thank for your reply, I manage to change the subquery inside the update statement ,
```
CREATE TRIGGER updater
AFTER UPDATE
ON Rental
FOR EACH ROW
WHEN OLD.rental_cost IS NULL AND
NEW.date_back IS NOT NULL
BEGIN
UPDATE Rental
SET rental_cost = 3 + (
SELECT price
FROM Rental
JOIN
License USING (
license_id
)
JOIN
Title USING (
title
)
)
* 0.05 * (date_out) - (date_back)
WHERE rental_cost = NULL;
END;
```
however, the new problem is the trigger not detecting the UPDATE statement and not fire. I believe is something wrong with my WHEN statement