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MongoDB query for counting number of unique fields grouped by another field?
For this, use aggregate() and group with $group. Let us create a collection with documents −
> db.demo354.insertOne({"Name1":"Chris","Name2":"David"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685a6f8647eb59e5620c0") } > db.demo354.insertOne({"Name1":"Chris","Name2":"David"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685a9f8647eb59e5620c1") } > db.demo354.insertOne({"Name1":"Chris","Name2":"Bob"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685aff8647eb59e5620c2") } > db.demo354.insertOne({"Name1":"Carol","Name2":"John"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685c4f8647eb59e5620c3") } > db.demo354.insertOne({"Name1":"Carol","Name2":"John"}); { "acknowledged" : true, "insertedId" : ObjectId("5e5685c5f8647eb59e5620c4") }
Display all documents from a collection with the help of find() method −
> db.demo354.find();
This will produce the following output −
{ "_id" : ObjectId("5e5685a6f8647eb59e5620c0"), "Name1" : "Chris", "Name2" : "David" } { "_id" : ObjectId("5e5685a9f8647eb59e5620c1"), "Name1" : "Chris", "Name2" : "David" } { "_id" : ObjectId("5e5685aff8647eb59e5620c2"), "Name1" : "Chris", "Name2" : "Bob" } { "_id" : ObjectId("5e5685c4f8647eb59e5620c3"), "Name1" : "Carol", "Name2" : "John" } { "_id" : ObjectId("5e5685c5f8647eb59e5620c4"), "Name1" : "Carol", "Name2" : "John" }
Following is the query for counting number of unique fields grouped by another field −
> db.demo354.aggregate([ ... { ... $group: { ... _id: { ... "Name1": "$Name1", ... "Name2": "$Name2" ... }, ... } ... }, ... { ... $group: { ... _id: "$_id.Name1", ... "Name2": { $sum: 1 } ... } ... }, ... { ... $project: { ... _id: 0, ... Name1: "$_id", ... Name2: 1 ... } ... } ... ])
This will produce the following output −
{ "Name2" : 2, "Name1" : "Chris" } { "Name2" : 1, "Name1" : "Carol" }
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