MySQL 8.0

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create table loans  (book_id int
                    , student_id int
                    , date_taken date
                    , number_od_days int
                    , return_date date);
CREATE TRIGGER rentcheck 
BEFORE INSERT ON loans 
FOR EACH ROW 
begin
    if new.number_od_days < 0 or new.number_od_days is null then
      signal sqlstate '45000';
    else
      set new.return_date = (new.date_taken + interval new.number_od_days day);
    end if;
end;
insert into loans (book_id, student_id, date_taken, number_od_days) 
            values (1, 201, '2018/10/02', 5);
insert into loans (book_id, student_id, date_taken, number_od_days) 
            values (1, 201, '2018/10/02', 0);
insert into loans (book_id, student_id, date_taken, number_od_days) 
            values (1, 201, '2018/10/02', -2);
Unhandled user-defined exception condition

insert into loans (book_id, student_id, date_taken, number_od_days) 
            values (1, 201, '2018/10/02', null);
Unhandled user-defined exception condition

select * from loans;


book_id
student_id
date_taken
number_od_days
return_date


1
201
2018-10-02
5
2018-10-07

1
201
2018-10-02
0
2018-10-02