Up and Running with NoSQL Databases
Illustration by John Hersey

Up and Running with NoSQL Databases

with Joseph LeBlanc

Video: Reducing data

Sometimes you just want a summary of a particular dataset. Many NoSQL databases, including CouchDB, allow you to define a reduce function to do this on the server side. Let's navigate to the restaurant database. If you're not already at the All Documents view, go there. I've already loaded in several documents that we're going to apply a Map and a Reduce function to. These documents each represent an item that's going to be on the menu for the restaurant.

Start your free trial now, and begin learning software, business and creative skills—anytime, anywhere—with video instruction from recognized industry experts.

Start Your Free Trial Now
please wait ...
Watch the Online Video Course Up and Running with NoSQL Databases
56m 57s Intermediate Nov 30, 2012

Viewers: in countries Watching now:

Explore the possibilities NoSQL databases offer developers for unparalleled flexibility and performance. In this course, author Joseph LeBlanc shows how to build an entire web application with CouchDB and introduces techniques such as querying with JavaScript, working with nested data, and creating views. He also explains some of the trade-offs you might encounter with NoSQL databases, such as the complexity of partitioning data across multiple servers, and presents five commonly used NoSQL databases: CouchDB, MongoDB, Cassandra, Riak, and Redis.

Topics include:
  • What is NoSQL?
  • Installing CouchDB on Mac, Windows, and Linux
  • Storing data
  • Nesting document data
  • Specifying search criteria
  • Reducing data
  • Attaching and retrieving images
  • Deploying applications
  • Securing CouchDB
  • Understanding the CAP theorem
Subjects:
Developer IT
Software:
NoSQL CouchDB MongoDB Cassandra Riak Redis
Author:
Joseph LeBlanc

Reducing data

Sometimes you just want a summary of a particular dataset. Many NoSQL databases, including CouchDB, allow you to define a reduce function to do this on the server side. Let's navigate to the restaurant database. If you're not already at the All Documents view, go there. I've already loaded in several documents that we're going to apply a Map and a Reduce function to. These documents each represent an item that's going to be on the menu for the restaurant.

Each document has the name of the menu item as well as the category that that menu item belongs to. What I'd like to do with this data is create a reduce function to tell me how many items are in each category. Let's do this by going to the Temporary View. If you already have a function defined here for Map Function, you can just paste over it.

Now I am going to paste in the function and click Run. This Map Function is detecting whether or not the category property is set on the document. We're not going to get the other documents that are pages in the system, we just want the menu items that have the category property. Next, we emit the category as the key, and the document as the value. You'll notice that we have three documents that each have the key entree. This is important because the reduce function is going to be called once for entree, once for dessert, once for beverages, and once for side dish even though entree is listed three times. The reduce function is going to receive all three of these documents in an array.

Now let's paste in a reduce function that will allow us to count the items in each category. Paste the function in. There's a small quirk in the Futon interface that doesn't allow me to run the reduce function until I have both the map function and the reduce function before I hit run. So, click the Refresh button, and then paste in that reduce function and click Run. Now, click Refresh again, and then click Run again.

Now, you'll notice a Reduce check box. Click the check box and the reduce function runs. So the reduce function takes three arguments: key, values, and rereduce. The reduce function gets called once for each key that the map function supplies. So even though three documents had the key of entree, the reduce function was called only once for entree. The values argument is an array that contains all the values that match up to that key.

So, in this case, the values array had three documents for the key entree. Finally, we are returning the value that we want for each key. In this case, we're just returning the length of the array. You can use whatever logic you want here to return sums or other statistics on your values, but at the end, you just want to return one value. Also, whenever you return more than one value, the reduce function is called recursively and rereduce is set to true.

This can help you reduce complex nested datasets. Reduce functions work together with map functions. The map function first retrieves a keyed set of data. Then, the reduce function takes the values mapped to each key, and transforms them into a single value. Using this combination allows you to retrieve summaries directly from the database, saving time and bandwidth.

There are currently no FAQs about Up and Running with NoSQL Databases.

 
Share a link to this course

What are exercise files?

Exercise files are the same files the author uses in the course. Save time by downloading the author's files instead of setting up your own files, and learn by following along with the instructor.

Can I take this course without the exercise files?

Yes! If you decide you would like the exercise files later, you can upgrade to a premium account any time.

Become a member Download sample files See plans and pricing

Please wait... please wait ...
Upgrade to get access to exercise files.

Exercise files video

How to use exercise files.

Learn by watching, listening, and doing, Exercise files are the same files the author uses in the course, so you can download them and follow along Premium memberships include access to all exercise files in the library.


Exercise files

Exercise files video

How to use exercise files.

For additional information on downloading and using exercise files, watch our instructional video or read the instructions in the FAQ .

This course includes free exercise files, so you can practice while you watch the course. To access all the exercise files in our library, become a Premium Member.

* Estimated file size

Are you sure you want to mark all the videos in this course as unwatched?

This will not affect your course history, your reports, or your certificates of completion for this course.


Mark all as unwatched Cancel

Congratulations

You have completed Up and Running with NoSQL Databases.

Return to your organization's learning portal to continue training, or close this page.


OK
Become a member to add this course to a playlist

Join today and get unlimited access to the entire library of video courses—and create as many playlists as you like.

Get started

Already a member ?

Exercise files

Learn by watching, listening, and doing! Exercise files are the same files the author uses in the course, so you can download them and follow along. Exercise files are available with all Premium memberships. Learn more

Get started

Already a Premium member?

Exercise files video

How to use exercise files.

Ask a question

Thanks for contacting us.
You’ll hear from our Customer Service team within 24 hours.

Please enter the text shown below:

The classic layout automatically defaults to the latest Flash Player.

To choose a different player, hold the cursor over your name at the top right of any lynda.com page and choose Site preferences from the dropdown menu.

Continue to classic layout Stay on new layout
Exercise files

Access exercise files from a button right under the course name.

Mark videos as unwatched

Remove icons showing you already watched videos if you want to start over.

Control your viewing experience

Make the video wide, narrow, full-screen, or pop the player out of the page into its own window.

Interactive transcripts

Click on text in the transcript to jump to that spot in the video. As the video plays, the relevant spot in the transcript will be highlighted.

Learn more, save more. Upgrade today!

Get our Annual Premium Membership at our best savings yet.

Upgrade to our Annual Premium Membership today and get even more value from your lynda.com subscription:

“In a way, I feel like you are rooting for me. Like you are really invested in my experience, and want me to get as much out of these courses as possible this is the best place to start on your journey to learning new material.”— Nadine H.

Thanks for signing up.

We’ll send you a confirmation email shortly.


Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

Keep up with news, tips, and latest courses with emails from lynda.com.

Sign up and receive emails about lynda.com and our online training library:

Here’s our privacy policy with more details about how we handle your information.

   
submit Lightbox submit clicked
Terms and conditions of use

We've updated our terms and conditions (now called terms of service).Go
Review and accept our updated terms of service.