Throw an error preventing a table update in a MySQL trigger

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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
declare msg varchar(128);
if < 0 then
set msg = concat(‘MyTriggerError: Trying to insert a negative value in trigger_test: ‘, cast( as char));
signal sqlstate ‘45000’ set message_text = msg;
end if;

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;


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