Mysql-how to assign value from a field of select into local variable?

I'm trying to assign a value from a field of a select statement to a local variable in my trigger. Does it have any way to do that? Any help would be very appreciated My trigger

DELIMITER $$

CREATE TRIGGER update_cart_total
before insert 
ON core_cartdetails FOR EACH ROW
BEGIN
    declare total DOUBLE;
    select Total into total from core_cart where CartID = NEW.CartID;
    
END $$
DELIMITER ;