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 easy to read and hard 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 Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Wide tables with a column per period are easy to read and hard 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.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
