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$avg (aggregation)

Definition

$avg

Returns the average value of the numeric values. $avg ignores non-numeric values.

Changed in version 3.2: $avg is available in the $group and $project stages. In previous versions of MongoDB, $avg is available in the $group stage only.

When used in the $group stage, $avg has the following syntax and returns the collective average of all the numeric values that result from applying a specified expression to each document in a group of documents that share the same group by key:

{ $avg: <expression> }

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

  • $avg has one specified expression as its operand:

    { $avg: <expression> }
    
  • $avg has a list of specified expressions as its operand:

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

For more information on expressions, see Expressions.

Behavior

Non-numeric Values

$avg ignores non-numeric values. If all operands for the average are non-numeric, $avg returns null.

Array Operand

In the $group stage, if the expression resolves to an array, $avg 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, $avg 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, $avg does not traverse into the array but instead treats the array as a non-numerical value.

Examples

Use in $group Stage

Consider a sales collection with the following documents:

{ "_id" : 1, "item" : "abc", "price" : 10, "quantity" : 2, "date" : ISODate("2014-01-01T08:00:00Z") }
{ "_id" : 2, "item" : "jkl", "price" : 20, "quantity" : 1, "date" : ISODate("2014-02-03T09:00:00Z") }
{ "_id" : 3, "item" : "xyz", "price" : 5, "quantity" : 5, "date" : ISODate("2014-02-03T09:05:00Z") }
{ "_id" : 4, "item" : "abc", "price" : 10, "quantity" : 10, "date" : ISODate("2014-02-15T08:00:00Z") }
{ "_id" : 5, "item" : "xyz", "price" : 5, "quantity" : 10, "date" : ISODate("2014-02-15T09:12:00Z") }

Grouping the documents by the item field, the following operation uses the $avg accumulator to compute the average amount and average quantity for each grouping.

db.sales.aggregate(
   [
     {
       $group:
         {
           _id: "$item",
           avgAmount: { $avg: { $multiply: [ "$price", "$quantity" ] } },
           avgQuantity: { $avg: "$quantity" }
         }
     }
   ]
)

The operation returns the following results:

{ "_id" : "xyz", "avgAmount" : 37.5, "avgQuantity" : 7.5 }
{ "_id" : "jkl", "avgAmount" : 20, "avgQuantity" : 1 }
{ "_id" : "abc", "avgAmount" : 60, "avgQuantity" : 6 }

Use in $project Stage

A collection students contains the following documents:

{ "_id": 1, "quizzes": [ 10, 6, 7 ], "labs": [ 5, 8 ], "final": 80, "midterm": 75 }
{ "_id": 2, "quizzes": [ 9, 10 ], "labs": [ 8, 8 ], "final": 95, "midterm": 80 }
{ "_id": 3, "quizzes": [ 4, 5, 5 ], "labs": [ 6, 5 ], "final": 78, "midterm": 70 }

The following example uses the $avg in the $project stage to calculate the average quiz scores, the average lab scores, and the average of the final and the midterm:

db.students.aggregate([
   {
     $project: {
       quizAvg: { $avg: "$quizzes"},
       labAvg: { $avg: "$labs" },
       examAvg: { $avg: [ "$final", "$midterm" ] }
     }
   }
])

The operation results in the following documents:

{ "_id" : 1, "quizAvg" : 7.666666666666667, "labAvg" : 6.5, "examAvg" : 77.5 }
{ "_id" : 2, "quizAvg" : 9.5, "labAvg" : 8, "examAvg" : 87.5 }
{ "_id" : 3, "quizAvg" : 4.666666666666667, "labAvg" : 5.5, "examAvg" : 74 }

In the $project stage:

  • With a single expression as its operand, if the expression resolves to an array, $avg 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, $avg does not traverse into the array but instead treats the array as a non-numerical value.