$avg - Amazon DocumentDB (original) (raw)

The $avg aggregation operator in Amazon DocumentDB calculates the average value of the specified expression across the documents that are input to the stage. This operator is useful for computing the average of a numeric field or expression across a set of documents.

Parameters

expression: The expression to use to calculate the average. This can be a field path (e.g. "$field") or an expression (e.g. { <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>m</mi><mi>u</mi><mi>l</mi><mi>t</mi><mi>i</mi><mi>p</mi><mi>l</mi><mi>y</mi><mo>:</mo><mo stretchy="false">[</mo><mi mathvariant="normal">"</mi></mrow><annotation encoding="application/x-tex">multiply: ["</annotation></semantics></math>multiply:["field1", "$field2"] }).

Example (MongoDB Shell)

The following example demonstrates how to use the $avg operator to calculate the average score across a set of student documents.

Create sample documents

db.students.insertMany([
  { name: "John", score: 85 },
  { name: "Jane", score: 92 },
  { name: "Bob", score: 78 },
  { name: "Alice", score: 90 }
]);

Query example

db.students.aggregate([
  { $group: { 
    _id: null,
    avgScore: { <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mi>v</mi><mi>g</mi><mo>:</mo><mi mathvariant="normal">&quot;</mi></mrow><annotation encoding="application/x-tex">avg: &quot;</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.625em;vertical-align:-0.1944em;"></span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.03588em;">vg</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">:</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord">&quot;</span></span></span></span>score" }
  }}
]);

Output

[
  {
    "_id": null,
    "avgScore": 86.25
  }
]

Code examples

To view a code example for using the $avg command, choose the tab for the language that you want to use:

Node.js

const { MongoClient } = require('mongodb');

async function calculateAvgScore() {
  const client = await MongoClient.connect('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');
  const db = client.db('test');
  const result = await db.collection('students').aggregate([
    { $group: {
      _id: null,
      avgScore: { <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>a</mi><mi>v</mi><mi>g</mi><msup><mo>:</mo><mo mathvariant="normal" lspace="0em" rspace="0em">′</mo></msup></mrow><annotation encoding="application/x-tex">avg: &#x27;</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.9463em;vertical-align:-0.1944em;"></span><span class="mord mathnormal">a</span><span class="mord mathnormal" style="margin-right:0.03588em;">vg</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel"><span class="mrel">:</span><span class="msupsub"><span class="vlist-t"><span class="vlist-r"><span class="vlist" style="height:0.7519em;"><span style="top:-3.063em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mtight"><span class="mord mtight">′</span></span></span></span></span></span></span></span></span></span></span></span>score' }
    }}
  ]).toArray();
  console.log(result);
  await client.close();
}

calculateAvgScore();

Python

from pymongo import MongoClient

def calculate_avg_score():
    client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
    db = client.test
    result = list(db.students.aggregate([
        { '$group': {
            '_id': None,
            'avgScore': { '$avg': '$score' }
        }}
    ]))
    print(result)
    client.close()

calculate_avg_score()