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You have a wide table with one column per month. How do you unpivot it into tidy (key, month, value) rows?

Wide tables with a column per period are painless to read and painful to query. The signal is unpivoting to long format with UNPIVOT or a UNION/cross-join trick, and knowing how NULLs and types bite you.

Updated Aug 2026 · Grounded in real Applied AI Engineer interview loops and written to a senior-engineer editorial bar.

Wide tables with a column per period are painless to read and painful to query. The signal is unpivoting to long format with UNPIVOT or a UNION/cross-join trick, and knowing how NULLs and types bite you.

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