Calculating Relative Percentages in PostgreSQL

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When working with vote counts and percentages, understanding how to calculate relative percentages is crucial for making informed decisions. In this article, we will delve into the world of PostgreSQL and explore how to find relative percentages using aggregate functions and filters.

Understanding the Problem

Let’s take a look at a sample dataset with vote counts:

We want to calculate the relative percentage of users who voted “yes” and those who voted “no”. To achieve this, we need to find the total count of votes and then calculate the percentage for each answer.

Solution Overview

The solution involves using PostgreSQL’s aggregate functions and filters to calculate the relative percentages. We will use the FILTER clause, which was introduced in Postgres 9.4, to exclude rows with a specific value. We will also use the round() function to prettify the results.

Step 1: Calculate Total Count

To start, we need to calculate the total count of votes:

SELECT count(*) AS total FROM fast_survey;

This query returns the total number of rows in the fast_survey table.

Step 2: Filter and Group by Answer

Next, we need to filter the rows based on the value of the answer column. We will use the FILTER clause to exclude rows where the answer is not “yes” or “no”. Then, we group the remaining rows by the answer column:

SELECT 
  answer,
  round(count(*) FILTER (WHERE answer = 'yes'::text) * 100.0 / count(*), 2) AS pct_yes,
  round(count(*) FILTER (WHERE answer = 'no'::text) * 100.0 / count(*), 2) AS pct_no
FROM   fast_survey;

In this query, we use the FILTER clause to filter rows where the answer column is either “yes” or “no”. We then calculate the percentage for each answer by dividing the count of votes for that answer by the total count.

Step 3: Round and Format Results

Finally, we round the results using the round() function to get the final percentages:

SELECT 
  answer,
  round(count(*) FILTER (WHERE answer = 'yes'::text) * 100.0 / count(*), 2) AS pct_yes,
  round(count(*) FILTER (WHERE answer = 'no'::text) * 100.0 / count(*), 2) AS pct_no
FROM   fast_survey;

Example Use Case

Here is an example of how to use this query:

Suppose we have the following data in our fast_survey table:

Running the query above will give us the following results:

This shows that 80% of users voted “yes” and 20% voted “no”.

Conclusion

Calculating relative percentages in PostgreSQL can be achieved using aggregate functions and filters. By following the steps outlined above, you can find the total count of votes and calculate the percentage for each answer.

References

• Calculating rates in PostgreSQL
• How can I simplify this game statistics query?

Note: The code snippets provided are written in SQL and are intended to be used in a PostgreSQL database.

Last modified on 2023-12-26