Sakila re-calculating paiment amount (by days and hours)

sakila_paiment_amount.sql

SELECT i.inventory_id, 
       f.title, 
       @rental_duration_in_days := f.rental_duration as 'rental_duration_in_DAYS',
       @rental_duration_in_hours := (@rental_duration_in_days * 24) as 'rental_duration_in_HOURS',
       f.rental_rate,
       p.amount as 'amount_paid',
       r.rental_date,
       r.return_date,
       @total_days_rented := DATEDIFF(r.return_date, r.rental_date) AS 'total_DAYS_rented',
       @rental_ratio_days := CAST((@total_days_rented / @rental_duration_in_days) AS DECIMAL(11,4)) AS 'rental_ratio_by_DAYS',    
       CAST((@rental_ratio_days * f.rental_rate) as decimal(11,4)) as 'calculated_amount_by_DAYS',                                
       @total_hours_rented := TIMESTAMPDIFF(HOUR,r.rental_date,r.return_date) AS 'total_HOURS_rented',                            
       @rental_ratio_hours := CAST((@total_hours_rented / @rental_duration_in_hours) AS DECIMAL(11,4)) AS 'rental_ratio_by_HOURS',
       CAST((@rental_ratio_hours * f.rental_rate) as decimal(11,4)) as 'calculated_amount_by_HOURS'

FROM payment AS p
JOIN rental AS r ON(p.rental_id = r.rental_id AND r.customer_id = 187 AND DATE(r.rental_date) = '2005-08-18')
JOIN inventory AS i ON(r.inventory_id = i.inventory_id)
JOIN film AS f ON(i.film_id = f.film_id)