Join Audit Cards
Remember the four checks that make joins trustworthy.
Check 1: row preservation Compare row counts before and after the join. Unexpected drops or multiplication need explanation. Check 2: unmatched keys List left-only and right-only keys by source and value. Unmatched records show coverage and timing issues. One-to-one join vs many-to-many join Never aggregate money after an accidental many-to-many join. “The join ran, so it is fine.” A join running only proves syntax. I need unmatched, duplicate-key, and metric-variance checks before we trust it. SQL success can hide business failure. It distinguishes technical execution from data correctness.
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