Calculate the Median Stock Price by Month in PostgreSQL 14

A step-by-step guide on how to extract the median stock price for each month using PostgreSQL 14, including the proper use of SQL functions and grouping techniques.
---
This video is based on the question https://stackoverflow.com/q/69962973/ asked by the user 'moth' ( https://stackoverflow.com/u/8176763/ ) and on the answer https://stackoverflow.com/a/69963208/ provided by the user 'LukStorms' ( https://stackoverflow.com/u/4003419/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

Visit these links for original content and any more details, such as alternate solutions, latest updates/developments on topic, comments, revision history etc. For example, the original title of the Question was: get median value from a group by query in postgresql 14

Also, Content (except music) licensed under CC BY-SA https://meta.stackexchange.com/help/licensing
The original Question post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license, and the original Answer post is licensed under the 'CC BY-SA 4.0' ( https://creativecommons.org/licenses/by-sa/4.0/ ) license.

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
How to Calculate the Median Stock Price by Month in PostgreSQL 14

When working with financial data, one common requirement is to analyze stock prices to derive meaningful statistics. If you're using PostgreSQL 14 and want to calculate the median stock price for every month in a specific year (like 2000), you might run into some challenges. In this post, we'll guide you through the process of accurately extracting this information from your database.

Understanding the Problem

You have a dataset that includes two important columns:

trading_day: Contains date-type information for each trading day.

stock_price: Contains stock prices listed as double precision numbers.

Your goal is to find the median stock price for each month throughout the year 2000. The median is a valuable statistical measure because it provides a better idea of central tendency when dealing with outliers or highly skewed data.

The Initial Query

You might start off with a query like this:

[[See Video to Reveal this Text or Code Snippet]]

However, while your logic is correct in grouping by month, the use of median() might not work as expected depending on your PostgreSQL version. So, let’s explore a more accurate way to calculate the median using PostgreSQL 14.

The Solution

In PostgreSQL 14, a standard SQL method for calculating the median is through the use of the percentile_cont() function. This function computes the continuous percentile, allowing you to determine the median with precision. Here’s how you can rewrite your query:

[[See Video to Reveal this Text or Code Snippet]]

Breakdown of the Query

date_trunc('month', trading_day): This function truncates the trading day to the first day of the month, effectively grouping your data by month.

percentile_cont(0.5) WITHIN GROUP (ORDER BY stock_price): This is the key function that calculates the median. By specifying 0.5, you are requesting the 50th percentile, which gives you the median stock price.

WHERE clause: This condition filters out records to only include those from the year 2000.

GROUP BY dt_trunc: This groups the results by the truncated trading day, allowing you to compute the median for each month.

Conclusion

By using the percentile_cont() function, you not only ensure a more accurate calculation of the median but also align with best practices in SQL, especially for newer versions like PostgreSQL 14. With this approach, you can confidently analyze your stock price data and derive meaningful insights. If you have any questions or need further clarification, feel free to reach out!

Видео Calculate the Median Stock Price by Month in PostgreSQL 14 канала vlogize