Knew immediately this was a window function thing but second-guessed myself and started writing a self-join first.
Clarify that the cumulative sum should be computed over all values less than or equal to each number, including duplicates. Use a window function with an ORDER BY on the number and a RANGE frame to include all equal values, or a self-join if window functions are unavailable. Then write a clean SQL query that returns each number and its cumulative sum.
Pro tip: Mention that using RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW correctly handles duplicates by including all rows with the same value, which is often a subtle point. Also, note that if the table has duplicate numbers, the cumulative sum for each duplicate should be the same.
Confirm that the cumulative sum includes all values less than or equal to the current number, and that duplicates should be handled by including all equal values. Ask about nulls, negative numbers, and whether the output should be sorted.
Decide between using a window function (e.g., SUM() OVER (ORDER BY num RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)) or a self-join with GROUP BY. Window functions are more efficient and standard in modern SQL.
Construct the SQL query, ensuring the window frame is correctly specified to include all rows with values <= current value. If using a self-join, join the table to itself on t1.num >= t2.num and group by t1.num.
Mentally or verbally test the query with a small dataset containing duplicates and out-of-order values to verify correctness. Explain how the result changes with duplicates.
Mention that window functions are generally more efficient than self-joins, especially for large datasets. If the database doesn't support window functions, describe the self-join approach.
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