**Thanks so much for your clarification, it means a lot.**
Would you please explain the subselect? And,
The date_back and rental_cost is default to be NULL, date_back will change until someone update new datetime string,and the rental_cost will be update by the trigger because of someone UPDATE the table.
The calculation of the rental cost is using the following formula
rental cost = 3 + price × 0.05 × days rented
which the days rented is done by subtraction of two rows date_out - date_back
date_out & date_back is two TEXT datatype with YYYY-MM-DD HH:MM: SS string format
My friend suggested I use julianday() to do it
And here is my edited version
`
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 IS NULL;
END;
`
Once again, thanks so much for your help