title: MongoDb sum query
body: <p>For example I have the following data in MongoDB:</p>
  
  <pre><code>{ "_id" : ObjectId("524091f99c49c4c3f66b0e46"), "hour" : 10, "incoming", 100}
  { "_id" : ObjectId("5240a045dbeff33c7333aa51"), "hour" : 11, "incoming", 200}
  { "_id" : ObjectId("5240a2ecda0d37f35c618aca"), "hour" : 12, "incoming", 300}
  </code></pre>
  
  <p>Now I want to query "SUM the number of incoming between 11 - 12" (the result should be 500), how could I do this using Mongo Shell?  </p>