Throw an error preventing a table update in a MySQL trigger

If I have a trigger before the update on a table, how can I throw an error that prevents the update on that table?

As of MySQL 5.5, you can use the SIGNAL syntax to throw an exception:

signal sqlstate '45000' set message_text = 'My Error Message';

State 45000 is a generic state representing "unhandled user-defined exception".

Here is a more complete example of the approach:

delimiter //
use test//
create table trigger_test
(
    id int not null
)//
drop trigger if exists trg_trigger_test_ins //
create trigger trg_trigger_test_ins before insert on trigger_test
for each row
begin
    declare msg varchar(128);
    if new.id < 0 then
        set msg = concat('MyTriggerError: Trying to insert a negative value in trigger_test: ', cast(new.id as char));
        signal sqlstate '45000' set message_text = msg;
    end if;
end
//

delimiter ;
-- run the following as seperate statements:
insert into trigger_test values (1), (-1), (2); -- everything fails as one row is bad
select * from trigger_test;
insert into trigger_test values (1); -- succeeds as expected
insert into trigger_test values (-1); -- fails as expected
select * from trigger_test;

Here is one hack that may work. It isn't clean, but it looks like it might work:

Essentially, you just try to update a column that doesn't exist.