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db.collection.distinct()

Definition

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

Important

mongo Shell Method

This page documents a mongo method. This is not the documentation for database commands or language-specific drivers, such as Node.js. To use the database command, see the distinct command.

For MongoDB API drivers, refer to the language-specific MongoDB driver documentation.

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

Compatibility

You can use db.collection.distinct() for deployments hosted in the following environments:

  • MongoDB Atlas: The fully managed service for MongoDB deployments in the cloud

Syntax

This method takes the following parameters:

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.

Note

Results must not be larger than the maximum BSON size. If your results exceed the maximum BSON size, use the aggregation pipeline to retrieve distinct values using the $group operator, as described in Retrieve Distinct Values with the Aggregation Pipeline.

Options

{ collation: <document> }
Field
Type
Description

collation

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.

You cannot specify multiple collations for an operation. For example, you cannot specify different collations per field, or if performing a find with a sort, you cannot use one collation for the find and another for the sort.

New in version 3.4.

Behavior

In a sharded cluster, the distinct command may return orphaned documents.

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.

Transactions

To perform a distinct operation within a transaction:

Important

In most cases, a distributed transaction incurs a greater performance cost over single document writes, and the availability of distributed transactions should not be a replacement for effective schema design. For many scenarios, the denormalized data model (embedded documents and arrays) will continue to be optimal for your data and use cases. That is, for many scenarios, modeling your data appropriately will minimize the need for distributed transactions.

For additional transactions usage considerations (such as runtime limit and oplog size limit), see also Production Considerations.

Client Disconnection

Starting in MongoDB 4.2, if the client that issued db.collection.distinct() disconnects before the operation completes, MongoDB marks db.collection.distinct() for termination using killOp.

Replica Set Member State Restriction

Starting in MongoDB 4.4, to run on a replica set member, distinct operations require the member to be in PRIMARY or SECONDARY state. If the member is in another state, such as STARTUP2, the operation errors.

In previous versions, the operations also run when the member is in STARTUP2. The operations wait until the member transitioned to RECOVERING.

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" ]

Tip

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.

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