-- Календарь от первого до последнего факта, отсутствующие дни дают 0.
-- На пустой БД результат пуст; изменение первого дня NULL (нет предшественника).
WITH RECURSIVE calendar(day) AS (
    SELECT MIN(order_date) FROM facts HAVING COUNT(*) > 0
    UNION ALL
    SELECT date(day, '+1 day') FROM calendar
    WHERE day < (SELECT MAX(order_date) FROM facts)
), totals AS (
    SELECT order_date, SUM(revenue_kopecks) AS revenue_kopecks
    FROM daily_category GROUP BY order_date
), filled AS (
    SELECT day AS order_date, COALESCE(revenue_kopecks, 0) AS revenue_kopecks
    FROM calendar LEFT JOIN totals ON totals.order_date = calendar.day
)
SELECT order_date, revenue_kopecks,
       revenue_kopecks - LAG(revenue_kopecks) OVER (ORDER BY order_date)
           AS change_kopecks
FROM filled ORDER BY order_date;