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SQL query with LAG function.

SQLmedium0.4 min readPremium

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82 wordsInterview-ready

LAG: access previous row. SELECT id, amount, LAG(amount) OVER (ORDER BY date) AS prev_amount, amount - LAG(amount) OVER (ORDER BY date) AS diff FROM sales. Default: LAG(amount, 1, 0) for 0 when no previous. PARTITION BY for per-group: LAG(amount) OVER (PARTITION BY customer_id ORDER BY date). Use for: period-over-period, running differences. **Why it matters**: Design choices compound at scale—wrong approach can cause 100× overhead....

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