Aggregation operations group values from multiple documents together, and can perform a variety of operations on the grouped data to return a single result.
To aggregate in MongoDB, use aggregate(). Let us create a collection with documents −
> db.demo620.insertOne({"Country":"IND","City":"Delhi",state:"Delhi"}); { "acknowledged" : true, "insertedId" : ObjectId("5e9a8de96c954c74be91e6a1") } > db.demo620.insertOne({"Country":"IND","City":"Bangalore",state:"Karnataka"}); { "acknowledged" : true, "insertedId" : ObjectId("5e9a8e336c954c74be91e6a3") } > db.demo620.insertOne({"Country":"IND","City":"Mumbai",state:"Maharashtra"}); { "acknowledged" : true, "insertedId" : ObjectId("5e9a8e636c954c74be91e6a4") }
Display all documents from a collection with the help of find() method −
> db.demo620.find();
This will produce the following output −
{ "_id" : ObjectId("5e9a8de96c954c74be91e6a1"), "Country" : "IND", "City" : "Delhi", "state" : "Delhi" } { "_id" : ObjectId("5e9a8e336c954c74be91e6a3"), "Country" : "IND", "City" : "Bangalore", "state" : "Karnataka" } { "_id" : ObjectId("5e9a8e636c954c74be91e6a4"), "Country" : "IND", "City" : "Mumbai", "state" : "Maharashtra" }
Following is the query to aggregate by country, state and city −
> db.demo620.aggregate([ ... { "$group": { ... "_id": { ... "Country": "$Country", ... "state": "$state" ... }, ... "City": { ... "$addToSet": { ... "City": "$City" ... } ... } ... }}, ... { "$group": { ... "_id": "$_id.Country", ... "states": { ... "$addToSet": { ... "state": "$_id.state", ... "City": "$City" ... } ... } ... }} ... ]).pretty();
This will produce the following output −
{ "_id" : "IND", "states" : [ { "state" : "Delhi", "City" : [ { "City" : "Delhi" } ] }, { "state" : "Maharashtra", "City" : [ { "City" : "Mumbai" } ] }, { "state" : "Karnataka", "City" : [ { "City" : "Bangalore" } ] } ] }