distinct (database command)（数据库命令）

Array Fields数组字段

If the value of the specified field is an array, distinct considers each element of the array as a separate value.如果指定field的值是一个数组，distinct会将数组的每个元素视为一个单独的值。

For instance, if a field has as its value [ 1, [1], 1 ], then distinct considers 1, [1], and 1 as separate values.例如，如果一个字段的值为[ 1, [1], 1 ]，则distinct将1、[1]和1视为单独的值。

Starting in MongoDB 6.0, the distinct command returns the same results for collections and views when using arrays.从MongoDB 6.0开始，当使用数组时，distinct命令对集合和视图返回相同的结果。

For examples, see:例如，请参阅：

• Return Distinct Values for an Array Field为数组字段返回不同的值
• Arrays in Collections and Views集合和视图中的数组

Index Use索引使用

When possible, distinct operations can use indexes.如果可能，distinct操作可以使用索引。

Indexes can also cover distinct operations. See Run Covered Queries for more information on queries covered by indexes.索引还可以涵盖distinct操作。有关索引覆盖的查询的更多信息，请参阅运行覆盖的查询。

Transactions事务

To perform a distinct operation within a transaction:要在事务中执行不同的操作，请执行以下操作：

• For unsharded collections, you can use the db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage.对于未记录的集合，您可以使用db.collection.distinct()方法/distinct命令以及$group阶段的聚合管道。

• For sharded collections, you cannot use the db.collection.distinct() method or the distinct command.对于分片集合，不能使用 db.collection.distinct()方法或distinct命令。

  To find the distinct values for a sharded collection, use the aggregation pipeline with the $group stage instead. 要查找分片集合的不同值，请使用带有$group阶段的聚合管道。See Distinct Operation for details.详见“Distinct操作”。

Client Disconnection客户端断开连接

If the client that issued distinct disconnects before the operation completes, MongoDB marks distinct for termination using killOp.如果发出distinct的客户端在操作完成之前断开连接，MongoDB将使用killOp标记distinct以终止。

Replica Set Member State Restriction副本集成员状态限制

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.要在副本集成员上运行，distinct操作要求该成员处于PRIMARY或SECONDARY状态。如果成员处于另一种状态，如STARTUP2，则操作错误。

Index Filters and Collations索引筛选器和排序

Starting in MongoDB 6.0, an index filter uses the collation previously set using the planCacheSetFilter command.从MongoDB 6.0开始，索引筛选器使用之前使用planCacheSetFilter命令设置的排序规则。

Starting in MongoDB 8.0, use query settings instead of adding index filters. Index filters are deprecated starting in MongoDB 8.0.从MongoDB 8.0开始，使用查询设置，而不是添加索引筛选器。索引筛选器从MongoDB 8.0开始就被弃用。

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. 查询设置比索引筛选器具有更多功能。此外，索引筛选器不是持久的，您无法轻松为所有集群节点创建索引筛选器。To add query settings and explore examples, see setQuerySettings.要添加查询设置并探索示例，请参阅setQuerySettings。

Query Settings查询设置

You can use query settings to set index hints, set operation rejection filters, and other fields. 您可以使用查询设置来设置索引提示、设置操作拒绝筛选器和其他字段。The settings apply to the query shape on the entire cluster. The cluster retains the settings after shutdown.这些设置适用于整个集群上的查询形状。集群在关闭后保留设置。

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.查询优化器在查询规划期间使用查询设置作为额外输入，这会影响为运行查询而选择的计划。您还可以使用查询设置来阻止查询形状。

To add query settings and explore examples, see setQuerySettings.要添加查询设置并探索示例，请参阅setQuerySettings。

You can add query settings for find, distinct, and aggregate commands.您可以为find、distinct和aggregate命令添加查询设置。

Query settings have more functionality and are preferred over deprecated index filters.查询设置具有更多功能，并且优于已弃用的索引筛选器。

To remove query settings, use removeQuerySettings. 要删除查询设置，请使用removeQuerySettings。To obtain the query settings, use a $querySettings stage in an aggregation pipeline.要获取查询设置，请在聚合管道中使用$querySettings阶段。

Return Distinct Values for a Field为字段返回不同的值

The following example returns the distinct values for the field dept from all documents in the inventory collection:以下示例返回inventory集合中所有文档的字段dept的不同值：

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

The command returns a document with a field named values that contains the distinct dept values:该命令返回一个文档，其中包含一个名为values的字段，该字段包含不同的dept值：

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

Return Distinct Values for an Embedded Field为嵌入式字段返回不同的值

The following example returns the distinct values for the field sku, embedded in the item field, from all documents in the inventory collection:以下示例返回inventory集合中所有文档中嵌入在项目字段中的字段sku的不同值：

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

The command returns a document with a field named values that contains the distinct sku values:该命令返回一个文档，其中包含一个名为values的字段，该字段包含不同的sku值：

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

Return Distinct Values for an Array Field为数组字段返回不同的值

The following example returns the distinct values for the field sizes from all documents in the inventory collection:以下示例返回inventory集合中所有文档的字段sizes的不同值：

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

The command returns a document with a field named values that contains the distinct sizes values:该命令返回一个文档，其中包含一个名为values的字段，该字段包含不同的sizes值：

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

For information on distinct and array fields, see the Behavior section.有关distinct字段和数组字段的信息，请参阅行为部分。

Arrays in Collections and Views集合和视图中的数组

Starting in MongoDB 6.0, the distinct command returns the same results for collections and views when using arrays.从MongoDB 6.0开始，当使用数组时，distinct命令对集合和视图返回相同的结果。

The following example creates a collection named sensor with an array of temperature values for each document:以下示例创建了一个名为sensor的集合，其中包含每个文档的温度值数组：

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:以下示例从sensor（传感器）集合中创建了一个名为sensorView的视图：

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

The following example uses distinct to return the unique values from the temperatures array in the sensor collection:以下示例使用distinct从sensor（传感器）集合中的temperatures（温度）数组返回唯一值：

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

The 1 in temperatures.1.value specifies the temperatures array index.temperatures.1.value中的1指定了temperatures（温度）数组索引。

Example output:输出示例：

[ 4, 8, 12 ]

Example for sensorView:sensorView示例：

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

Example output:输出示例：

• [ 4, 8, 12 ] starting in MongoDB 6.0 (identical to result returned from the sensor collection).从MongoDB 6.0开始（与sensor（传感器）集合返回的结果相同）。
• [] in MongoDB versions earlier than 6.0.[]在早于6.0的MongoDB版本中。

Specify Query with distinct用distinct指定查询

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":以下示例从dept等于"A"的文档中返回嵌入在item字段中的字段sku的不同值：

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:该命令返回一个文档，其中包含一个名为values的字段，该字段包含不同的sku值：

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

Specify a Collation指定排序规则

Collation排序规则 allows users to specify language-specific rules for string comparison, such as rules for lettercase and accent marks.允许用户为字符串比较指定特定于语言的规则，例如字母大小写和重音标记的规则。

A collection myColl has the following documents:myColl集合有以下文件：

{ _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:以下聚合操作包括排序规则选项：

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

For descriptions on the collation fields, see Collation Document.有关排序规则字段的说明，请参阅排序规则文档。

Override Default Read Concern覆盖默认读取关注

To override the default read concern level of "local", use the readConcern option.要覆盖默认的读取关注级别"local"，请使用readConcern选项。

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.对副本集的以下操作指定读取关注为"majority"，以读取已确认已写入大多数节点的数据的最新副本。

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.为了确保单个线程可以读取自己的写入，请对副本集的主线程使用"majority"读取关注和"majority"写入关注。

Specify an Index指定索引

You can specify an index name or pattern using the hint option.您可以使用提示选项指定索引名称或模式。

To specify a hint based on an index name:要基于索引名称指定提示，请执行以下操作：

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

To specify a hint based on an index pattern:要基于索引模式指定提示，请执行以下操作：

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