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Getting a list of values by using MongoDB $group?
To get a list of values, use $group aggregation along with $push operator. Let us first create a collection with documents −
> db.groupByDemo.insertOne({"UserName":"John","Subject":"MongoDB"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f0457806ebf1256f136")
}
> db.groupByDemo.insertOne({"UserName":"Larry","Subject":"MySQL"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f0657806ebf1256f137")
}
> db.groupByDemo.insertOne({"UserName":"John","Subject":"Java"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f0d57806ebf1256f138")
}
> db.groupByDemo.insertOne({"UserName":"John","Subject":"C"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f1357806ebf1256f139")
}
> db.groupByDemo.insertOne({"UserName":"Larry","Subject":"SQL Server"});
{
"acknowledged" : true,
"insertedId" : ObjectId("5cd69f1c57806ebf1256f13a")
}Following is the query to display all documents from a collection with the help of find() method −
> db.groupByDemo.find().pretty();
This will produce the following output −
{
"_id" : ObjectId("5cd69f0457806ebf1256f136"),
"UserName" : "John",
"Subject" : "MongoDB"
}
{
"_id" : ObjectId("5cd69f0657806ebf1256f137"),
"UserName" : "Larry",
"Subject" : "MySQL"
}
{
"_id" : ObjectId("5cd69f0d57806ebf1256f138"),
"UserName" : "John",
"Subject" : "Java"
}
{
"_id" : ObjectId("5cd69f1357806ebf1256f139"),
"UserName" : "John",
"Subject" : "C"
}
{
"_id" : ObjectId("5cd69f1c57806ebf1256f13a"),
"UserName" : "Larry",
"Subject" : "SQL Server"
}Following is the query to get list of values by using $group and $push −
> db.groupByDemo.aggregate({$group: { '_id': '$UserName', 'Subject': { $push: '$Subject'}}});This will produce the following output −
{ "_id" : "Larry", "Subject" : [ "MySQL", "SQL Server" ] }
{ "_id" : "John", "Subject" : [ "MongoDB", "Java", "C" ] }
Updated on: 30-Jul-2019
1K+ Views
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