distinct¶

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Definition
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.

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:

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 option has the following syntax: readConcern: { level: <value> } Possible read concern values are: `"local"`. This is the default read concern level. `"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.
`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. 3.4 新版功能.

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:

{ "_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" ],
   "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:

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" ],
  "stats" : { ... },
  "ok" : 1
}

参见

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" ],
  "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":

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" ],
  "stats" : { ... },
  "ok" : 1
}

Specify a Collation¶

3.4 新版功能.

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.

注解

To use read concern level of "majority",
- you must start the mongod instances with the --enableMajorityReadConcern command line option (or the replication.enableMajorityReadConcern set to true if using a configuration file).
- replica sets must use WiredTiger storage engine and election protocol version 1.
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.

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