filters the resulting documents to only return items with atotalSaleAmount
greater than or equal to 100.
The operation returns the following result:
{ "_id" : "abc", "totalSaleAmount" : Decimal128("170") } { "_id" : "xyz", "totalSaleAmount" : Decimal128("150") } { "_id" : "def", "totalSaleAmount" : Decimal128("112.5") }
This aggregation operation is equivalent to the following SQL statement:
SELECT item, Sum(( price * quantity )) AS totalSaleAmount FROM sales GROUP BY item HAVING totalSaleAmount >= 100
Calculate Count, Sum, and Average
In mongosh
, create a sample collection named
sales
with the following documents:
db.sales.insertMany([ { "_id" : 1, "item" : "abc", "price" : Decimal128("10"), "quantity" : Int32("2"), "date" : ISODate("2014-03-01T08:00:00Z") }, { "_id" : 2, "item" : "jkl", "price" : Decimal128("20"), "quantity" : Int32("1"), "date" : ISODate("2014-03-01T09:00:00Z") }, { "_id" : 3, "item" : "xyz", "price" : Decimal128("5"), "quantity" : Int32( "10"), "date" : ISODate("2014-03-15T09:00:00Z") }, { "_id" : 4, "item" : "xyz", "price" : Decimal128("5"), "quantity" : Int32("20") , "date" : ISODate("2014-04-04T11:21:39.736Z") }, { "_id" : 5, "item" : "abc", "price" : Decimal128("10"), "quantity" : Int32("10") , "date" : ISODate("2014-04-04T21:23:13.331Z") }, { "_id" : 6, "item" : "def", "price" : Decimal128("7.5"), "quantity": Int32("5" ) , "date" : ISODate("2015-06-04T05:08:13Z") }, { "_id" : 7, "item" : "def", "price" : Decimal128("7.5"), "quantity": Int32("10") , "date" : ISODate("2015-09-10T08:43:00Z") }, { "_id" : 8, "item" : "abc", "price" : Decimal128("10"), "quantity" : Int32("5" ) , "date" : ISODate("2016-02-06T20:20:13Z") }, ])
Group by Day of the Year
The following pipeline calculates the total sales amount, average sales quantity, and sale count for each day in the year 2014:
db.sales.aggregate([ // First Stage { $match : { "date": { $gte: new ISODate("2014-01-01"), $lt: new ISODate("2015-01-01") } } }, // Second Stage { $group : { _id : { $dateToString: { format: "%Y-%m-%d", date: "$date" } }, totalSaleAmount: { $sum: { $multiply: [ "$price", "$quantity" ] } }, averageQuantity: { $avg: "$quantity" }, count: { $sum: 1 } } }, // Third Stage { $sort : { totalSaleAmount: -1 } } ])
- First Stage:
- The
$match
stage filters the documents to only pass documents from the year 2014 to the next stage. - Second Stage:
- The
$group
stage groups the documents by date and calculates the total sale amount, average quantity, and total count of the documents in each group. - Third Stage:
- The
$sort
stage sorts the results by the total sale amount for each group in descending order.
The operation returns the following results:
{ "_id" : "2014-04-04", "totalSaleAmount" : Decimal128("200"), "averageQuantity" : 15, "count" : 2 } { "_id" : "2014-03-15", "totalSaleAmount" : Decimal128("50"), "averageQuantity" : 10, "count" : 1 } { "_id" : "2014-03-01", "totalSaleAmount" : Decimal128("40"), "averageQuantity" : 1.5, "count" : 2 }
This aggregation operation is equivalent to the following SQL statement:
SELECT date, Sum(( price * quantity )) AS totalSaleAmount, Avg(quantity) AS averageQuantity, Count(*) AS Count FROM sales WHERE date >= '01/01/2014' AND date < '01/01/2015' GROUP BY date ORDER BY totalSaleAmount DESC
Tip
See also:
db.collection.countDocuments()
which wraps the$group
aggregation stage with a$sum
expression.
Group by null
The following aggregation operation specifies a group _id
of
null
, calculating the total sale amount, average quantity, and count of
all documents in the collection.
db.sales.aggregate([ { $group : { _id : null, totalSaleAmount: { $sum: { $multiply: [ "$price", "$quantity" ] } }, averageQuantity: { $avg: "$quantity" }, count: { $sum: 1 } } } ])
The operation returns the following result:
{ "_id" : null, "totalSaleAmount" : Decimal128("452.5"), "averageQuantity" : 7.875, "count" : 8 }
This aggregation operation is equivalent to the following SQL statement:
SELECT Sum(price * quantity) AS totalSaleAmount, Avg(quantity) AS averageQuantity, Count(*) AS Count FROM sales
Tip
See also:
db.collection.countDocuments()
which wraps the$group
aggregation stage with a$sum
expression.
Pivot Data
In mongosh
, create a sample collection named
books
with the following documents:
db.books.insertMany([ { "_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.
db.books.aggregate([ { $group : { _id : "$author", books: { $push: "$title" } } } ])
The operation returns the following documents:
{ "_id" : "Homer", "books" : [ "The Odyssey", "Iliad" ] } { "_id" : "Dante", "books" : [ "The Banquet", "Divine Comedy", "Eclogues" ] }
Group Documents by author
The following aggregation operation groups documents by author
:
db.books.aggregate([ // First Stage { $group : { _id : "$author", books: { $push: "$$ROOT" } } }, // Second Stage { $addFields: { totalCopies : { $sum: "$books.copies" } } } ])
- First Stage:
$group
uses the$$ROOT
system variable to group the entire documents by authors. This stage passes the following documents to the next stage:{ "_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 } ] } - Second Stage:
$addFields
adds a field to the output containing the total copies of books for each author.Note
The resulting documents must not exceed the BSON Document Size limit of 16 megabytes.
The operation returns the following documents:
{ "_id" : "Homer", "books" : [ { "_id" : 7000, "title" : "The Odyssey", "author" : "Homer", "copies" : 10 }, { "_id" : 7020, "title" : "Iliad", "author" : "Homer", "copies" : 10 } ], "totalCopies" : 20 } { "_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 } ], "totalCopies" : 5 }
Additional Resources
The Aggregation with the Zip Code Data Set
tutorial provides an extensive example of the $group
operator in a common use case.