A. DELIMITER //
CREATE TRIGGER age check AFTER INSERT ON people FOR EACH ROW IF NEW. age < 0 THEN SET NEW.age =0; END IF;// DELIMITER;

B. DELIMITER //
CREATE TRIGGER agecheck BEFORE INSERT ON people FOR EACH ROW IF NEW. age < 0 THEN SET NEW.age =0; END IF;// DELIMITER ;

C. ALTER TABLE people ADD CONSTRAINT check_age CHECK (ABS(age)>=0);

D. ALTER TABLE people ADD COLUMN valid_age=ABS(check_age) GENERATED ALWAYS;