What is a MERGE (upsert), and how do you use it for incremental loads and SCDs?
MERGE does insert, update, or delete in a single statement, the foundation of incremental loads and slowly-changing dimensions. The signal is the matched/not-matched logic and what keeps a retry safe.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
MERGE does insert, update, or delete in a single statement, the foundation of incremental loads and slowly-changing dimensions. The signal is the matched/not-matched logic and what keeps a retry safe.
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.
