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What is a window function in SQL Server, and how is it different from a regular aggregate function used with GROUP BY? What is the difference between RANK, DENSE_RANK, and ROW_NUMBER when there are tied values in the ordering column? How do LAG and LEAD window functions let you access values from a previous or following row without needing a self join? How does the PARTITION BY clause within a window function differ from GROUP BY, and why is this distinction useful? How does the ROWS versus RANGE frame specification within an OVER clause affect the result of a window function like a running total? How would you use window functions together to build a comprehensive sales analytics report showing running totals, rankings, and period over period comparisons all in a single query?
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What does the ROW_NUMBER function do, and what is a common practical use case for it?

Beginner

Answer

ROW_NUMBER assigns a unique, sequential number to each row within a defined partition, based on a specified order, which is commonly used for tasks like identifying and removing duplicate records, or implementing pagination by numbering rows so you can easily retrieve a specific range, such as the next set of results for a page of a report.
SELECT OrderId, CustomerId, OrderDate,
  ROW_NUMBER() OVER (ORDER BY OrderDate DESC) AS RowNum
FROM Orders;
Real-world example A pagination feature in a reporting application uses ROW_NUMBER to number all orders by date, letting it easily retrieve exactly rows twenty one through forty for the second page of results.

Common follow-ups: What is the difference between ROW_NUMBER, RANK, and DENSE_RANK?;Does ROW_NUMBER guarantee the same numbering every time if there are ties in the ORDER BY column?

Common Table Expressions (CTEs);Pivoting & Unpivoting Data
All 7 Window Functions questions