SQL doesn't warn you when a JOIN multiplies your rows — it just quietly hands back more of them than you expected, and every SUM() and COUNT() downstream inherits the mistake. Here's the exact mechanism, and how to catch it before it ships. Adapted from the SQL Essentials Companion Guide . You write a query to total up each customer's orders, and it runs fine — no error, no red text, just a result set: SELECT customers . name , SUM ( orders . amount ) AS tota...