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distinct

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.

The command takes the following form:

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{ distinct: "<collection>", key: "<field>", query: <query> }

The command contains 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 default level is "local". To use a read concern level of "majority", you must use the WiredTiger storage engine and start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern setting if using a configuration file). Only replica sets using protocol version 1 support "majority" read concern. Replica sets running protocol version 0 do not support "majority" read concern. 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. New in version 3.2.

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

Behavior

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.

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

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.

Examples

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

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{ "_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:

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

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

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{
   "values" : [ "A", "B" ],
   "stats" : { ... },
   "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:

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

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

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

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:

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

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

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

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":

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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:

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

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

• To use a read concern level of "majority", you must use the WiredTiger storage engine and start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern setting if using a configuration file).

  Only replica sets using protocol version 1 support "majority" read concern. Replica sets running protocol version 0 do not support "majority" read concern.

• 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.

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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.

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