Question

I am having trouble creating a BEFORE INSERT trigger.

table schema:

CREATE TABLE IF NOT EXISTS `myReferenceTable` (
  `id` bigint(20) NOT NULL AUTO_INCREMENT,
  `linkFrom` bigint(20) NOT NULL,
  `linkTo` bigint(20) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `linkFrom` (`linkFrom`,`linkTo`),
  KEY `linkTo` (`linkTo`)
) ENGINE=InnoDB 

inserting data:

INSERT INTO `myReferenceTable` (`id`, `linkFrom`, `linkTo`) VALUES
(1, 1, 2), // allowed
(2, 2, 1); // allowed

My failed attempt at creating a BEFORE INSERT trigger which will not allow linkFrom and linkTo to equal each another. This table articleReferncesTable can not have any article refering to it self,

/* This fails */
create trigger myReferenceTable_noDuplicate
BEFORE INSERT 
ON myReferenceTable
FOR EACH ROW
BEGIN
 IF NEW.linkFrom = NEW.linkTo
  insert ignore()
 END IF;
END;

Example:

INSERT INTO `myReferenceTable` (`id`, `linkFrom`, `linkTo`) VALUES
(3, 1, 1), // should fail
(4, 2, 2); // should fail

the above data is not allowed. So i want this table to be a “set” table the following data is allowed:

INSERT INTO `myReferenceTable` (`id`, `linkFrom`, `linkTo`) VALUES
    (3, 1, 2),  // allowed 
    (4, 1, 3); // allowed

Answer 1

INSERT ignore() doesn’t cancel the insert or cause it to fail. There are a few ways to do this, but the simplest is to cause an error:

...
IF NEW.linkFrom = NEW.linkTo
    DECLARE dummy INT;
    SELECT LINKFROM_EQUALS_LINKTO INTO dummy FROM links
    WHERE links.id = new.id;
END IF;
...

see TRIGGERs that cause INSERTs to fail? Possible?