How Can I Calculate Percentages of Distinct Values in SQL?

Mastering Percentage Calculations in SQL: A Practical Guide

Analyzing data distributions is a fundamental aspect of data analysis. A key task involves calculating the percentage of each distinct value within a specific column. This is especially useful for understanding the frequency of different categories or responses.

This guide explores various SQL techniques for calculating percentages, particularly when dealing with text-based data like grades, where unexpected values might occur.

Method 1: Leveraging the OVER() Clause

The OVER() clause offers an efficient way to calculate percentages within a defined window of rows. Here's how to calculate the percentage of each grade relative to the total number of grades:

SELECT Grade, COUNT(*) * 100.0 / SUM(COUNT(*)) OVER ()
FROM MyTable
GROUP BY Grade;

Method 2: A Universal Approach for Broad Compatibility

This method provides broader SQL compatibility by calculating percentages based on the total number of rows in the table:

SELECT Grade, COUNT(*) * 100.0 / (SELECT COUNT(*) FROM MyTable)
FROM MyTable
GROUP BY Grade;

Method 3: Utilizing Common Table Expressions (CTEs)

While less efficient, CTEs provide another approach to calculate percentages:

WITH t(Grade, GradeCount) AS (
    SELECT Grade, COUNT(*)
    FROM MyTable
    GROUP BY Grade
)
SELECT Grade, GradeCount * 100.0 / (SELECT SUM(GradeCount) FROM t)
FROM t;

These techniques offer flexibility in handling diverse data and provide accurate percentage distributions. Data analysts can use these methods to effectively summarize and interpret data within their tables.

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