Aggregations

An aggregation represents a query to a collection that aggregates on records.

They are simple three keys objects:

• An operation that specifies how the data should be aggregated (Count, Sum, Avg, Max, Min)

• A field, that specifies the data that should be aggregated

• Groups, which may be rounded when they are Dates

Supported group rounding operations are Year, Month, Week, Day, and null (let the field as it is).

Count records

The simplest possible query is to count records from a collection.

{ "operation": "Count", "field": null, "groups": [] }

Equivalent in SQL: SELECT COUNT(*) FROM books

Equivalent in Mongo: { $count: 'value' }

Average rating

{ "operation": "Average", "field": "rating", "groups": [] }

Equivalent in SQL: SELECT AVG(rating) FROM books

Equivalent in Mongo: { $group: { _id: null, rating: { $avg: "$rating" } } }

Average rating by author

{ "operation": "Average", "field": "rating", "groups": [{ "field": "author:name" }] }

Equivalent in SQL: SELECT authorName, AVG(rating) FROM books GROUP BY 1

Equivalent in Mongo: { $group: { _id: "$authorName", rating: { $avg: "$rating" } } }

Average rating by author and year

{
  "operation": "Average",
  "field": "rating",
  "groups": [{ "field": "authorName" }, { "field": "createdAt", "operation": "Year" }]
}

Equivalent in SQL: SELECT authorName, TO_YEAR(createdAt), AVG(rating) FROM books GROUP BY 1, 2

Equivalent in Mongo: { $group: { _id: { author: "$authorName", created: { $year: "$createdAt" }, rating: { $avg: "$rating" } } }