What is a Window Function?

Concepts covered: sqlPartitionBy

What Window Functions Solve Consider a common analytical question: "Show me each employee's salary alongside the average salary for their department." This requires two things at once: individual row detail (each employee's name and salary) and aggregate context (the department average). Before window functions existed, you had to write a subquery or a self-join to combine these. Window functions let you do it in a single, clean pass. Comparing Approaches Here is our employee data: GROUP BY Result (Collapsed) Only three rows remain. All employee-level detail is gone. You cannot tell who earns what. Preserved Rows in Results The OVER Clause

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This section is part of the Window Functions: Beginner lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live PostgreSQL database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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