MySQL: how to create a trigger that add not-null constraint? - mysql

Question

I have created the customer table that has a trigger that not allow null value on the customer_id column. it looks like:
create table customer(
customer_id varchar(20) UNIQUE,
customer_name varchar(15) NOT NULL,
password varchar(10) NOT NULL,
social_number varchar(14) not null,
phone_number varchar(13) NOT NULL,
email varchar(30) NOT NULL,
address varchar(50) NOT NULL,
primary key ( social_number )
);
But, creating the following trigger causes this error.
create trigger null_checker
on customer
after insert
as
delete from customer
where customer_id is null;
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'on customer after insert as delete from customer where customer_id is null' at line 1
What's wrong with? I could add not-null constraint using not null specifier. but I am not allowed to do it because creating the trigger is a part of my assignment. Any help would be appreciated.

Answer 1

You need to use right syntax:
create trigger null_checker
after insert on customer
for each row
delete from customer
where customer_id is null;
But you cannot modify the same table from trigger.
A solution, without triggers, may be is change the sql_mode to avoid null values defined in your columns.
set sql_mode = 'TRADITIONAL';
That command says to MySQL “give an error instead of a warning” when inserting an incorrect value into a column, like inserting NULL to a 'NOT NULL column.
More info: http://dev.mysql.com/doc/refman/5.0/en/sql-mode.html