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Count unique items in array-based fields across all MongoDB documents?
To count unique items in array-based fields, use $group along with aggregate(). Let us create a collection with documents −
> db.demo493.insertOne({"SubjectName":["MySQL","MongoDB","Java"]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e849f97b0f3fa88e22790c4")
}
> db.demo493.insertOne({"SubjectName":["C++","MongoDB","C"]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e849fa4b0f3fa88e22790c5")
}
> db.demo493.insertOne({"SubjectName":["MySQL","MongoDB","C"]});{
"acknowledged" : true,
"insertedId" : ObjectId("5e849fb2b0f3fa88e22790c6")
}Display all documents from a collection with the help of find() method −
> db.demo493.find();
This will produce the following output −
{ "_id" : ObjectId("5e849f97b0f3fa88e22790c4"), "SubjectName" : [ "MySQL", "MongoDB",
"Java" ] }
{ "_id" : ObjectId("5e849fa4b0f3fa88e22790c5"), "SubjectName" : [ "C++", "MongoDB", "C" ] }
{ "_id" : ObjectId("5e849fb2b0f3fa88e22790c6"), "SubjectName" : [ "MySQL", "MongoDB", "C"
] }Following is the query to count unique items in array-based fields across all documents −
> db.demo493.aggregate([
... { $unwind: "$SubjectName" },
... { $group: { _id: "$SubjectName", Frequency: { $sum : 1 } } }
... ]
... );This will produce the following output −
{ "_id" : "C++", "Frequency" : 1 }
{ "_id" : "C", "Frequency" : 2 }
{ "_id" : "Java", "Frequency" : 1 }
{ "_id" : "MySQL", "Frequency" : 2 }
{ "_id" : "MongoDB", "Frequency" : 3 }
Updated on: 13-May-2020
623 Views
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