How to Calculate Percentages of Values in a SQL Server Table?

SQL Server Percentage Calculations: Three Approaches

This guide demonstrates three effective methods for calculating percentages of values within a SQL Server table. Each method achieves the same result, offering flexibility depending on your preference and database structure.

Method 1: Window Function Efficiency

This approach leverages the OVER() window function for a concise and efficient solution:

<code class="language-sql">SELECT Grade, COUNT(*) * 100.0 / SUM(COUNT(*)) OVER() AS Percentage
FROM MyTable
GROUP BY Grade;</code>

The OVER() clause calculates the total count across all rows, providing a direct basis for the percentage calculation.

Method 2: Universal Approach

A more straightforward method, suitable for various SQL dialects, involves a subquery:

<code class="language-sql">SELECT Grade, COUNT(*) * 100.0 / (SELECT COUNT(*) FROM MyTable) AS Percentage
FROM MyTable
GROUP BY Grade;</code>

This directly divides the count of each grade by the total row count obtained from the subquery.

Method 3: Common Table Expression (CTE)

For enhanced readability and modularity, a CTE can be used:

<code class="language-sql">WITH GradeCounts AS (
    SELECT Grade, COUNT(*) AS GradeCount
    FROM MyTable
    GROUP BY Grade
)
SELECT Grade, GradeCount * 100.0 / (SELECT SUM(GradeCount) FROM GradeCounts) AS Percentage
FROM GradeCounts;</code>

The CTE first calculates the count for each grade, then the main query computes the percentage using the total count derived from the CTE.

All three methods yield the same output format:

<code>Grade | Percentage
------------------
A      | 5%
B      | 15%
C      | 40%
D      | 40%</code>

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