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distinct

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  • Definition
  • Compatibility
  • Syntax
  • Command Fields
  • Behavior
  • Examples

Definition

distinct

Finds the distinct values for a specified field across a single collection. distinct returns a document that contains an array of the distinct values. The return document also contains an embedded document with query statistics and the query plan.

Tip

In mongosh, this command can also be run through the db.collection.distinct() helper method.

Helper methods are convenient for mongosh users, but they may not return the same level of information as database commands. In cases where the convenience is not needed or the additional return fields are required, use the database command.

Compatibility

This command is available in deployments hosted in the following environments:

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

Important

This command has limited support in M0, M2, and M5 clusters. For more information, see Unsupported Commands.

Syntax

The command has the following syntax:

db.runCommand(
   {
     distinct: "<collection>",
     key: "<field>",
     query: <query>,
     readConcern: <read concern document>,
     collation: <collation document>,
     comment: <any>,
     hint: <string or document>
   }
)

Command Fields

The command takes the following fields:

Field
Type
Description

distinct

string

The name of the collection to query for distinct values.

key

string

The field for which to return distinct values.

query

document

Optional. A query that specifies the documents from which to retrieve the distinct values.

readConcern

document

Optional. Specifies the read concern.

The readConcern option has the following syntax: readConcern: { level: <value> }

Possible read concern levels are:

For more formation on the read concern levels, see Read Concern Levels.

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.

comment

any

Optional. A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations:

A comment can be any valid BSON type (string, integer, object, array, etc).

hint

string or document

Optional. Specify the index name, either as a string or a document. If specified, the query planner only considers plans using the hinted index. For more details, see Specify an Index.

New in version 7.1.

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.

MongoDB also provides the shell wrapper method db.collection.distinct() for the distinct command. Additionally, many MongoDB drivers provide a wrapper method. Refer to the specific driver documentation.

Behavior

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

For time series collections, the distinct command can't make efficient use of indexes. Instead, use a $group aggregation to group documents by distinct values. For details, see Time Series Limitations.

Array Fields

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

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

Starting in MongoDB 6.0, the distinct command returns the same results for collections and views when using arrays.

For examples, see:

Index Use

When possible, distinct operations can use indexes.

Indexes can also cover 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 distinct disconnects before the operation completes, MongoDB marks distinct for termination using killOp.

Replica Set Member State Restriction

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.

Index Filters and Collations

Starting in MongoDB 6.0, an index filter uses the collation previously set using the planCacheSetFilter command.

Starting in MongoDB 8.0, use query settings instead of adding index filters. Index filters are deprecated starting in MongoDB 8.0.

Query settings have more functionality than index filters. Also, index filters aren't persistent and you cannot easily create index filters for all cluster nodes. To add query settings and explore examples, see setQuerySettings.

Query Settings

New in version 8.0.

You can use query settings to set index hints, set operation rejection filters, and other fields. The settings apply to the query shape on the entire cluster. The cluster retains the settings after shutdown.

The query optimizer uses the query settings as an additional input during query planning, which affects the plan selected to run the query. You can also use query settings to block a query shape.

To add query settings and explore examples, see setQuerySettings.

You can add query settings for find, distinct, and aggregate commands.

Query settings have more functionality and are preferred over deprecated index filters.

To remove query settings, use removeQuerySettings. To obtain the query settings, use a $querySettings stage in an aggregation pipeline.

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.runCommand ( { distinct: "inventory", key: "dept" } )

The command returns a document with a field named values that contains the distinct dept values:

{
   "values" : [ "A", "B" ],
   "ok" : 1
}

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.runCommand ( { distinct: "inventory", key: "item.sku" } )

The command returns a document with a field named values that contains the distinct sku values:

{
  "values" : [ "111", "222", "333" ],
  "ok" : 1
}

Tip

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.runCommand ( { distinct: "inventory", key: "sizes" } )

The command returns a document with a field named values that contains the distinct sizes values:

{
  "values" : [ "M", "S", "L" ],
  "ok" : 1
}

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

Arrays in Collections and Views

Starting in MongoDB 6.0, the distinct command returns the same results for collections and views when using arrays.

The following example creates a collection named sensor with an array of temperature values for each document:

db.sensor.insertMany( [
   { _id: 0, temperatures: [ { value: 1 }, { value: 4 } ] },
   { _id: 1, temperatures: [ { value: 2 }, { value: 8 } ] },
   { _id: 2, temperatures: [ { value: 3 }, { value: 12 } ] },
   { _id: 3, temperatures: [ { value: 1 }, { value: 4 } ] }
] )

The following example creates a view named sensorView from the sensor collection:

db.createView( "sensorView", "sensor", [] )

The following example uses distinct to return the unique values from the temperatures array in the sensor collection:

db.sensor.distinct( "temperatures.1.value" )

The 1 in temperatures.1.value specifies the temperatures array index.

Example output:

[ 4, 8, 12 ]

Example for sensorView:

db.sensorView.distinct( "temperatures.1.value" )

Example output:

  • [ 4, 8, 12 ] starting in MongoDB 6.0 (identical to result returned from the sensor collection).

  • [] in MongoDB versions earlier than 6.0.

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.runCommand ( { distinct: "inventory", key: "item.sku", query: { dept: "A"} } )

The command returns a document with a field named values that contains the distinct sku values:

{
  "values" : [ "111", "333" ],
  "ok" : 1
}

Specify a Collation

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

For descriptions on the collation fields, see Collation Document.

Override Default Read Concern

To override the default read concern level of "local", use the readConcern option.

The following operation on a replica set specifies a Read Concern of "majority" to read the most recent copy of the data confirmed as having been written to a majority of the nodes.

Note

Regardless of the read concern level, the most recent data on a node may not reflect the most recent version of the data in the system.

db.runCommand(
   {
     distinct: "restaurants",
     key: "rating",
     query: { cuisine: "italian" },
     readConcern: { level: "majority" }
   }
)

To ensure that a single thread can read its own writes, use "majority" read concern and "majority" write concern against the primary of the replica set.

Specify an Index

You can specify an index name or pattern using the hint option.

To specify a hint based on an index name:

db.runCommand ( { distinct: "inventory", key: "dept", hint: "sizes" } )

To specify a hint based on an index pattern:

db.runCommand ( { distinct: "inventory", key: "dept", hint: { sizes: 1 } } )

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