Difference between revisions of "Sum and Count"

Revision as of 15:33, 3 December 2015

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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}}

)

pp.pprint(temp['results'])

@@ 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="min-height:25em">
@@ Line 22: / Line 6: @@
 mapReduce can produce a sum or a count during the results stage by using JavaScript.
 </div>
-<div class="q nonum" data-lang="py3">.count()
+<div class="q nonum" data-lang="mongo">.count()
-<div class="def">print(db.world.count({"continent":"Africa"}))
+<div class="def">db.world.count({"continent":"Africa"})
 </div>
 </div>
-<div class="q nonum" data-lang="py3">$sum
+<div class="q nonum" data-lang="mongo">$sum
-<div class="def">pp.pprint(list(
+<div class="def">
-    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}
-        }}
+    }}
-    ])
+])
-))
 </div>
 </div>
-<div class="q nonum" data-lang="py3">Array.sum()
+<div class="q nonum" data-lang="mongo">Array.sum()
-<div class="def">temp = db.world.map_reduce(
+<div class="def">
-     map=Code("function(){emit(this.continent, this.population)}"),
+db.world.mapReduce(
-     reduce=Code("""function(k,v){
+     function(){emit(this.continent, this.population)},
-                       return Array.sum(v)
+     function(k,v){ return Array.sum(v) },
-                   }
+     {out:{inline:1}}
-                """),
-     out={"inline":1}
 )