Choose a window function instead of GROUP BY when the output must keep one row per event, order, or user.
The row grain is the asset. Protect it. Why GROUP BY feels right and still breaks the report GROUP BY is perfect when the final output truly should be one row per group. It is the wrong tool when the business still wants order-level, event-level, or user-level detail in the final result. In that case, grouping early destroys evidence you still need. What a window changes A window function performs the calculation across related rows while leaving the current row in place. partition by defines the peer set. order by defines sequence. The result is a metric attached to each…
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