How would you calculate a running total of sales for each product?
This question tests knowledge of window functions for analytical calculations.
Why Interviewers Ask This
Running totals are common in reporting. Interviewers evaluate your ability to use analytic functions to accumulate data over time or partitions efficiently.
How to Answer This Question
Identify the need for SUM() OVER(). Specify PARTITION BY for grouping by product and ORDER BY for time sequence. Explain ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW.
Key Points to Cover
- Window functions
- Partitioning logic
- Ordering sequence
- Accumulation method
Sample Answer
To calculate a running total, I use a window function like SUM(amount) OVER (PARTITION BY product_id ORDER BY sale_date). This accumulates the sum from the start up to the current row for each product, keeping row detail…
Common Mistakes to Avoid
- Using GROUP BY incorrectly
- Forgetting ORDER BY
- Not partitioning properly
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