$covariancePop (aggregation operator)（聚合运算符）

Definition定义

$covariancePop

Returns the population covariance of two numeric expressions that are evaluated using documents in the $setWindowFields stage window.返回使用$setWindowFields阶段window中的文档计算的两个数值表达式的总体协方差。

$covariancePop is only available in the $setWindowFields stage.仅在$setWindowFields阶段可用。

$covariancePop syntax:

{
   $covariancePop:
   [
      <numeric expression 1>,
      <numeric expression 2>
   ]
}

Behavior行为

$covariancePop behavior: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 contains one document, returns null. 如果窗口包含一个文档，则返回null。(Compare to $covarianceSamp, which returns null if the window contains one document.)（与$covarianceSamp相比，如果窗口包含一个文档，则返回null。）
• 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 NaN.如果窗口包含NaN值，则返回NaN。
• If the window contains one or more Infinity value(s) that are all positive or all negative, returns Infinity. The returned Infinity value has the same sign as the Infinity values in the window.如果窗口包含一个或多个全正或全负的Infinity，则返回Infinity。返回的Infinity值与窗口中的Infinity值具有相同的符号。
• If the window contains Infinity values with different signs, returns NaN.如果窗口包含具有不同符号的Infinity值，则返回NaN。
• If the window contains a decimal value, returns a decimal value.如果窗口包含decimal值，则返回decimal值。
• 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。

The returned values in order of precedence are as follows:返回值的优先级顺序如下：

• NaN
• Infinity
• decimal
• double

Example示例

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 $covariancePop in the $setWindowFields stage to output the population covariance values for the cake sales orderDate year and quantity values:此示例在$setWindowFields阶段使用$covariancePop输出蛋糕销售orderDate的年份和quantity值的总体协方差值：

db.cakeSales.aggregate( [
   {
      $setWindowFields: {
         partitionBy: "$state",
         sortBy: { orderDate: 1 },
         output: {
            covariancePopForState: {
               $covariancePop: [ { $year: "$orderDate" }, "$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.按状态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 population covariance values for the orderDate year and quantity values using $covariancePop run in a documents window.output使用文档窗口中的$covariancePop函数设置orderDate年份和quantity值的总体协方差值。

  The window contains documents between an unbounded lower limit and the current document in the output. This means $covariancePop sets the covariancePopForState field to the population covariance values for the documents between the beginning of the partition and the current document.该window包含的文档位于无限制的下限和输出中的当前文档之间。这意味着$covariancePop将covarianceOpForState字段设置为分区开始和当前文档之间的文档的总体协方差值。

In this output, the population covariance is shown in the covariancePopForState field:在此输出中，总体协方差显示在covariancePopForState字段中：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "covariancePopForState" : 0 }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "covariancePopForState" : -10.5 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "covariancePopForState" : -5.666666666666671 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "covariancePopForState" : 0 }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "covariancePopForState" : -7.5 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "covariancePopForState" : 2 }