Implement Slowly Changing Dimension Type 2, keep full history when a customer's attributes change.
Everyone can define SCD2; very few can write the MERGE that maintains it correctly. The expire-and-insert dance, the one-statement trick, and the late-arriving-change trap that breaks naive implementations.
Updated Sep 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.
Everyone can define SCD2; very few can write the MERGE that maintains it correctly. The expire-and-insert dance, the one-statement trick, and the late-arriving-change trap that breaks naive implementations.
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.
