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MongoDB: average value of field

To calculate the average value of a field in MongoDB using the shell, the aggregation framework with the $avg operator is utilized.
Example 1: Calculating the average of a field across the entire collection
Assume a collection named products with documents like:
Code
{ "_id": 1, "item": "A", "price": 10 }
{ "_id": 2, "item": "B", "price": 15 }
{ "_id": 3, "item": "C", "price": 20 }
{ "_id": 4, "item": "D", "price": 12 }
To find the average price of all products:
JavaScript
db.products.aggregate([
  {
    $group: {
      _id: null, // Group all documents into a single group
      averagePrice: { $avg: "$price" } // Calculate the average of the 'price' field
    }
  }
])
This query would return a result similar to:
Code
{ "_id": null, "averagePrice": 14.25 }
Example 2: Calculating the average of a field grouped by another field
To find the average price grouped by item:
JavaScript
db.products.aggregate([
  {
    $group: {
      _id: "$item", // Group documents by the 'item' field
      averagePrice: { $avg: "$price" } // Calculate the average of 'price' for each group
    }
  }
])
If the products collection contains:
Code
{ "_id": 1, "item": "A", "price": 10 }
{ "_id": 2, "item": "B", "price": 15 }
{ "_id": 3, "item": "A", "price": 20 }
{ "_id": 4, "item": "B", "price": 12 }
This query would return a result similar to:
Code
[
  { "_id": "B", "averagePrice": 13.5 },
  { "_id": "A", "averagePrice": 15 }
]

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