mongoDB : $avg (aggregation)$avg (aggregation)

$avg returns the average value of 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.

Syntax

{ $avg: <expression> }

> db.products.find()
{ "_id" : 1, "name" : "Nokia X3", "category" : "cellphone", "manufacturer" : "Nokia", "price" : 103 }
{ "_id" : 2, "name" : "Dell Inspiron i3147-3750", "category" : "laptop", "manufacturer" : "DELL", "price" : 359 }
{ "_id" : 3, "name" : "Nokia Lumia 521", "category" : "cellphone", "manufacturer" : "Nokia", "price" : 65 }
{ "_id" : 4, "name" : "Google Nexus 9 tablet", "category" : "tablet", "manufacturer" : "Google", "price" : 194 }
{ "_id" : 5, "name" : "Apple MacBook Air MD711LL/B", "category" : "laptop", "manufacturer" : "Apple", "price" : 854 }
{ "_id" : 6, "name" : "Apple MacBook Air MD760LL/B", "category" : "laptop", "manufacturer" : "Apple", "price" : 859 }
{ "_id" : 7, "name" : "Samsung Galaxy S III", "category" : "cellphone", "manufacturer" : "Samsung", "price" : 199 }
{ "_id" : 8, "name" : "Samsung Galaxy S5, White", "category" : "cellphone", "manufacturer" : "Samsung", "price" : 569 }
{ "_id" : 9, "name" : "Samsung Galaxy S2, White", "category" : "cellphone", "manufacturer" : "Samsung", "price" : 469 }
{ "_id" : 10, "name" : "Google Nexus 10 tablet", "category" : "tablet", "manufacturer" : "Google", "price" : 254 }
{ "_id" : 11, "name" : "Nokia x2", "category" : "cellphone", "manufacturer" : "Nokia", "price" : 93 }
{ "_id" : 12, "name" : "Apple iPhone 4", "category" : "cellphone", "manufacturer" : "Apple", "price" : 121 }
{ "_id" : 13, "name" : "Apple iPhone 5", "category" : "cellphone", "manufacturer" : "Apple", "price" : 409 }
{ "_id" : 14, "name" : "Apple iPhone 6", "category" : "cellphone", "manufacturer" : "Apple", "price" : 695 }

1. Get average sum of all products by category

> db.products.aggregate([{"$group" : {"_id" : "$category", "Total amount" : {"$avg" : "$price"}}}])
{ "_id" : "laptop", "Total amount" : 690.6666666666666 }
{ "_id" : "tablet", "Total amount" : 224 }
{ "_id" : "cellphone", "Total amount" : 302.55555555555554 }

2.Get average sum of all products by manufacturer.

> db.products.aggregate([{"$group" : {"_id" : "$manufacturer", "Total amount" : {"$avg" : "$price"}}}])
{ "_id" : "Samsung", "Total amount" : 412.3333333333333 }
{ "_id" : "Apple", "Total amount" : 587.6 }
{ "_id" : "Google", "Total amount" : 224 }
{ "_id" : "DELL", "Total amount" : 359 }
{ "_id" : "Nokia", "Total amount" : 87 }

3.Get average sum of all products by manufacturer and category.

> db.products.aggregate([{"$group" : {"_id" : {"manufacturer" : "$manufacturer", "product_category" : "$category"}, "Total amount" : {"$avg" : "$price"}}}])
{ "_id" : { "manufacturer" : "Samsung", "product_category" : "cellphone" }, "Total amount" : 412.3333333333333 }
{ "_id" : { "manufacturer" : "Apple", "product_category" : "laptop" }, "Total amount" : 856.5 }
{ "_id" : { "manufacturer" : "Google", "product_category" : "tablet" }, "Total amount" : 224 }
{ "_id" : { "manufacturer" : "Apple", "product_category" : "cellphone" }, "Total amount" : 408.3333333333333 }
{ "_id" : { "manufacturer" : "DELL", "product_category" : "laptop" }, "Total amount" : 359 }
{ "_id" : { "manufacturer" : "Nokia", "product_category" : "cellphone" }, "Total amount" : 87 }

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