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Check for duplicates in an array in MongoDB?
To check for duplicates in an array, use aggregate() in MongoDB. Let us create a collection with documents −
> db.demo756.insertOne({"SubjectName":["MySQL","MongoDB","Java"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5eb01e0d5637cd592b2a4add")
}
> db.demo756.insertOne({"SubjectName":["MongoDB","MySQL","MongoDB","C","C+","MySQL"]});
{
"acknowledged" : true,
"insertedId" : ObjectId("5eb01e2b5637cd592b2a4ade")
}Display all documents from a collection with the help of find() method −
> db.demo756.find();
This will produce the following output −
{ "_id" : ObjectId("5eb01e0d5637cd592b2a4add"), "SubjectName" : [ "MySQL", "MongoDB", "Java" ] }
{ "_id" : ObjectId("5eb01e2b5637cd592b2a4ade"), "SubjectName" : [ "MongoDB", "MySQL", "MongoDB", "C", "C+", "MySQL" ] }Following is the query to check for duplicates in an array −
> db.demo756.aggregate([
... {"$project": {"SubjectName":1}},
... {"$unwind":"$SubjectName"},
... {"$group": {"_id":{"_id":"$_id", "Name":"$SubjectName"}, "count":{"$sum":1}}},
... {"$match": {"count":{"$gt":1}}},
... {"$group": {"_id": "$_id._id", "SubjectName":{"$addToSet":"$_id.Name"}}}
... ])This will produce the following output −
{ "_id" : ObjectId("5eb01e2b5637cd592b2a4ade"), "SubjectName" : [ "MongoDB", "MySQL" ] }
Updated on: 01-Jul-2020
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