Window Function

Definitions

1. A SQL function that performs a calculation across a set of rows related to the current row, without collapsing them into a single result (unlike GROUP BY). Functions include ROW_NUMBER(), RANK(), LAG(), LEAD(), SUM() OVER(), and running totals. The PARTITION BY clause defines groups and ORDER BY defines the window frame.

   In plain English: SQL functions that can look at surrounding rows while keeping every row in the result — like having GROUP BY without actually grouping.

   Example: "SELECT name, salary, RANK() OVER (PARTITION BY department ORDER BY salary DESC) — ranks employees within each department without losing individual rows."

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