db.command.aggregate.stdDevSamp

1. Operator Description

Function: Calculates the sample standard deviation of the input values. If the input values represent the entire population of data, or do not generalize to a larger dataset, use db.command.aggregate.stdDevPop instead.

Declaration: db.command.aggregate.stdDevSamp(<expression>)

2. Operator Parameters

Field	Type	Required	Description
-	Expression	Required	The expression is passed a specified field. stdDevSamp automatically ignores non-numeric values. If all values in the specified field are non-numeric, the result returns null.

3. Sample Code

Suppose the collection students contains the following records:

{ "score": 80 }
{ "score": 100 }

stdDevSamp can be used to calculate the sample standard deviation of scores. The code is as follows:

// Sample code in the Cloud Function environment
const tcb = require('@cloudbase/node-sdk')
const app = tcb.init({
  env: 'xxx'
})

const db = app.database()
const $ = db.command.aggregate
const _ = db.command

exports.main = async (event, context) => {
  const res = await db
    .collection('students')
    .aggregate()
    .group({
      _id: null,
      ageStdDev: $.stdDevSamp('$score')
    })
    .end()
  console.log(res.data)
}

The returned data result is as follows:

{ "_id": null, "ageStdDev": 14.142135623730951 }

If a new record is added to the collection students with its score field being of type string:

{ "score": "aa" }

When calculating the sample standard deviation with the above code, stdDevSamp automatically ignores records whose type is not number, and the returned result remains unchanged.