$stdDevPop (accumulator operator)（累加器运算符）

Result Type

$stdDevPop returns the population standard deviation of the input values as a decimal.以decimal形式返回输入值的总体标准偏差。

Non-numeric Values非数值

$stdDevPop ignores non-numeric values. If all operands for a $stdDevPop are non-numeric, $stdDevPop returns null.$stdDevPop忽略非数字值。如果$stdDevPop的所有操作数都是非数字的，则$stdDevPop将返回null。

Single Value单一值

If the sample consists of a single numeric value, $stdDevPop returns 0.如果样本由单个数值组成，$stdDevPop将返回0。

Array Operand数组操作对象

In the $group and $setWindowFields stages, if the expression resolves to an array, $stdDevPop treats the operand as a non-numerical value and has no effect on the calculation.在$group和$setWindowFields阶段，如果表达式解析为数组，$stdDevPop会将操作数视为非数值，对计算没有影响。

In the other supported stages:在其他支持的阶段：

• With a single expression as its operand, if the expression resolves to an array, $stdDevPop traverses into the array to operate on the numeric elements of the array to return a single value.以单个表达式作为操作数，如果表达式解析为数组，$stdDevPop将遍历数组以对数组的数字元素进行操作，从而返回单个值。
• With a list of expressions as its operand, if any of the expressions resolves to an array, $stdDevPop does not traverse into the array but instead treats the array as a non-numeric value.使用表达式列表作为操作数，如果任何表达式解析为数组，$stdDevPop不会遍历到数组中，而是将数组视为非数值。

Window Values窗口值

Behavior with values in a $setWindowFields stage window:在$setWindowFields阶段窗口中使用值的行为：

• Ignores non-numeric values, null values, and missing fields in a window.忽略窗口中的非数值、null值和缺失字段。
• If the window is empty, returns null.如果窗口为空，则返回null。
• If the window contains a NaN value, returns null.如果窗口包含NaN值，则返回null。
• If the window contains Infinity values, returns null.如果窗口包含Infinity值，则返回null。
• If none of the previous points apply, returns a double value.如果前面的点都不适用，则返回一个double值。

Use in $group Stage在$group阶段中使用

Create a collection called users with the following documents:使用以下文档创建一个名为users的集合：

db.users.insertMany( [
   { _id : 1, name : "dave123", quiz : 1, score : 85 },
   { _id : 2, name : "dave2", quiz : 1, score : 90 },
   { _id : 3, name : "ahn", quiz : 1, score : 71 },
   { _id : 4, name : "li", quiz : 2, score : 96 },
   { _id : 5, name : "annT", quiz : 2, score : 77 },
   { _id : 6, name : "ty", quiz : 2, score : 82 }
] )

The following example calculates the standard deviation of each quiz:以下示例计算每个测验的标准偏差：

db.users.aggregate( [
   { $group: { _id: "$quiz", stdDev: { $stdDevPop: "$score" } } }
] )

The operation returns the following results:该操作返回以下结果：

{ "_id" : 2, "stdDev" : 8.04155872120988 }
{ "_id" : 1, "stdDev" : 8.04155872120988 }

Use in $project Stage在$project阶段使用

Create an example collection named quizzes with the following documents:使用以下文档创建一个名为quizzes的示例集合：

db.quizzes.insertMany( [
   {
      _id : 1,
      scores : [
         { name : "dave123", score : 85 },
         { name : "dave2", score : 90 },
         { name : "ahn", score : 71 }
      ]
   },
   {
      _id : 2,
      scores : [
         { name : "li", quiz : 2, score : 96 },
         { name : "annT", score : 77 },
         { name : "ty", score : 82 }
      ]
   }
] )

The following example calculates the standard deviation of each quiz:以下示例计算每个测验的标准偏差：

db.quizzes.aggregate( [
   { $project: { stdDev: { $stdDevPop: "$scores.score" } } }
] )

The operation returns the following results:该操作返回以下结果：

{ _id : 1, stdDev : 8.04155872120988 }
{ _id : 2, stdDev : 8.04155872120988 }

Use in $setWindowFields Stage在$setWindowFields阶段中使用

Create a cakeSales collection that contains cake sales in the states of California (CA) and Washington (WA):创建一个cakeSales集合，其中包含加利福尼亚州（CA）和华盛顿州（WA）的蛋糕销售：

db.cakeSales.insertMany( [
   { _id: 0, type: "chocolate", orderDate: new Date("2020-05-18T14:10:30Z"),
     state: "CA", price: 13, quantity: 120 },
   { _id: 1, type: "chocolate", orderDate: new Date("2021-03-20T11:30:05Z"),
     state: "WA", price: 14, quantity: 140 },
   { _id: 2, type: "vanilla", orderDate: new Date("2021-01-11T06:31:15Z"),
     state: "CA", price: 12, quantity: 145 },
   { _id: 3, type: "vanilla", orderDate: new Date("2020-02-08T13:13:23Z"),
     state: "WA", price: 13, quantity: 104 },
   { _id: 4, type: "strawberry", orderDate: new Date("2019-05-18T16:09:01Z"),
     state: "CA", price: 41, quantity: 162 },
   { _id: 5, type: "strawberry", orderDate: new Date("2019-01-08T06:12:03Z"),
     state: "WA", price: 43, quantity: 134 }
] )

This example uses $stdDevPop in the $setWindowFields stage to output the population standard deviation of the cake sales quantity for each state:此示例在$setWindowFields阶段使用$stdDevPop输出每个state的蛋糕销售quantity的总体标准偏差：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            stdDevPopQuantityForState: {
               $stdDevPop: "$quantity",
               window: {
                  documents: [ "unbounded", "current" ]
               }
            }
         }
      }
   }
] )

In the example:在示例中：

• partitionBy: "$state" partitions the documents in the collection by state. There are partitions for CA and WA.按state对集合中的文档进行分区。CA和WA有分区。
• sortBy: { orderDate: 1 } sorts the documents in each partition by orderDate in ascending order (1), so the earliest orderDate is first.按orderDate升序（1）对每个分区中的文档进行排序，因此最早的orderDate位居前列。

• output sets the stdDevPopQuantityForState field to the quantity population standard deviation value using $stdDevPop that is run in a documents window.使用在documents窗口中运行的$stdDevPop将stdDevPopQuantityForState字段设置为quantity的总体标准偏差值。

  The window contains documents between an unbounded lower limit and the current document in the output. This means $stdDevPop returns the quantity population standard deviation value for the documents between the beginning of the partition and the current document.该窗口包含的文档位于unbounded下限和输出中的current文档之间。这意味着$stdDevPop返回分区开始和当前文档之间的文档quantity的总体标准偏差值。

In this example output, the quantity population standard deviation value for CA and WA is shown in the stdDevPopQuantityForState field:在此示例输出中，CA和WA的quantity的总体标准偏差值显示在stdDevPopQuantityForState字段中：

{ _id : 4, type : "strawberry", orderDate : ISODate("2019-05-18T16:09:01Z"),
  state : "CA", price : 41, quantity : 162, stdDevPopQuantityForState : 0 }
{ _id : 0, type : "chocolate", orderDate : ISODate("2020-05-18T14:10:30Z"),
  state : "CA", price : 13, quantity : 120, stdDevPopQuantityForState : 21 }
{ _id : 2, type : "vanilla", orderDate : ISODate("2021-01-11T06:31:15Z"),
  state : "CA", price : 12, quantity : 145, stdDevPopQuantityForState : 17.249798710580816 }
{ _id : 5, type : "strawberry", orderDate : ISODate("2019-01-08T06:12:03Z"),
  state : "WA", price : 43, quantity : 134, stdDevPopQuantityForState : 0 }
{ _id : 3, type : "vanilla", orderDate : ISODate("2020-02-08T13:13:23Z"),
  state : "WA", price : 13, quantity : 104, stdDevPopQuantityForState : 15 }
{ _id : 1, type : "chocolate", orderDate : ISODate("2021-03-20T11:30:05Z"),
  state : "WA", price : 14, quantity : 140, stdDevPopQuantityForState : 15.748015748023622 }