Select two fields and return a sorted array with their distinct values in MongoDB?

To select two fields and return a sorted array with their distinct values in MongoDB, use the aggregation framework with the $setUnion operator. This operator combines arrays and automatically removes duplicates while maintaining sorted order.

Syntax

db.collection.aggregate([
    { $group: {
        "_id": null,
        "field1Array": { $push: "$field1" },
        "field2Array": { $push: "$field2" }
    }},
    { $project: {
        "_id": 0,
        "distinctValues": { $setUnion: ["$field1Array", "$field2Array"] }
    }}
]);

Sample Data

db.sortedArrayWithDistinctDemo.insertMany([
    { value1: 4, value2: 5 },
    { value1: 5, value2: 6 },
    { value1: 7, value2: 4 }
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5cc690b99cb58ca2b005e666"),
        ObjectId("5cc690b99cb58ca2b005e667"),
        ObjectId("5cc690b99cb58ca2b005e668")
    ]
}

Display all documents to verify the data ?

db.sortedArrayWithDistinctDemo.find();

{ "_id": ObjectId("5cc690b99cb58ca2b005e666"), "value1": 4, "value2": 5 }
{ "_id": ObjectId("5cc690b99cb58ca2b005e667"), "value1": 5, "value2": 6 }
{ "_id": ObjectId("5cc690b99cb58ca2b005e668"), "value1": 7, "value2": 4 }

Example

Select the "value1" and "value2" fields and return a sorted array with their distinct values ?

db.sortedArrayWithDistinctDemo.aggregate([
    { $group: {
        "_id": null,
        "value1": { $push: "$value1" },
        "value2": { $push: "$value2" }
    }},
    { $project: {
        "_id": 0,
        "bothValues": { $setUnion: ["$value1", "$value2"] }
    }}
]);

{ "bothValues": [4, 5, 6, 7] }

How It Works

• $group with $push collects all values from both fields into separate arrays
• $setUnion combines the arrays, removes duplicates, and sorts the result automatically
• The duplicate value 4 and 5 appear only once in the final output

Conclusion

The $setUnion operator efficiently combines multiple field values into a single sorted array with distinct values. This approach is ideal for creating unified value sets from different document fields.