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MongoDB is an open-source Document-oriented database.

Description

MongoDB is a database which holds documents in the JSON-like Language:BSON format. A single database may consist of several collections in which documents are organized.

To query a database collection, Language:BSON objects are passed which are matched against collection entries. Certain object keys may also have a special use as operators. A typical query may look like this: 

> db.employees.find({"salary" : {"$lt" : 2000}});

MongoDB also realizes Technology:Data Aggregation using an internal MapReduce implementation based on Language:JavaScript. This works on collection level by passing JavaScript functions for map and reduce steps to the database.

A simple example of aggregating statistical data may be the counting of tags attributed to a blog post or product in a database collection. If the collection contains documents of the form 

{..., tags: ["foo", "bar", ...], ...}

A map function that emits a count for each tag might look like this: 

> var mapfun = function() {
  this.tags.forEach(function(tag) {
    this.emit(tag, {"count" : 1});
  });
}

And a corresponding reduce function that emits the summed up tags will look like this: 

> var reducefun = function(key, values) {
  var total = 0;
  values.forEach(function(value) {
    total += value.count;
  });
  return total;
}

To run everything a MapReduce query is formulated: 

> db.blogposts.mapReduce(mapfun, reducefun {"out" : "output"});

This will return an object containing data about the processed request and save all results to the new collection "output".

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Marcel Heinz edited this article at Wed, 05 Jul 2017 15:06:55 +0200
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