$stdDevPop (aggregation)¶

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Definition
Behavior
Examples

Definition¶

$stdDevPop¶

3.2 新版功能.

Calculates the population standard deviation of the input values. Use if the values encompass the entire population of data you want to represent and do not wish to generalize about a larger population. $stdDevPop ignores non-numeric values.

If the values represent only a sample of a population of data from which to generalize about the population, use $stdDevSamp instead.

$stdDevPop is available in the $group and $project stages.

When used in the $group stage, $stdDevPop returns the population standard deviation of the specified expression for a group of documents that share the same group by key and has the following syntax:

$stdDevPop has one specified expression as its operand:
```
{ $stdDevPop: <expression> }
```

When used in the $project stage, $stdDevPop returns the standard deviation of the specified expression or list of expressions for each document and has one of two syntaxes:

$stdDevPop has one specified expression as its operand:
```
{ $stdDevPop: <expression> }
```

$stdDevPop has a list of specified expressions as its operand:

{ $stdDevPop: [ <expression1>, <expression2> ... ]  }

The argument for $stdDevPop can be any expression as long as it resolves to an array. For more information on expressions, see Expressions

Behavior¶

Non-numeric Values¶

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

Single Value¶

If the sample consists of a single numeric value, $stdDevPop returns 0.

Array Operand¶

In the $group stage, if the expression resolves to an array, $stdDevPop treats the operand as a non-numerical value.

In the $project stage:

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.
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.

Examples¶

Use in `$group` Stage¶

A collection named users contains the following documents:

{ "_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:

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

The operation returns the following results:

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

Use in `$project` Stage¶

A collection named quizzes contains the following documents:

{
   "_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
      }
   ]
}