TRIGGERs that cause INSERTs to fail? Possible?

Question

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?

Answer 1

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;

Answer 2

This will abort your INSERT by raising an exception (from http://www.experts-exchange.com/Database/MySQL/Q_23788965.html)

DROP PROCEDURE IF EXISTS `MyRaiseError`$$

CREATE PROCEDURE `MyRaiseError`(msg VARCHAR(62))
BEGIN
DECLARE Tmsg VARCHAR(80);
SET Tmsg = msg;
IF (CHAR_LENGTH(TRIM(Tmsg)) = 0 OR Tmsg IS NULL) THEN
SET Tmsg = 'ERROR GENERADO';
END IF;
SET Tmsg = CONCAT('@@MyError', Tmsg, '@@MyError');
SET @MyError = CONCAT('INSERT INTO', Tmsg);
PREPARE stmt FROM @MyError;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END$$

Usage:

call MyRaiseError('Here error message!');

Answer 3

in MS SQL you could make it work using proper syntax:

IF UPDATE(column_name)
BEGIN
  RAISEERROR
  ROLLBACK TRAN
  RETURN
END

http://msdn.microsoft.com/en-us/magazine/cc164047.aspx

Answer 4

Here is the way I did it. Note the SET NEW='some error'; MySQL will tell you "Variable 'new' can't be set to the value of 'Error: Cannot delete this item. There are records in the sales table with this item.'"

You can trap this in your code and then show the resulting error :)

DELIMITER $$ DROP TRIGGER IF EXISTS before_tblinventoryexceptionreasons_delete $$ CREATE TRIGGER before_tblinventoryexceptionreasons_delete BEFORE DELETE ON tblinventoryexceptionreasons FOR EACH ROW BEGIN IF (SELECT COUNT(*) FROM tblinventoryexceptions WHERE tblinventoryexceptions.idtblinventoryexceptionreasons = old.idtblinventoryexceptionreasons) > 0 THEN SET NEW='Error: Cannot delete this item. There are records in the inventory exception reasons table with this item.'; END IF; END$$ DELIMITER ;

DELIMITER $$ DROP TRIGGER IF EXISTS before_storesalesconfig_delete $$ CREATE TRIGGER before_storesalesconfig_delete BEFORE DELETE ON tblstoresalesconfig FOR EACH ROW BEGIN IF (SELECT COUNT() FROM tblstoresales WHERE tblstoresales.idtblstoresalesconfig=old.idtblstoresalesconfig) > 0 THEN SET NEW='Error: Cannot delete this item. There are records in the sales table with this item.'; END IF; IF (SELECT COUNT() FROM tblinventory WHERE tblinventory.idtblstoresalesconfig=old.idtblstoresalesconfig) > 0 THEN SET NEW='Error: Cannot delete this item. There are records in the inventory table with this item.'; END IF; IF (SELECT COUNT() FROM tblinventoryexceptions WHERE tblinventoryexceptions.idtblstoresalesconfig=old.idtblstoresalesconfig) > 0 THEN SET NEW='Error: Cannot delete this item. There are records in the inventory exceptions table with this item.'; END IF; IF (SELECT COUNT() FROM tblinvoicedetails WHERE tblinvoicedetails.idtblstoresalesconfig=old.idtblstoresalesconfig) > 0 THEN SET NEW='Error: Cannot delete this item. There are records in the inventory details table with this item.'; END IF; END$$ DELIMITER ;

DELIMITER $$ DROP TRIGGER IF EXISTS before_tblinvoice_delete $$ CREATE TRIGGER before_tblinvoice_delete BEFORE DELETE ON tblinvoice FOR EACH ROW BEGIN IF (SELECT COUNT(*) FROM tblinvoicedetails WHERE tblinvoicedetails.idtblinvoice = old.idtblinvoice) > 0 THEN SET NEW='Error: Cannot delete this item. There are records in the inventory details table with this item.'; END IF; END$$ DELIMITER ;