MongoDB: db.collection.group() method

Last update on August 19 2022 21:51:25 (UTC/GMT +8 hours)

db.collection.group

The db.collection.group() method is used to Groups documents in a collection by the specified keys and performs simple aggregation functions such as computing counts and sums.

Syntax:

db.collection.group({ key, reduce, initial [, keyf] [, cond] [, finalize] })

Parameters:

The db.collection.group() accepts a single document that contains the following:

Name	Description	Required / Optional	Type
key	The field or fields to the group. Returns a key object for use as the grouping key.	Required	document
reduce	An aggregation function that operates on the documents during the grouping operation. These functions may return a sum or a count. The function takes two arguments: the current document and an aggregation result document for that group.	Required	function
initial	Initializes the aggregation result document.	Required	initial
keyf	Alternative to the key field. Specifies a function that creates a key object for use as the grouping key. Use keyf instead of key to group by calculated fields rather than existing document fields.	Optional	function
cond	The selection criteria to determine which documents in the collection to process. If you omit the cond field, db.collection.group() processes all the documents in the collection for the group operation.	Required	document
finalize	A function that runs each item in the result set before db.collection.group() returns the final value. This function can either modify the result document or replace the result document as a whole.	Optional	function

Assume that the sample collection is empdetails:


{
        "_id" : ObjectId("56750affdaac17575df77f3e"),
        "emp_code" : "E005",
        "emp_name" : "Alan Hogg",
        "date_of_join" : "15/09/2013",
        "salary" : 19000,
        "gross" : {
                "basic" : 12000,
                "da" : 5000,
                "hra" : 2000
        },
        "deduction" : {
                "pf" : 2000,
                "pt" : 300,
                "it" : 200
        }
}
{
        "_id" : ObjectId("567532f0f61afaaed2aae48c"),
        "emp_code" : "E006",
        "emp_name" : "Karlos Mint",
        "date_of_join" : "23/05/2010",
        "salary" : 17000,
        "gross" : {
                "basic" : 11000,
                "da" : 4500,
                "hra" : 1500
        },
        "deduction" : {
                "pf" : 3000,
                "pt" : 300,
                "it" : 400
        }
}
{
        "_id" : ObjectId("567532fbf61afaaed2aae48d"),
        "emp_code" : "E007",
        "emp_name" : "Burg Hence",
        "date_of_join" : "27/08/2011",
        "salary" : 20000,
        "gross" : {
                "basic" : 14000,
                "da" : 5000,
                "hra" : 1000
        },
        "deduction" : {
                "pf" : 2500,
                "pt" : 300,
                "it" : 200
        }
}
{
        "_id" : ObjectId("56753307f61afaaed2aae48e"),
        "emp_code" : "E004",
        "emp_name" : "Kim Nail",
        "date_of_join" : "16/10/2010",
        "salary" : 17000,
        "gross" : {
                "basic" : 14000,
                "da" : 3000,
                "hra" : 0
        },
        "deduction" : {
                "pf" : 2000,
                "pt" : 300,
                "it" : 200
        }
}

Example: Group by Two Fields

The following example groups by the date_of_join and gross.basic fields those documents that have date_of_join greater than 01/01/2009.


db.empdetails.group(
   {
     key: { "date_of_join": 1, "gross.basic": 1 },
     cond: { "date_of_join": { $gt: "01/01/2009" } },
     reduce: function ( curr, result ) { },
     initial: { }
   }
);

Output

[
        {
                "date_of_join" : "15/09/2013",
                "gross.basic" : 12000
        },
        {
                "date_of_join" : "23/05/2010",
                "gross.basic" : 11000
        },
        {
                "date_of_join" : "27/08/2011",
                "gross.basic" : 14000
        },
        {
                "date_of_join" : "16/10/2010",
                "gross.basic" : 14000
        }
]

Example: Group by Two Fields

The following example groups by the date_of_join and gross.basic fields, those documents that have date_of_join greater than 01/01/2009 and calculates the sum of the basic field for each grouping:

db.empdetails.group(
   {
     key: { "date_of_join": 1, "gross.basic": 1 },
     cond: { "date_of_join": { $gt: "01/01/2009" } },
     reduce: function ( curr, result ) {
                 result.total += curr.gross.basic;
             },
     initial: { total : 0 }
   }
);

Output:

[
        {
                "date_of_join" : "15/09/2013",
                "gross.basic" : 12000,
                "total" : 12000
        },
        {
                "date_of_join" : "23/05/2010",
                "gross.basic" : 11000,
                "total" : 11000
        },
        {
                "date_of_join" : "27/08/2011",
                "gross.basic" : 14000,
                "total" : 14000
        },
        {
                "date_of_join" : "16/10/2010",
                "gross.basic" : 14000,
                "total" : 14000
        }
]

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