Window Functions are the "Swiss Army Knife" of T-SQL. They allow you to perform calculations across a set of table rows that are related to the current row, without collapsing the rows into a single result (unlike GROUP BY).
SELECT Name, Salary,
RANK() OVER(ORDER BY Salary DESC) as Rank
FROM EmployeesHow do you compare 'Today's Sales' with 'Yesterday's Sales' in one row? You use LAG() to peek at the previous row. This eliminates the need for expensive self-joins.
Q: "What is the purpose of the 'PARTITION BY' clause in a window function?"
Architect Answer: "It is like a 'Local Group By'. Instead of ranking everyone in the company, `PARTITION BY DepartmentID` will reset the ranking for every department. Department A will have a Rank 1, and Department B will also have a Rank 1. It allows you to group logic while still seeing every individual row."