SQL JOIN Fan-Out Silently Doubled Refund Totals in Store Report

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A developer's SQL query ran without errors, yet one customer's refund total came out at $60 when only $30 had been refunded. The cause was a JOIN that matched one customer against multiple rows, so the results multiplied rather than staying one row per customer. The article reproduces the issue with a small sample dataset of customers, orders, and refunds for an online store. It explains that a JOIN's result grain can change when it links to the many side of a one-to-many relationship. The author notes that the mistake comes from forgetting that multiplication occurred, not from a syntax problem.

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