Avg sessions per user in 30 days: SELECT user_id, COUNT(DISTINCT session_id)/30.0 AS avg_sessions_per_day FROM events WHERE event_date >= CURRENT_DATE - 30 GROUP BY user_id. For 30-day window per user: use date range in WHERE. If sessions span dates: COUNT(DISTINCT session_id) / 30. Or use date diff for actual active days. **Why it matters**: Design choices compound at scale—wrong approach can cause 100× overhead....
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