SQL Window Functions are a feature of SQL that allow you to perform calculations across rows within a set of rows, called a "window". The window is defined using the "OVER" clause and can be used to perform operations such as ranking, cumulative sums, running totals, and more. These functions provide a more concise and efficient way to perform calculations that would otherwise require complex subqueries or self-joins.
Here are a few examples of how SQL Window Functions can be used:
- RANKING: Assigning a unique rank to each row within a set of rows. For example, to assign a rank to each player based on their scores in a game:
SELECT player, score, RANK() OVER (ORDER BY score DESC) as rank FROM scores;
- CUMULATIVE SUM: Calculating the sum of values for a given column across a set of rows. For example, to calculate the cumulative sum of sales for each day in a month:
SELECT date, sales, SUM(sales) OVER (ORDER BY date) as cumulative_sales FROM sales_data;
- RUNNING TOTAL: Similar to cumulative sum, but only considers the rows up to and including the current row. For example, to calculate the running total of expenses for each month:
SELECT month, expenses, SUM(expenses) OVER (ORDER BY month) as running_total FROM expenses_data;
- ROW NUMBER: Assigning a unique number to each row within a set of rows. For example, to assign a unique number to each order in a database:
SELECT order_id, ROW_NUMBER() OVER (ORDER BY order_id) as order_number FROM orders;
These are just a few examples of the many types of calculations that can be performed using SQL Window Functions.