Window Function windowing options – with examples in PostgreSQL

More window functions here on the blog? What can I say? The deeper I get into them, the more I realize their power. Yet, I have not covered any of the Windowing options available in the OVER() clause so far and that ends with this post. I will share what I have learned, and what I am still learning. Join me and we can learn together…

Note: All data, names or naming found within the database presented in this post, are strictly used for practice, learning, instruction, and testing purposes. It by no means depicts actual data belonging to or being used by any party or organization.

• Xubuntu Linux 18.04.2 LTS (Bionic Beaver)
• PostgreSQL 11.2

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I’ll use a couple of tables from the PostgreSQL practice DVD Rental database for the example queries below.

Throughout this post, let’s assume we have this CTE that returns the below query results:

Using the FIRST_VALUE() window function, this query returns the ‘first_name’ value for what I like to think of as, the big spender:

However, behind the scenes, there is a Windowing clause in the OVER() clause that can make a significant impact on returned results

Let’s incorporate one in this next query:

Via the explicit, ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING, windowing clause, you can now see what is, default windowing behavior when using ROWS in this instance.

Notice the change in this next query where I use an integer value:

With ROWS BETWEEN 2 PRECEDING, the FIRST_VALUE() function is applied to the current row and that specified number of rows back (preceding) from it.

The CURRENT ROW portion of the windowing clause is what enables (or prohibits) any returned rows beyond the current row, in the results set.

How about 5 ROWS PRECEDING?

Finally, one last example with 15 ROWS PRECEDING:

Up to this point, we have established: 1) Specifying UNBOUNDED PRECEDING envelopes all rows back as far as possible, and the current row. 2) An explicit (integer) ROWS value includes the current row and number of rows preceding.

Once again, I have thoroughly enjoyed learning and writing this post. Look for more blog posts to come here on Digital Owl’s Prose regarding Window Functions and SQL goodness.

Like what you have read? See anything incorrect? Please comment below and thanks for reading!!!

Explore the official PostgreSQL 11 On-line Documentation for more information.

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Josh Otwell has a passion to study and grow as a SQL Developer and blogger. Other favorite activities find him with his nose buried in a good book, article, or the Linux command line. Among those, he shares a love of tabletop RPG games, reading fantasy novels, and spending time with his wife and two daughters.

Disclaimer: The examples presented in this post are hypothetical ideas of how to achieve similar types of results. They are not the utmost best solution(s). The majority, if not all, of the examples provided, is performed on a personal development/learning workstation-environment and should not be considered production quality or ready. Your particular goals and needs may vary. Use those practices that best benefit your needs and goals. Opinions are my own.