MongoDB aggregation to combine or merge fields and then count?

To combine or merge fields and then perform count, use $group along with $sum and $sort. This aggregation approach groups documents by field values and counts occurrences in each group.

Syntax

db.collection.aggregate([
    { $group: { "_id": "$fieldName", "COUNT": { $sum: 1 } } },
    { $sort: { "COUNT": -1 } },
    { $limit: n }
]);

Sample Data

db.demo647.insertMany([
    {"Subject": "MySQL"},
    {"Subject": "MongoDB"},
    {"Subject": "MySQL"},
    {"Subject": "SQL Server"},
    {"Subject": "MongoDB"},
    {"Subject": "PL/SQL"},
    {"Subject": "MongoDB"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e9c86316c954c74be91e6ee"),
        ObjectId("5e9c86356c954c74be91e6ef"),
        ObjectId("5e9c86376c954c74be91e6f0"),
        ObjectId("5e9c86406c954c74be91e6f1"),
        ObjectId("5e9c86436c954c74be91e6f2"),
        ObjectId("5e9c864b6c954c74be91e6f3"),
        ObjectId("5e9c86c16c954c74be91e6f4")
    ]
}

Display all documents from the collection ?

db.demo647.find();

{ "_id" : ObjectId("5e9c86316c954c74be91e6ee"), "Subject" : "MySQL" }
{ "_id" : ObjectId("5e9c86356c954c74be91e6ef"), "Subject" : "MongoDB" }
{ "_id" : ObjectId("5e9c86376c954c74be91e6f0"), "Subject" : "MySQL" }
{ "_id" : ObjectId("5e9c86406c954c74be91e6f1"), "Subject" : "SQL Server" }
{ "_id" : ObjectId("5e9c86436c954c74be91e6f2"), "Subject" : "MongoDB" }
{ "_id" : ObjectId("5e9c864b6c954c74be91e6f3"), "Subject" : "PL/SQL" }
{ "_id" : ObjectId("5e9c86c16c954c74be91e6f4"), "Subject" : "MongoDB" }

Example: Group, Count and Sort

Following is the query to combine or merge fields then count ?

db.demo647.aggregate([
    { "$group": { "_id": "$Subject", "COUNT": { "$sum": 1 } } },
    { "$sort": { "COUNT": -1 } },
    { "$limit": 2 }
]);

{ "_id" : "MongoDB", "COUNT" : 3 }
{ "_id" : "MySQL", "COUNT" : 2 }

How It Works

• $group combines documents by the Subject field value
• $sum: 1 counts each document in the group
• $sort: { "COUNT": -1 } orders results by count in descending order
• $limit: 2 returns only the top 2 results

Conclusion

Use MongoDB's aggregation pipeline with $group and $sum to combine documents by field values and count occurrences. Add $sort and $limit to organize and restrict results.