aggregate() - Retrieve JSON documents command

Runs a sequence of tasks to retrieve documents from a collection, including attributes and calculated values. Tasks can occur multiple times.

Syntax diagram

Command parameters

task

This parameter specifies one or more of the following options:

$distinct: Retrieves distinct values for specific fields.
$group: Sets a grouping key and collects grouped values.
$limit: Returns at most the specified number of documents from the result set.
$match: Filters documents.
$project: Selects fields for the document to retrieve. You can specify fields to include, not exclude.
$skip: Skips the specified number of documents.
$sort: Sorts the data on the specified fields.
$unwind: Returns a stream of individual documents from the elements of an array.

Example

Calculate the average price of books per author in a category.

db.books.aggregate(
      {$match:   { category: "Fantasy" }}, 
      {$project: {author:1, price:1}},
      {$group:   {_id: {author:1}, avgPrice:{$avg:"$price"}}} 
   )

Sample output that is as follows:

Row 1:
 {
"_id":"Tolkien, J.R",
"avgPrice":15.5
 }
Row 2:
 {
"_id":"Verne, Jules",
"avgPrice":6.35
 }
2 rows returned