How to Use the CASE Statement in SQL for Conditional Logic

The CASE statement in SQL allows you to implement conditional logic within queries, making it a powerful tool for handling complex data transformations and classifications.

Basic Syntax

The CASE statement works like an IF-ELSE structure, evaluating conditions and returning corresponding values:

SELECT column_name,
       CASE
           WHEN condition1 THEN result1
           WHEN condition2 THEN result2
           ELSE default_result
       END AS alias_name
FROM table_name;

Example: Categorizing Data

Assume we have an Employees table with an Age column, and we want to categorize employees based on their age groups:

SELECT Name, Age,
       CASE
           WHEN Age < 25 THEN 'Young'
           WHEN Age BETWEEN 25 AND 40 THEN 'Mid-Age'
           ELSE 'Senior'
       END AS AgeCategory
FROM Employees;

Output Example:

Using CASE in Aggregations

CASE is often used in aggregate functions to perform conditional counting or summing:

SELECT
    SUM(CASE WHEN Age < 25 THEN 1 ELSE 0 END) AS YoungCount,
    SUM(CASE WHEN Age BETWEEN 25 AND 40 THEN 1 ELSE 0 END) AS MidAgeCount,
    SUM(CASE WHEN Age > 40 THEN 1 ELSE 0 END) AS SeniorCount
FROM Employees;

Using CASE in ORDER BY

You can use CASE to customize sorting order dynamically:

SELECT Name, Age
FROM Employees
ORDER BY
    CASE
        WHEN Age < 25 THEN 1
        WHEN Age BETWEEN 25 AND 40 THEN 2
        ELSE 3
    END;

Conclusion

The CASE statement is a versatile tool in SQL for implementing conditional logic in SELECT, WHERE, ORDER BY, and aggregate functions. It enhances query flexibility, making data classification and transformation more efficient.