1. sum
{ "_id" : ObjectId("50b1aa983b3d0043b51b2c52"), "name" : "Nexus 7", "category" : "Tablets", "manufacturer" : "Google", "price" : 199 }
select manufacturer,count(*) from xx
group by manufacturer
db.products.aggregate([
{$group:
{
_id:"$manufacturer",
num_products:{$sum:1}
}
}
])
result
{ "_id" : "Amazon", "num_products" : 2 }
{ "_id" : "Sony", "num_products" : 1 }
{ "_id" : "Samsung", "num_products" : 2
{ "_id" : "Google", "num_products" : 1 }
{ "_id" : "Apple", "num_products" : 4 }