How do you calculate a running total in SQL?
This question tests your knowledge of window functions. It specifically looks for the ability to use SUM with OVER clauses to accumulate values.
Why Interviewers Ask This
Running totals are common in financial and analytical reports. Interviewers ask this to verify you know modern SQL features beyond basic aggregation. Using window functions is often more efficient and cleaner than self-joins for this task.
How to Answer This Question
Explain that window functions allow calculations across a set of rows related to the current row. Describe using SUM() with an OVER clause partitioned by category and ordered by time. Mention the frame specification like UNBOUNDED PRECEDING AND CURRENT ROW.
Key Points to Cover
- Use SUM with OVER clause
- Partition by category
- Order by time for accumulation
Sample Answer
To calculate a running total, I use the SUM window function combined with the OVER clause. I partition the data by the relevant category, such as product ID, and order it by the transaction date. By specifying the frame…
Common Mistakes to Avoid
- Using GROUP BY instead of window functions
- Forgetting to specify the ORDER BY in OVER
- Writing inefficient self-joins
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