How to calculate percentages in sql

There are three main ways to calculate percentages using SQL: direct calculation, use window functions, and use CASE expressions. Direct calculations are obtained by dividing the numerator by the denominator and multiplying by 100; the window function allows the percentage to be calculated by the denominator value based on the group; and the CASE expression can calculate different percentages based on the conditions.

How to calculate percentages using SQL

Direct calculation

The most direct way is to use the following syntax:

 <code class="sql">Percentage = (numerator / denominator) * 100</code>

For example, to calculate sales as a percentage of revenue:

 <code class="sql">Sales_Percentage = (Sales / Revenue) * 100</code>

Using window functions

Window functions allow us to use denominator values ​​when calculating percentages by group. The OVER clause specifies the range of the calculation.

For example, use the ROW_NUMBER() function to calculate the percentage of sales in total sales for each department:

 <code class="sql">SELECT Department, Sales, SUM(Sales) OVER() AS TotalSales, (Sales / TotalSales) * 100 AS Sales_Percentage FROM Sales GROUP BY Department</code>

Using CASE expressions

CASE expressions can be used to calculate different percentages based on conditions. For example, calculate the discount percentage based on customer type:

 <code class="sql">SELECT CustomerType, CASE WHEN CustomerType = 'Gold' THEN 0.1 WHEN CustomerType = 'Silver' THEN 0.05 ELSE 0 END AS Discount_Percentage FROM Customers</code>

Things to note

• Make sure the denominator is not zero, otherwise a zero-division error will occur.
• Round or truncate the calculation results as needed.
• Use the appropriate alias as percentage columns for easy identification.

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