2nd latest joining per department: WITH ranked AS (SELECT *, DENSE_RANK() OVER (PARTITION BY dept ORDER BY join_date DESC) rn FROM employees) SELECT * FROM ranked WHERE rn=2. Use DENSE_RANK so ties get same rank; ROW_NUMBER gives unique rank. Alternative: NTH_VALUE or LAG. **Why it matters**: Design choices compound at scale—wrong approach can cause 100× overhead. **Scalability trade-offs**: Profile before optimizing; validate on sample then full....
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