The Chasm Trap: Missing Relationships

Concepts covered: dmManyToMany

Your chasm trap answer: 'A chasm trap is the inverse. Instead of too many rows, I get too few. An INNER JOIN through a sparse dimension silently drops entities that exist on only one side. Bob has orders but no shipments. The INNER JOIN drops Bob entirely. My total orders metric just lost 70% of its value.' Pair this with the fan trap to show you check for both. The Tell: Words That Signal This Pattern The Scenario the Interviewer Will Draw on the Whiteboard Set up the scenario for the interviewer: 'Orders has alice, bob, carol. Shipments has alice, dave. An INNER JOIN returns only alice. Bob and carol are silently dropped. Dave is silently dropped. Your report shows 1 customer instead of 4.' Walk through it on the whiteboard. The visual makes the problem undeniable. Narrate the impact: 'I

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This section is part of the Grain and Fan Traps: Advanced lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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