$stdDevSamp (operador de acumulación)

Non-numeric Values

$stdDevSamp ignores non-numeric values. If all operands for a sum are non-numeric, $stdDevSamp returns null.

Valor único

If the sample consists of a single numeric value, $stdDevSamp returns null.

Arreglo Operand

In the $group and $setWindowFields stages, if the expression resolves to an array, $stdDevSamp treats the operand as a non-numerical value.

En las otras etapas admitidas:

• With a single expression as its operand, if the expression resolves to an array, $stdDevSamp traverses into the array to operate on the numeric elements of the array to return a single value.

• With a list of expressions as its operand, if any of the expressions resolves to an array, $stdDevSamp does not traverse into the array but instead treats the array as a non-numeric value.

Window Values

Comportamiento con valores en una $setWindowFields etapa ventana:

• Ignora los valores no numéricos, valores null y los campos faltantes en una ventana.

• Si la ventana está vacía, devuelve null.

• Si la ventana contiene un valor de NaN, devuelve null.

• Si la ventana contiene Infinity valores, retorna null.

• Si ninguno de los puntos anteriores aplica, devuelve un valor de double.

Uso en la etapa $group

Una colección users contiene documentos con los siguientes campos:

db.users.insertMany( [
   { _id: 0, username: "user0", age: 20 },
   { _id: 1, username: "user1", age: 42 },
   { _id: 2, username: "user2", age: 28 }
] )

Para calcular la desviación estándar de una muestra de usuarios, la siguiente operación de agregación primero utiliza la canalización para $sample muestrear 100 usuarios y luego utiliza para $stdDevSamp calcular la desviación estándar para los usuarios muestreados.

db.users.aggregate(
   [
      { $sample: { size: 100 } },
      { $group: { _id: null, ageStdDev: { $stdDevSamp: "$age" } } }
   ]
)

La operación devuelve un resultado como el siguiente:

{ _id: null, ageStdDev: 7.811258386185771 }

Uso en la etapa $setWindowFields

Crea una colección de cakeSales que incluya ventas de pasteles en el estado de California (CA) y Washington (WA):

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 $stdDevSamp in the $setWindowFields stage to output the sample standard deviation of the quantity values of the cake sales for each state:

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

En el ejemplo:

• partitionBy: "$state" particiona los documentos de la colección en state. Existen particiones para CA y WA.

• sortBy: { orderDate: 1 } ordena los documentos de cada partición por orderDate en orden ascendente (1), de modo que el orderDate más antiguo sea el primero.

• output sets the stdDevSampQuantityForState field to the sample standard deviation of the quantity values using $stdDevSamp that is run in a documents window.

  The window contains documents between an unbounded lower limit and the current document in the output. This means $stdDevSamp returns the sample standard deviation of the quantity values for the documents between the beginning of the partition and the current document.

En esta salida, se muestra el valor quantity de la desviación estándar de la muestra para CA y WA en el campo stdDevSampQuantityForState:

{ _id: 4, type: "strawberry", orderDate: ISODate("2019-05-18T16:09:01Z"),
  state: "CA", price: 41, quantity: 162, stdDevSampQuantityForState: null }
{ _id: 0, type: "chocolate", orderDate: ISODate("2020-05-18T14:10:30Z"),
  state: "CA", price: 13, quantity: 120, stdDevSampQuantityForState: 29.698484809834994 }
{ _id: 2, type: "vanilla", orderDate: ISODate("2021-01-11T06:31:15Z"),
  state: "CA", price: 12, quantity: 145, stdDevSampQuantityForState: 21.1266025033211 }
{ _id: 5, type: "strawberry", orderDate: ISODate("2019-01-08T06:12:03Z"),
  state: "WA", price: 43, quantity: 134, stdDevSampQuantityForState: null }
{ _id: 3, type: "vanilla", orderDate: ISODate("2020-02-08T13:13:23Z"),
  state: "WA", price: 13, quantity: 104, stdDevSampQuantityForState: 21.213203435596427 }
{ _id: 1, type: "chocolate", orderDate: ISODate("2021-03-20T11:30:05Z"),
  state: "WA", price: 14, quantity: 140, stdDevSampQuantityForState: 19.28730152198591 }