In cleaning up this answer I learnt a bit about TRIGGERs and stored procedures in MySQL, but was stunned that, while BEFORE INSERT and BEFORE UPDATE triggers could modify data, they seemingly couldn't cause the insert/update to fail (ie. validation). In this particular case I was able to get this to work by manipulating the data in such a way as to cause a primary key duplicate, which in this particular case made sense, but doesn't necessarily make sense in a general sense.
Is this sort of functionality possible in MySQL? In any other RDBMS (my experience is limited to MySQL sadly)? Perhaps a THROW EXCEPTION style syntax?
From this blog post
MySQL Triggers: How do you abort an INSERT, UPDATE or DELETE with a trigger? On EfNet’s #mysql someone asked:
How do I make a trigger abort the operation if my business rule fails?
In MySQL 5.0 and 5.1 you need to resort to some trickery to make a trigger fail and deliver a meaningful error message. The MySQL Stored Procedure FAQ says this about error handling:
SP 11. Do SPs have a “raise” statement to “raise application errors”? Sorry, not at present. The SQL standard SIGNAL and RESIGNAL statements are on the TODO.
Perhaps MySQL 5.2 will include SIGNAL statement which will make this hack stolen straight from MySQL Stored Procedure Programming obsolete. What is the hack? You’re going to force MySQL to attempt to use a column that does not exist. Ugly? Yes. Does it work? Sure.
CREATE TRIGGER mytabletriggerexample BEFORE INSERT FOR EACH ROW BEGIN IF(NEW.important_value) < (fancy * dancy * calculation) THEN DECLARE dummy INT; SELECT Your meaningful error message goes here INTO dummy FROM mytable WHERE mytable.id=new.id END IF; END;