MongoDB聚合运算符：$covarianceSamp

$covarianceSamp聚合运算符返回使用$setWindowFields阶段窗口中的文档计算的两个数值表达式的样本协方差。

$covarianceSamp运算符只能用于$setWindowFields阶段。

语法

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

使用

$covarianceSamp使用时遵守以下规则：

• 会忽略窗口中非数值、null和缺失字段的值
• 如果窗口只有一个文档，则返回null
• 如果窗口为空，返回null
• 如果窗口包含NaN则返回NaN
• 如果窗口包含一个或多个全是正无穷或全是负无穷值，返回的无穷值的符号与窗口值一致
• 如果窗口包含的无穷值符号不同，返回NaN
• 如果窗口包含小数值，返回小数值
• 如果以上都不适用，返回双精度数

返回值的优先顺序：

• NaN
• 无穷
• 小数
• 双精度数

举例

使用下面的命令创建cakeSales，它包含了在加利福尼亚California (CA)和华盛顿Washington (WA)的蛋糕销售记录：

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 }
] )

下面的例子在$setWindowFields阶段使用$covarianceSamp操作符输出蛋糕销售orderDate年度和quantity的样本协方差：

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

在本例中：

• partitionBy: "$state"依据state对集合文档进行分区，有两个区：CA和WA
• sortBy: { orderDate: 1 }根据orderDate对每个分区的文档按正序进行排序，orderDate最早的日期排在最前面
• 在文档窗口中使用$covarianceSamp输出年度orderDate和quantity的样本协方差

该窗口包含的文档介于无限制的下限和输出的当前文档之间，这意味着$covarianceSamp将covariancePopForState字段设置为分区开始和当前文档之间文档的样本协方差值。

在输出中，样本协方差显示在covariancePopForState字段中：

{ "_id" : 4, "type" : "strawberry", "orderDate" : ISODate("2019-05-18T16:09:01Z"),
  "state" : "CA", "price" : 41, "quantity" : 162, "covarianceSampForState" : null }
{ "_id" : 0, "type" : "chocolate", "orderDate" : ISODate("2020-05-18T14:10:30Z"),
  "state" : "CA", "price" : 13, "quantity" : 120, "covarianceSampForState" : -21 }
{ "_id" : 2, "type" : "vanilla", "orderDate" : ISODate("2021-01-11T06:31:15Z"),
  "state" : "CA", "price" : 12, "quantity" : 145, "covarianceSampForState" : -8.500000000000007 }
{ "_id" : 5, "type" : "strawberry", "orderDate" : ISODate("2019-01-08T06:12:03Z"),
  "state" : "WA", "price" : 43, "quantity" : 134, "covarianceSampForState" : null }
{ "_id" : 3, "type" : "vanilla", "orderDate" : ISODate("2020-02-08T13:13:23Z"),
  "state" : "WA", "price" : 13, "quantity" : 104, "covarianceSampForState" : -15 }
{ "_id" : 1, "type" : "chocolate", "orderDate" : ISODate("2021-03-20T11:30:05Z"),
  "state" : "WA", "price" : 14, "quantity" : 140, "covarianceSampForState" : 3 }