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Difference between revisions of "Sum and Count"

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<code>count()</code> is a cursor method that takes a query and returns a number equal to the amount of documents that matched the query.<br/>
 
<code>count()</code> is a cursor method that takes a query and returns a number equal to the amount of documents that matched the query.<br/>
 
<code>$sum</code> is an aggregation operator availible in the <code>$group</code> stage, that can be used to both sum values and count the number of documents.<br/>
 
<code>$sum</code> is an aggregation operator availible in the <code>$group</code> stage, that can be used to both sum values and count the number of documents.<br/>
mapReduce can produce a sum or a count during the results stage by using JavaScript.
+
<code>mapReduce</code> can produce a sum or a count during the results stage by using JavaScript.
</div>
 
<div class="q nonum" data-lang="mongo">.count()
 
<div class="def">db.world.count({"continent":"Africa"})
 
 
</div>
 
</div>
 +
<div class="q nonum" data-lang="mongo"><code>.count()</code>
 +
<pre class="def"><nowiki>db.world.count({"continent": "Africa"});</nowiki></pre>
 
</div>
 
</div>
  
<div class="q nonum" data-lang="mongo">$sum
+
<div class="q nonum" data-lang="mongo"><code>$sum</code>
<div class="def">
+
<pre class="def"><nowiki>
 
db.world.aggregate([
 
db.world.aggregate([
    {"$group":{
+
  {"$group": {
        "_id":"$continent",
+
      "_id": "$continent",
        "sum of populations":{$sum:"$population"},
+
      "sum of populations": {$sum: "$population"},
        "count of countries":{$sum:1}
+
      "count of countries": {$sum: 1}
    }}
+
  }}
])
+
]);</nowiki></pre>
</div>
 
 
</div>
 
</div>
  
<div class="q nonum" data-lang="mongo">Array.sum()
+
<div class="q nonum" data-lang="mongo"><code>Array.sum()</code>
<div class="def">
+
<pre class="def"><nowiki>
 
db.world.mapReduce(
 
db.world.mapReduce(
    function(){emit(this.continent, this.population)},
+
  function () {
    function(k,v){ return Array.sum(v) },
+
    emit(this.continent, this.population);
     {out:{inline:1}}
+
  },
)
+
  function (k, v) {
 
+
    return Array.sum(v);
pp.pprint(temp['results'])
+
  },
</div>
+
  {
 +
     out: {inline: 1}
 +
  }
 +
);
 +
</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}
  }
);