Window Functions in SQL

SQL window functions are essential for advanced data analysis and database management. It is a type of function that allows us to perform calculations across a specific set of rows related to the current row. These calculations happen within a defined window of data and they are particularly useful for aggregates, rankings and cumulative totals without modifying the dataset.

The OVER clause is a key for defining this window. It partitions the data into different sets (using the PARTITION BY clause) and orders them (using the ORDER BY clause). These windows enable functions like SUM(), AVG(), ROW_NUMBER(), RANK() and DENSE_RANK() to be applied in an organized manner.

Syntax 

SELECT column_name1,
window_function(column_name2)
OVER([PARTITION BY column_name1] [ORDER BY column_name3]) AS new_column
FROM table_name;

Key Terms

• window_function= any aggregate or ranking function
• column_name1= column to be selected
• column_name2= column on which window function is to be applied
• column_name3= column on whose basis partition of rows is to be done
• new_column= Name of new column
• table_name= Name of table

Types of Window Functions in SQL

SQL window functions can be categorized into two primary types: aggregate window functions and ranking window functions. These two types serve different purposes but share a common ability to perform calculations over a defined set of rows while retaining the original data. The employee table contains details about employees, such as their name, age, department and salary.

Employees Table

1. Aggregate Window Function

Aggregate window functions calculate aggregates over a window of rows while retaining individual rows. Common aggregate functions include:

• SUM(): Sums values within a window.
• AVG(): Calculates the average value within a window.
• COUNT(): Counts the rows within a window.
• MAX(): Returns the maximum value in the window.
• MIN(): Returns the minimum value in the window.

Example: Using AVG() to Calculate the Average Salary within each department

SELECT Name, Age, Department, Salary, 
 AVG(Salary) OVER( PARTITION BY Department) AS Avg_Salary
 FROM employee

Output

Explanation:

• The AVG() function calculates the average salary for each department using the PARTITION BY Department clause.
  
• The average salary is repeated for all rows in the respective department.

2. Ranking Window Functions

These functions provide rankings of rows within a partition based on specific criteria. Common ranking functions include:

• RANK(): Assigns ranks to rows, skipping ranks for duplicates.
• DENSE_RANK(): Assigns ranks to rows without skipping rank numbers for duplicates.
• ROW_NUMBER(): Assigns a unique number to each row in the result set.

RANK() Function

The RANK() function assigns ranks to rows within a partition, with the same rank given to rows with identical values. If two rows share the same rank, the next rank is skipped. 

Example: Using RANK() to Rank Employees by Salary

SELECT Name, Department, Salary,
       RANK() OVER(PARTITION BY Department ORDER BY Salary DESC) AS emp_rank
FROM employee;

Output

Explanation:

Rows with the same salary (e.g., Ramesh and Suresh) are assigned the same rank. The next rank is skipped (e.g., rank 2) due to duplicate ranks.

DENSE_RANK() Function

It assigns rank to each row within partition. Just like rank function first row is assigned rank 1 and rows having same value have same rank. The difference between RANK() and DENSE_RANK() is that in DENSE_RANK(), for the next rank after two same rank, consecutive integer is used, no rank is skipped. 

Example:

SELECT Name, Department, Salary,
       DENSE_RANK() OVER(PARTITION BY Department ORDER BY Salary DESC) AS emp_dense_rank
FROM employee;

Output

Explanation: The DENSE_RANK() function works similarly to RANK(), but it doesn't skip rank numbers when there are ties. For example, if two employees have the same salary, both will receive rank 1, and the next employee will receive rank 2.

ROW_NUMBER() Function

ROW_NUMBER() gives e­ach row a unique number. It numbers rows from one­ to the total rows. The rows are put into groups base­d on their values. Each group is called a partition. In e­ach partition, rows get numbers one afte­r another. No two rows have the same­ number in a partition.

Example: Using ROW_NUMBER() for Unique Row Numbers

SELECT Name, Department, Salary,
       ROW_NUMBER() OVER(PARTITION BY Department ORDER BY Salary DESC) AS emp_row_no
FROM employee;

Output

Explanation: ROW_NUMBER() assigns a unique number to each employee based on their salary within the department. No two rows will have the same row number.

Practical Use Cases for Window Functions

Window functions are extremely versatile and can be used in a variety of practical scenarios. Below are some examples of how these functions can be applied in real-world data analysis.

Example 1: Calculating Running Totals

We want to calculate a running total of sales for each day without resetting the total, every time a new day starts.

SELECT Date, Sales,
       SUM(Sales) OVER(ORDER BY Date) AS Running_Total
FROM sales_data;

Explanation: This query calculates the cumulative total of sales for each day, ordered by date.

Example 2: Finding Top N Values in Each Category

We need to retrieve the top 3 employees in each department based on their salary.

WITH RankedEmployees AS (
    SELECT Name, Department, Salary,
           RANK() OVER(PARTITION BY Department ORDER BY Salary DESC) AS emp_rank
    FROM employee
)
SELECT Name, Department, Salary
FROM RankedEmployees
WHERE emp_rank <= 3;

Explanation: This query retrieves the top 3 employees per department based on salary. It uses RANK() to rank employees within each department and filters for the top 3

Troubleshooting Common Issues with Window Functions

While SQL window functions are incredibly powerful, there are some common pitfalls and challenges that users may encounter:

• Partitioning Error: Ensure that the PARTITION BY clause is used correctly. If no partition is defined, the entire result set is treated as a single window.
  
• ORDER BY Within the Window: The ORDER BY clause within the window function determines the order of calculations. Always verify that it aligns with the logic of your calculation.
  
• Performance Considerations: Window functions can be computationally expensive, especially on large datasets. Always ensure that your window functions are optimized and, if necessary, combined with appropriate indexes.

Conclusion

SQL window functions are a crucial feature for advanced data analysis and provide flexibility when working with partitioned data. By mastering the OVER, PARTITION BY and ORDER BY clause, we can perform complex calculations like aggregate calculations, ranking and cumulative totals while preserving the row-level data. The combination of window functions with ORDER BY and PARTITION BY provides a flexible approach for data manipulation across different types of datasets.