distinct

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，则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，distinct命令在使用数组时为集合和视图返回相同的结果。

For examples, see:有关示例，请参阅：

• Return Distinct Values for an Array Field返回数组字段的剔除重复值
• 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 Covered Query for more information on queries covered by indexes.索引还可以涵盖distinct的操作。有关索引所涵盖查询的更多信息，请参阅覆盖查询。

Transactions事务

To perform a distinct operation within a transaction:要在事务中执行不同的操作：

• For unsharded collections, you can use the db.collection.distinct() method/the distinct command as well as the aggregation pipeline with the $group stage对于未排序的集合，可以使用db.collection.distinct()方法/distinct命令以及带有$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 $group stage instead. See Distinct Operation for details.要查找分片集合的不同值，请使用带有$group阶段的聚合管道。有关详细信息，请参阅剔除重复操作。

Client Disconnection客户端断开连接

Starting in MongoDB 4.2, if the client that issued distinct disconnects before the operation completes, MongoDB marks distinct for termination using killOp.从MongoDB 4.2开始，如果在操作完成之前发出distinct的客户端断开连接，MongoDB会使用killOp标记distinct以终止。

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

Starting in MongoDB 4.4, 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.从MongoDB 4.4开始，要在副本集成员上运行，distinct操作需要该成员处于PRIMARY或SECONDARY状态。如果成员处于其他状态，例如STARTUP2，则操作将出错。

In previous versions, the operations also run when the member is in STARTUP2. 在以前的版本中，成员处于STARTUP2时也会运行这些操作。The operations wait until the member transitioned to RECOVERING.操作将等待，直到成员转换到RECOVERING。

Index Filters and Collations索引筛选器和排序规则

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。

Return Distinct Values for a Field返回字段的不同值

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集合中所有文档中字段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:该命令返回一个具有名为values的字段的文档，该字段包含不同的dept值：

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

Return Distinct Values for an Embedded Field返回嵌入字段的不同值

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物料字段中的字段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:该命令返回一个具有名为values的字段的文档，该字段包含不同的sku值：

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

Return Distinct Values for an Array Field返回数组字段的不同值

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sizes的值：

{
  "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，distinct命令在使用数组时为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:以下示例为每个文档创建一个名为sensor的集合，其中包含一组温度值：

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集合中创建一个名为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返回sensor集合中temperatures数组的唯一值：

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temperatures数组索引。

Example output:示例输出：

[ 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开始为[ 4, 8, 12 ]（与sensor集合返回的结果相同）。
• [] in MongoDB versions earlier than 6.0.在低于6.0的MongoDB版本中为[]。

Specify Query with distinct指定具有distinct的查询

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:该命令返回一个具有名为values的字段的文档，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 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:集合myColl包含以下文档：

{ _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collation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.以下对副本集的操作指定"majority"的读取关注，以读取已确认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.要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。