Define Grain Before Joining

The key idea: make table grain inspectable. Name the contract For this topic, the contract is one row meaning, safe joins, and aggregation behavior. Naming it keeps the team from treating every data issue as a vague pipeline failure. Check the boundary Ask where the promise first stopped being true: extract, transform, or load. Boundary checks are faster than editing the first SQL file you see. Preserve evidence Counts, timestamps, reject reasons, keys, and run IDs make the lesson operational. Evidence lets another person verify the same conclusion.

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