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Given a complex nested query, how would you refactor it for better readability and efficiency?

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**Refactor steps**: (1) Extract nested SELECTs to CTEs (WITH)—named, reusable. (2) Replace correlated subqueries with JOINs—planner optimizes better. (3) Window functions instead of self-joins for row comparisons. (4) Materialize hot paths—temp table if CTE scanned multiple times. (5) Push predicates into CTEs. **Why**: Readability = maintainability; optimizer may flatten CTEs or may not—benchmark....

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