Keeping One Row Per Key

Concepts covered: sqlWindowDedup

ROW_NUMBER + filter only works if you can tell the engine which copy to pick. The tiebreaker in the ORDER BY is the business decision. Different tiebreakers produce different answers. The candidate at this level is being scored on whether they ask the tiebreaker question before writing the query, and whether they pick a deterministic tiebreaker that produces the same result on every run. The tiebreaker is a business decision 'Keep the most recent row' is the default for many dedup questions, but other tiebreakers come up: keep the row with the highest version number, keep the row with the most complete data (fewest NULLs), keep the row that came from the highest-trust source, keep the row with the largest amount, keep the row with the most recent status of 'active.' Each tiebreaker is a di

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This section is part of the Deduplication: Beginner 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 PostgreSQL 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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