how programming a business rule on a trigger in mysql - mysql

Question

I'm trying to programm a business rule in a trigger using MySQL, but it doesn't work.
What I want to do is that if the condition is false then the insert must fail and rollback it. The problem is that MySQL doesn't permit using rollback inside a trigger.
I've also tried to use SIGNAL and text message but it fails.
The code is:
CREATE TRIGGER tr_NewTeacher
AFTER INSERT ON teachers FOR EACH ROW
BEGIN
IF (salary > 1300 AND budget < 15000)
BEGIN
ROLLBACK TRANSACTION;
END
END

Answer 1

In MySQL trigger will rollback transaction if it fails.
So, you must raise exception within trigger body:
...
if [Test]
then
SIGNAL sqlstate '45001' set message_text = "No way ! You cannot do this !";
end if ;
...

Answer 2

You should be using before insert and for getting the column values you need to use new
So the trigger would be as
delimiter //
create trigger tr_NewTeacher before insert on teacher
for each row
begin
if new.salaRY>1300 AND new.budget<15000 then
signal sqlstate '45000' set message_text = 'Your error message';
end if;
end;//
delimiter ;
Here is a test case in mysql
mysql> create table teacher (id int auto_increment primary key,salaRY int , budget int);
Query OK, 0 rows affected (0.17 sec)
mysql> delimiter //
mysql> create trigger tr_NewTeacher before insert on teacher
-> for each row
-> begin
-> if new.salaRY>1300 AND new.budget<15000 then
-> signal sqlstate '45000' set message_text = 'Your error message';
-> end if;
-> end;//
Query OK, 0 rows affected (0.14 sec)
mysql> insert into teacher (salaRY,budget) values (1200,16000);
Query OK, 1 row affected (0.04 sec)
mysql> insert into teacher (salaRY,budget) values (1400,13000);
ERROR 1644 (45000): Your error message
mysql> select * from teacher ;
+----+--------+--------+
| id | salaRY | budget |
+----+--------+--------+
| 1 | 1200 | 16000 |
+----+--------+--------+
1 row in set (0.00 sec)