distinct

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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

{
  distinct: "<collection>",
  key: "<field>",
  query: <query>,
  readConcern: <read concern document>,
  collation: <collation document>
}

The command contains the following fields:

FieldTypeDescription
distinctstringThe name of the collection to query for distinct values.
keystringThe field for which to return distinct values.
querydocumentOptional. A query that specifies the documents from which to retrieve the distinct values.
readConcerndocumentOptional. Specifies the read concern.
The readConcern option has the following syntax:
Changed in version 3.6.
readConcern: { level: <value> }
Possible read concern levels are:
"local". This is the default read concern level.
"available". This is the default for reads against secondaries when Read Operations and Causally Consistent Sessions and “level” are unspecified. The query returns the instance’s most recent data.
"majority". Available for replica sets that use WiredTiger storage engine.
"linearizable". Available for read operations on the primary only.
For more formation on the read concern levels, see Read Concern Levels.
For "local" (default) or "majority" read concern level, you can specify the afterClusterTime option to have the read operation return data that meets the level requirement and the specified after cluster time requirement. For more information, see Read Operations and Causally Consistent Sessions.
collationdocumentOptional.
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.

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.

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:

{ "_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
}

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.

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

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.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

Starting in MongoDB 3.6, support for read concern "majority" is enabled by default. For MongoDB 3.6.1 - 3.6.x, you can disable read concern "majority". For more information, see Disable Read Concern Majority.

  • 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.