ClickHouse Window Functions: How They Work and When to Use Them

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ClickHouse window functions allow analysts to perform calculations across related rows — such as rankings, running totals, and moving averages — without collapsing the result set the way GROUP BY does. Every window function uses an OVER() clause, which defines row grouping via PARTITION BY, ordering via ORDER BY, and row scope via ROWS BETWEEN. Common functions covered include ROW_NUMBER, RANK, DENSE_RANK, and NTILE for ranking, as well as aggregate functions like SUM and AVG used in windowed mode for running totals and department-level comparisons. These functions are especially useful for analytics tasks like pagination, customer segmentation, and sales performance grading. The guide provides a practical employee sales dataset and step-by-step SQL examples to demonstrate each function's behavior in ClickHouse.

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