LAG — zmiana sprzedaży względem poprzedniego okresu

-- monthly_sales(product_id, month date, revenue numeric)
-- Jeden wiersz na produkt i miesiąc
WITH comparison AS (
  SELECT *, LAG(revenue) OVER (
    PARTITION BY product_id ORDER BY month
  ) AS previous_revenue
  FROM monthly_sales
)
SELECT product_id, month,
       revenue - previous_revenue AS difference,
       100.0 * (revenue - previous_revenue)
         / NULLIF(previous_revenue, 0) AS change_pct
FROM comparison;