Difference between revisions of "Sum and Count"

Latest revision as of 16:41, 18 July 2018

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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}
  }
);

@@ Line 1: / Line 1: @@
-<pre class=setup>
-#ENCODING
-import io
-import sys
-sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-16')
-#MONGO
-from pymongo import MongoClient
-client = MongoClient()
-client.progzoo.authenticate('scott','tiger')
-db = client['progzoo']
-#PRETTY
-import pprint
-pp = pprint.PrettyPrinter(indent=4)
-#CODE
-from bson.code import Code
-</pre>
 {{TopTenTips}}
-<div style="height:25em">
+<div style="min-height:25em">
 There are many ways to do this in MongoDB.<br/><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/>
-mapReduce can produce a sum or a count during the results stage.
+<code>mapReduce</code> can produce a sum or a count during the results stage by using JavaScript.
-</div>
-<div class=q data-lang="py3">.count()
-<div class="def">print(db.world.count({"continent":"Africa"}))
 </div>
+<div class="q nonum" data-lang="mongo"><code>.count()</code>
+<pre class="def"><nowiki>db.world.count({"continent": "Africa"});</nowiki></pre>
 </div>
-<div class=q data-lang="py3">$sum
+<div class="q nonum" data-lang="mongo"><code>$sum</code>
-<div class="def">pp.pprint(list(
+<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 class=q data-lang="py3">.count()
+<div class="q nonum" data-lang="mongo"><code>Array.sum()</code>
-<div class="def">temp = db.world.map_reduce(
+<pre class="def"><nowiki>
-    map=Code("function(){emit(this.name, this.population)}"),
+db.world.mapReduce(
-    reduce=Code("""function(k,v){
+  function () {
-                       return Array.sum(v)
+    emit(this.continent, this.population);
-                   }
+  },
-                """),
+  function (k, v) {
-     out={"inline":1}
+    return Array.sum(v);
-)
+  },
+  {
-pp.pprint(temp['results'])
+     out: {inline: 1}
-</div>
+  }
+);
+</nowiki></pre>
 </div>