Pivot data without date libraries

Q: Pivot data without date libraries

This is a Data Manipulation (SQL/Python) interview question from Instacart for Software Engineer roles. View the full question and solution on PracHub.

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Question

Transform a tall dataset into a wide, pivoted view without using any date/time libraries. Input: a list of records (date, store_id, metric, value) where date is an ISO 8601 string YYYY-MM-DD, metric ∈ {"sales", "refunds", "visits"}, and value is a non-negative integer. Output: for each (store_id, date), produce one row with columns [store_id, date, sales, refunds, visits], filling missing metric values with 0 and ordering rows by ascending date within each store. Requirements: (

Implement this in code (any language), but you may not import date/time packages; rely only on string operations for ordering dates. (
Ensure the solution is O(n log n) or better with respect to the number of records n. (
Explain how you would handle malformed dates, duplicate (store_id, date, metric) entries, and very large inputs that do not fit in memory.

Pivot data without date libraries

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