Difference between revisions of "Sum and Count"
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<div class="q nonum" data-lang="mongo"><code>.count()</code> | <div class="q nonum" data-lang="mongo"><code>.count()</code> | ||
| − | <pre class="def"><nowiki>db.world.count({"continent": "Africa"})</nowiki></pre> | + | <pre class="def"><nowiki>db.world.count({"continent": "Africa"});</nowiki></pre> |
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emit(this.continent, this.population); | emit(this.continent, this.population); | ||
}, | }, | ||
| − | function (k, v) { | + | function (k, v) { |
return Array.sum(v); | return Array.sum(v); | ||
}, | }, | ||
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out: {inline: 1} | out: {inline: 1} | ||
} | } | ||
| − | ) | + | ); |
</nowiki></pre> | </nowiki></pre> | ||
</div> | </div> | ||
Latest revision as of 16:41, 18 July 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}
}
);