Mastering SQL Window Functions: A Complete Guide

Introduction

Window functions are one of the most powerful features in SQL for analytical workloads. Unlike aggregate functions that collapse rows, window functions perform calculations across a set of rows related to the current row.

ROW_NUMBER vs RANK vs DENSE_RANK

Understanding the differences between these ranking functions is crucial for any data analyst.

SELECT 
  employee_name,
  department,
  salary,
  ROW_NUMBER() OVER (PARTITION BY department ORDER BY salary DESC) AS row_num,
  RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS rank,
  DENSE_RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS dense_rank
FROM employees;

Practical Use Cases

• Running totals and moving averages
• Top N per group analysis
• Period-over-period comparisons
• Cohort analysis

Conclusion

Mastering window functions will significantly improve your SQL proficiency and make complex analytical queries much more readable.