Explain unmatched records after a join

Inputs: [LEFT SCHEMA], [RIGHT SCHEMA], [MATCH KEY]. Define mutually exclusive matched, left-only, right-only and ambiguous buckets at a stated row or entity grain; do not double-count many-to-many matches. Propose read-only checks for key format, duplicates and timing. Output: A reconciliation table and investigation order. Check: Reconcile each input side separately to its allocated buckets and explain any multiplicity. If essential information is missing, identify it rather than guessing.