MongoDB聚合命令

📌 相关文章

📜 MongoDB聚合命令

📅 最后修改于: 2020-11-23 00:59:33 🧑 作者: Mango

聚合命令

Aggregation命令使用聚合管道执行聚合操作。聚合管道允许用户使用基于阶段的应用程序序列从记录或其他源执行数据处理。

句法：

{
  aggregate: "" || 1, pipeline: [ , <...>],
  explain: , allowDiskUse: ,
  cursor: ,
  maxTimeMS: ,
  bypassDocumentValidation: ,
  readConcern: ,
  collation: ,
  hint: ,
  comment: ,
  writeConcern: 
}

命令字段：

Fields	Type	Description
aggregate	string	It contains the name of the aggregation pipeline
pipeline	array	The array that transforms the list of documents as a part of the aggregation pipeline.
explain	boolean	The explain field is optional that is used to return the information on the processing of the pipeline.
allowDiskUse	boolean	It enables the command to write to the temporary files.
cursor	document	It addresses the documents that contains the control option for the creation of the cursor object.
maxTimeMS	non-negative integer	It defines a time limit for the processing operations on a cursor.
Bypass Document Validation	boolean	It is applicable only in case if you specify the out or merge aggregation stages.
readConcern	document	It specifies the read concern. The possible read concern levels are – local, available, majority, and linearizable.
collation	document	We can specify language-specific rules for string comparison using collation. Such as – rules for case and accent marks. collation: { locale: , caseLevel: , caseFirst: , strength: , numericOrdering: , alternate: , maxVariable: , backwards: }
hint	string or document	It declares the index either by the index name or by the index specification document.
comment	string	We can declare an arbitrary string to help trace the operation through the database profiler, currentOp, and logs.
writeConcern	document	It is set to use the default write concern with the $out or $merge pipeline stages.

例：

本文中包含以下文档：

{
   _id: ObjectId("52769ea0f3dc6ead47c9a1b2"),
   author: "Ankit",
   title: "JavaTpoint",
   tags: [ "Java Tutorial", "DBMS Tutorial", "mongodb"]
}

现在，我们将对articles集合执行聚合操作，以计算出现在集合中的tags数组中每个不同元素的计数。

db.runCommand( { aggregate: "articles",
   pipeline: [
      { $project: { tags: 1 } },
      { $unwind: "$tags" },
      { $group: { _id: "$tags", count: { $sum : 1 } } }
   ],
   cursor: { }
} )

MongoDB Count命令

MongoDB count命令对集合或视图中的文档数进行计数。它返回包含计数和命令状态的文档。

句法：

{
  count: ,
  query: ,
  limit: ,
  skip: ,
  hint: ,
  readConcern: ,
  collation: 
}

命令字段

Field	Type	Description
count	string	It is the name of the collection or view to count.
query	document	It is optional and used to select the document to count in the collection or view.
limit	integer	It is optional and used to limit the maximum number of matching documents to return.
skip	integer	It is optional and is used for matching documents to skip before returning results.
hint	string	It is used to define either the index name as a string or the index specification document.
readConcerndocument	It specifies the read concern. readConcern: { level: }
collation	document	It allows us to define language-specific rules for string comparison. Syntax: collation: { locale: , caseLevel: , caseFirst: , strength: , numericOrdering: , alternate: , maxVariable: , backwards: }

例子：

要计算集合中所有文档的数量：

db.runCommand( { count: 'orders' } )

MongoDB独特命令

此命令在单个集合中查找给定字段的不同值。它返回包含不同值数组的文档。返回文档包含带有查询统计信息和查询计划的嵌入式记录。

句法：

  distinct: "",
  key: "",
  query: ,
  readConcern: ,
  collation: 
  }

命令字段

Field	Type	Description
distinct	string	It is the name of the collection to query for different values
key	string	This is the field for which the command returns the distinct value.
query	document	It specifies the documents from where the distinct value will be retrieved.
readConcerndocument	It specifies the read concern. readConcern: { level: }
collation	document	It allows us to define language-specific rules for string comparison. Syntax: collation: { locale: , caseLevel: , caseFirst: , strength: , numericOrdering: , alternate: , maxVariable: , backwards: }

例：

下面的示例从库集合中的所有文档中为外业手簿返回不同的值：

db.runCommand ( { distinct: "library", key: "books" } )

MongoDB MapReduce命令

MapReduce命令允许我们在集合上运行map-reduce聚合操作。

句法：

    db.runCommand(
               {
                 mapReduce: ,
                 map: ,
                 reduce: ,
                 finalize: ,
                 out: ,
                 query: ,
                 sort: ,
                 limit: ,
                 scope: ,
                 jsMode: ,
                 verbose: ,
                 bypassDocumentValidation: ,
                 collation: ,
                 writeConcern: 
               }
             )

指令栏位

Field	Type	Description
MapReduce	collection	It is the name of the collection on which we want to perform the map-reduce operation.
map	function	It is a JavaScript function that associates or maps the key-value pair.
reduce	function	It is a JavaScript function that reduces to a single object all the values associated with a particular key.
out	string	It specifies where to store the output.
query	document	It specifies the selection criteria to determine the documents input to the map function.
sort	document	It sorts the input document.
limit	number	It specifies a maximum number of documents for the input into the map function.
finalize	function	It follows the reduce method to modify the output.
scope	document	It is an option and declares the global variables that are accessible on the map.
jsMode	boolean	The jsMode specifies whether to convert intermediate data into BSON format.
verbose	boolean	It specifies whether to include the timing information in the result or not.
bypass Document Validation	boolean	It enables map-reduce to bypass document validation during the operation.
collation	document	It allows us to specify language-specific rules for string comparison. collation: { locale: , caseLevel: , caseFirst: , strength: , numericOrdering: , alternate: , maxVariable: , backwards: }
writeConcern	document	It is a document that expresses the write concern to use when outputting to a collection.

例：

var mapFunction = function() { ... };
var reduceFunction = function(key, values) { ... };

db.runCommand(
               {
                 mapReduce: ,
                 map: mapFunction,
                 reduce: reduceFunction,
                 out: { merge:  },
                 query: 
               }
             )