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，如果字段的值为[ 1, [1], 1 ]，则distinct会将1、[1]和1视为单独的值。

For an example, see Return Distinct Values for an Array Field.有关示例，请参阅返回数组字段的不同值。

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 coverdistinct operations. 索引还可以覆盖distinct的操作。See Covered Query for more information on queries covered by indexes.有关索引涵盖的查询的更多信息，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，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 $group stage.对于非存储集合，可以使用db.collection.distinct()方法/distinct命令以及带有$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 $group stage instead. 要查找分片集合的不同值，请改用带有$group阶段的聚合管道。See Distinct Operation for details.有关详细信息，请参阅Distinct操作。

Client Disconnection客户端断开连接

Starting in MongoDB 4.2, if the client that issued the distinct disconnects before the operation completes, MongoDB marks the distinct for termination (i.e. killOp on the operation).从MongoDB 4.2开始，如果发出distinct命令的客户端在操作完成之前断开连接，MongoDB会将distinct标记为终止（即操作上的killOp）。

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. 从MongoDB 4.4开始，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。If the member is in another state, such as STARTUP2, the operation errors.如果成员处于其他状态，例如STARTUP2，则操作错误。

In previous versions, the operations can also be run when the member is in STARTUP2. 在以前的版本中，当成员位于STARTUP2中时，也可以运行这些操作。However, the operations wait until the member transitions to RECOVERING.但是，操作会一直等到成员转换到RECOVERING。

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:以下示例从inventory集合中的所有文档中返回字段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:以下示例从inventory集合中的所有文档中返回嵌入在item字段中的字段sku的不同值：

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

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:以下示例从inventory集合中的所有文档返回字段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。

Specify Query with distinct使用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: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.有关排序字段的描述，请参阅排序规则文档。

Override Default Read Concern覆盖默认读取问题

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将读取关注点指定为"majority"，以读取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.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"读取关注点和"majority"写入关注点。