Difference between revisions of "Sum and Count"
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Revision as of 02:36, 6 April 2018
There are many ways to do this in MongoDB.
count() is a cursor method that takes a query and returns a number equal to the amount of documents that matched the query.
$sum is an aggregation operator availible in the $group stage, that can be used to both sum values and count the number of documents.
mapReduce can produce a sum or a count during the results stage by using JavaScript.
.count()
db.world.count({"continent":"Africa"})
$sum
db.world.aggregate([
{"$group":{
"_id": "$continent",
"sum of populations": {$sum: "$population"},
"count of countries": {$sum: 1}
}}
])
Array.sum()
db.world.mapReduce(
function(){
emit(this.continent, this.population);
},
function(k,v){
return Array.sum(v);
},
{
out:{inline:1}
}
)