$stdDevPop (aggregation)

Non-numeric Values非数字值

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

Single Value单个值

If the sample consists of a single numeric value, $stdDevPop returns 0.如果样本包含单个数值，$stdDevPop将返回0。

Array Operand数组操作数

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$setWindowFields阶段中，如果表达式解析为数组，$stdDevPop会将操作数视为非数值，对计算没有影响。

In the other supported stages:在其他受支持的阶段中：

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

Window Values窗口值

Behavior with values in a $setWindowFields stage window:$setWindowFields阶段窗口中值的行为：

• Ignores non-numeric values, null values, and missing fields in a window.忽略窗口中的非数值、null值和缺少的字段。
• 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.如果窗口包含无限值，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

A collection named users contains the following documents:名为users的集合包含以下文档：

{ "_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阶段中使用

Create an example collection named quizzes with the following documents:使用以下文档创建名为quezzes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):创建包含加利福尼亚州（CA）和华盛顿州（WA）蛋糕销售的cakeSales集合：

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 $setWindowFields stage to output the population standard deviation of the cake sales quantity for each state:本例使用$setWindowFields阶段中的$stdDevPop输出每个州蛋糕销售数量的总体标准偏差：

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. 按state对集合中的文档进行分区。There are partitions for CA and WA.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以升序（1）对每个分区中的文档进行排序，因此最早的orderDate是第一个。

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

  The window contains documents between an unbounded lower limit and the current document in the output. 该窗口包含输出中处于unbounded下限和current文档之间的文档。This means $stdDevPop returns the quantity population standard deviation value for the documents between the beginning of the partition and the current document.这意味着$stdDevPop返回分区开始处和当前文档之间文档的数量填充标准偏差值。

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 }