We have various ways to achieve the result. However, we will discuss here two methods.

1. Distinct

db.collection.distinct(field, query, options)

Finds the distinct values for a specified field across a single collection or view and returns the results in an array.

This method takes the following parameters:

Examples

The examples use the inventory collection that contains the following documents:

{
    "_id": 1,
    "dept": "A",
    "item": {
        "sku": "111",
        "color": "red"
    },
    "sizes": ["S", "M"]
}{
    "_id": 2,
    "dept": "A",
    "item": {"sku": "111","color": "blue" },
    "sizes": [ "M", "L"]
}{
    "_id": 3,
    "dept": "B",
    "item": {"sku": "222", "color": "blue"},
    "sizes": "S"
}{
    "_id": 4,
    "dept": "A",
    "item": {"sku": "333", "color": "black"},
    "sizes": [ "S"]
}

Return Distinct Values for a Field

The following example returns the distinct values for the field dept from all documents in the inventory collection:

db.inventory.distinct( "dept")

The method returns the following array of distinct dept values:

[ "A", "B" ]

Return Distinct Values for an Embedded Field

The following example returns the distinct values for the field sku, embedded in the item field, from all documents in the inventory collection:

db.inventory.distinct("item.sku")

The method returns the following array of distinct sku values:

[ "111", "222", "333" ]

2. Aggregation Pipeline:

The following aggregation operation uses the $group stage to retrieve the distinct item values from the sales collection:

db.inventory .aggregate( [ { $group : { _id : "$dept" } } ] )

The operation returns the following result:

{ "_id" : "A" }{ "_id" : "B" }