create database triggers;
use triggers;
create table students (name varchar(30), age int, enrollement_date varchar(30));
select * from students;
delimiter //
create trigger age_verify
before insert on students
for each row
if new.age<0 then set new.age=0;
end if; //
insert into students values ('Mohamed', 33, '2019'),
('Salah', 33, '2019'),
('chaima', 25, '2019'),
('Mahmoud', -20, '2020');

select * from students;