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MongoDB query to count the size of an array distinctly?
Use DISTINCT for distinct elements and then length to get the size of array −
db.yourCollectionName.distinct('yourFieldName').length;Let us first create a collection with documents −
> db.countOrSizeDemo.insertOne({"StudentFirstName":"John"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd304f5b64f4b851c3a13dc")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd304fab64f4b851c3a13dd")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"David"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd304fcb64f4b851c3a13de")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"Carol"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd30500b64f4b851c3a13df")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"Sam"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd30505b64f4b851c3a13e0")
}
> db.countOrSizeDemo.insertOne({"StudentFirstName":"John"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd3050ab64f4b851c3a13e1")
}Following is the query to display all documents from a collection with the help of find() method −
> db.countOrSizeDemo.find();
This will produce the following output −
{ "_id" : ObjectId("5cd304f5b64f4b851c3a13dc"), "StudentFirstName" : "John" }
{ "_id" : ObjectId("5cd304fab64f4b851c3a13dd"), "StudentFirstName" : "David" }
{ "_id" : ObjectId("5cd304fcb64f4b851c3a13de"), "StudentFirstName" : "David" }
{ "_id" : ObjectId("5cd30500b64f4b851c3a13df"), "StudentFirstName" : "Carol" }
{ "_id" : ObjectId("5cd30505b64f4b851c3a13e0"), "StudentFirstName" : "Sam" }
{ "_id" : ObjectId("5cd3050ab64f4b851c3a13e1"), "StudentFirstName" : "John" }Following is the query to get the size of a query array −
> db.countOrSizeDemo.distinct('StudentFirstName').length;This will produce the following output −
4
Updated on: 30-Jul-2019
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