The Fan Trap: Joins That Inflate Metrics

Your fan trap answer: 'A fan trap happens when I join two facts through a shared dimension and one side has multiple rows per key. The join fans out, duplicating the other side. My SUM now counts revenue 3x because each order was duplicated once per shipment. No error message, no warning, just wrong numbers that look plausible.' Say 'no error message' explicitly. That is what makes fan traps dangerous and why interviewers test them. The Tell: Words That Signal This Pattern The Scenario the Interviewer Will Draw on the Whiteboard The interviewer gives you two tables with different row counts per join key and asks you to combine them. They are not testing your JOIN syntax. They are watching whether you see the fan trap before you write the query. Candidates who join directly and only notice