mongoDB : $min operator (aggregation)

$min returns the lowest value that results from applying an expression to each document in a group of documents that share the same group by key.

Syntax

{ $min: <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 minimum price by manufacturer.

> db.products.aggregate([{"$group":{"_id": {"manufacturer":"$manufacturer"}, "min_price" : {"$min" : "$price"}}}])
{ "_id" : { "manufacturer" : "Samsung" }, "min_price" : 199 }
{ "_id" : { "manufacturer" : "Apple" }, "min_price" : 121 }
{ "_id" : { "manufacturer" : "Google" }, "min_price" : 194 }
{ "_id" : { "manufacturer" : "DELL" }, "min_price" : 359 }
{ "_id" : { "manufacturer" : "Nokia" }, "min_price" : 65 }

2. Get minimum price by manufacturer and category

> db.products.aggregate([{"$group":{"_id": {"manufacturer":"$manufacturer","category": "$category"}, "min_price" : {"$min" : "$price"}}}])
{ "_id" : { "manufacturer" : "Samsung", "category" : "cellphone" }, "min_price" : 199 }
{ "_id" : { "manufacturer" : "Apple", "category" : "laptop" }, "min_price" : 854 }
{ "_id" : { "manufacturer" : "Google", "category" : "tablet" }, "min_price" : 194 }
{ "_id" : { "manufacturer" : "Apple", "category" : "cellphone" }, "min_price" : 121 }
{ "_id" : { "manufacturer" : "DELL", "category" : "laptop" }, "min_price" : 359 }
{ "_id" : { "manufacturer" : "Nokia", "category" : "cellphone" }, "min_price" : 65 }

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