MongoDB query to gather unique array item?

To gather unique array items from a MongoDB collection, use the distinct() method. This method returns an array of unique values for a specified field, automatically removing duplicates.

Syntax

db.collection.distinct("fieldName")

Sample Data

Let us create a collection with documents ?

db.demo588.insertOne({
    "CountryName": ["US", "AUS", "UK", "US", "UK", "AUS"]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e92bbd2fd2d90c177b5bccb")
}

Display all documents from the collection ?

db.demo588.find().pretty();

{
    "_id": ObjectId("5e92bbd2fd2d90c177b5bccb"),
    "CountryName": [
        "US",
        "AUS", 
        "UK",
        "US",
        "UK",
        "AUS"
    ]
}

Example: Get Unique Array Values

Following is the query to gather unique array items ?

db.demo588.distinct("CountryName");

["AUS", "UK", "US"]

Key Points

• The distinct() method automatically removes duplicate values from arrays.
• Results are returned as a sorted array of unique values.
• Works on both array fields and regular fields across multiple documents.

Conclusion

Use MongoDB's distinct() method to extract unique values from array fields. It efficiently handles duplicate removal and returns a clean array of distinct values.