db.collection.distinct()

Definition

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

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

Parameter Type Description
field string The field for which to return distinct values.
query document A query that specifies the documents from which to retrieve the distinct values.
options document Optional. A document that specifies the options. See Options.

The db.collection.distinct() method provides a wrapper around the distinct command. Results must not be larger than the maximum BSON size.

Options

{ collation: <document> }
Field Type Description
collations document

Optional.

Specifies the collation to use for the operation.

Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.

The collation option has the following syntax:

collation: {
   locale: <string>,
   caseLevel: <boolean>,
   caseFirst: <string>,
   strength: <int>,
   numericOrdering: <boolean>,
   alternate: <string>,
   maxVariable: <string>,
   backwards: <boolean>
}

When specifying collation, the locale field is mandatory; all other collation fields are optional. For descriptions of the fields, see Collation Document.

If the collation is unspecified but the collection has a default collation (see db.createCollection()), the operation uses the collation specified for the collection.

If no collation is specified for the collection or for the operations, MongoDB uses the simple binary comparison used in prior versions for string comparisons.

New in version 3.4.

Behavior

Array Fields

If the value of the specified field is an array, db.collection.distinct() considers each element of the array as a separate value.

For instance, if a field has as its value [ 1, [1], 1 ], then db.collection.distinct() considers 1, [1], and 1 as separate values.

For an example, see Return Distinct Values for an Array Field.

Index Use

When possible, db.collection.distinct() operations can use indexes.

Indexes can also cover db.collection.distinct() operations. See Covered Query for more information on queries covered by indexes.

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" ]

See also

Dot Notation for information on accessing fields within embedded documents

Return Distinct Values for an Array Field

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

db.inventory.distinct( "sizes" )

The method returns the following array of distinct sizes values:

[ "M", "S", "L" ]

For information on distinct() and array fields, see the Behavior section.

Specify Query with distinct

The following example returns the distinct values for the field sku, embedded in the item field, from the documents whose dept is equal to "A":

db.inventory.distinct( "item.sku", { dept: "A" } )

The method returns the following array of distinct sku values:

[ "111", "333" ]

Specify a Collation

New in version 3.4.

Collation allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.

A collection myColl has the following documents:

{ _id: 1, category: "café", status: "A" }
{ _id: 2, category: "cafe", status: "a" }
{ _id: 3, category: "cafE", status: "a" }

The following aggregation operation includes the Collation option:

db.myColl.distinct( "category", {}, { collation: { locale: "fr", strength: 1 } } )

For descriptions on the collation fields, see Collation Document.