MySQL触发器-实现问题

Question

I am developing a MySQL database using Workbench. I want two send two fields from a newly created record to another table. I would then like to update the original table with newly created data from the second table. I was looking to implement this with triggers, unless there is a better way of course :) My attempt was a fail when I went to upload it(see below)

Specifically, I would like tc_Event to send the ID & tc_EventTags_ID to tc_EventTags to fill in tc_Tag_ID & tc_Event_ID. Afterwards I want the ID of tc_EventTags sent back to tc_Event to the tc_EventTags_ID field.

Thanks for any help.

-- -----------------------------------------------------
-- Table `mcontest`.`tc_EventTags`
-- -----------------------------------------------------
CREATE  TABLE IF NOT EXISTS `mcontest`.`tc_EventTags` (
  `ID` INT NOT NULL AUTO_INCREMENT ,
  `tc_Tag_ID` INT NOT NULL ,
  `tc_Event_ID` INT NOT NULL ,
  PRIMARY KEY (`ID`) ,
  INDEX `fk_tc_EventTags_tc_Tag1` (`tc_Tag_ID` ASC) ,
  INDEX `fk_tc_EventTags_tc_Event1` (`tc_Event_ID` ASC) ,
  CONSTRAINT `fk_tc_EventTags_tc_Tag1`
    FOREIGN KEY (`tc_Tag_ID` )
    REFERENCES `mcontest`.`tc_Tag` (`ID` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION,
  CONSTRAINT `fk_tc_EventTags_tc_Event1`
    FOREIGN KEY (`tc_Event_ID` )
    REFERENCES `mcontest`.`tc_Event` (`ID` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = MyISAM;

-- -----------------------------------------------------
-- Table `mcontest`.`tc_Event`
-- -----------------------------------------------------
CREATE  TABLE IF NOT EXISTS `mcontest`.`tc_Event` (
  `ID` INT NOT NULL AUTO_INCREMENT ,
  `date` DATE NOT NULL ,
  `time` TIME NOT NULL ,
  `location` VARCHAR(45) NOT NULL ,
  `description` VARCHAR(45) NOT NULL ,
  `tc_EventTags_ID` INT NULL ,
  `tc_Orgs_ID` INT NOT NULL ,
  `tc_PersonEvent_ID` INT NOT NULL ,
  PRIMARY KEY (`ID`) ,
  INDEX `fk_tc_Event_tc_EventTags1` (`tc_EventTags_ID` ASC) ,
  INDEX `fk_tc_Event_tc_Orgs1` (`tc_Orgs_ID` ASC) ,
  INDEX `fk_tc_Event_tc_PersonEvent1` (`tc_PersonEvent_ID` ASC) ,
  CONSTRAINT `fk_tc_Event_tc_EventTags1`
    FOREIGN KEY (`tc_EventTags_ID` )
    REFERENCES `mcontest`.`tc_EventTags` (`ID` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION,
  CONSTRAINT `fk_tc_Event_tc_Orgs1`
    FOREIGN KEY (`tc_Orgs_ID` )
    REFERENCES `mcontest`.`tc_Orgs` (`ID` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION,
  CONSTRAINT `fk_tc_Event_tc_PersonEvent1`
    FOREIGN KEY (`tc_PersonEvent_ID` )
    REFERENCES `mcontest`.`tc_PersonEvent` (`ID` )
    ON DELETE NO ACTION
    ON UPDATE NO ACTION)
ENGINE = MyISAM;


    USE `mcontest`;

    DELIMITER $$
    USE `mcontest`$$


    CREATE TRIGGER eventTag_Trigger
    AFTER insert ON tc_Event

    FOR EACH ROW BEGIN 
    INSERT INTO tc_EventTags values('',NEW.tc_Event_ID);
    END;


    END$$


    DELIMITER ;


    SET SQL_MODE=@OLD_SQL_MODE;
    SET FOREIGN_KEY_CHECKS=@OLD_FOREIGN_KEY_CHECKS;
    SET UNIQUE_CHECKS=@OLD_UNIQUE_CHECKS;

Answer 1

Why have put a tc_EventTags_ID in table tc_Event ? What is the logic behind that?

I mean that the relationship between the 2 tables would be (I guess): 1 Event - many EventTags. This is already achieved by the tc_EventTags.tc_Event_ID which is a Foreign Key to tc_Event .

To answer your question:

As it is now, the Trigger tries for every row inserted in table Event, to add a row in table EventTag. But it will fail for 2 reasons:

1. EventTag has 2 Constraints (Foreign keys) which have to be fulfilled for the triggered insert to succeed. So, tc_Event_ID is ok but tc_Tag_ID has to be NOT NULL and reference the Tag table but the value you supply, '' , probably is not in table Tag.

EDIT: the value you supply, '' , is also a CHAR while it should be INT .

1. But trigger will probably not be started at all, since every time you insert a row in table Event, those constraints have to be fulfilled and one of them is the tc_EventTags_ID which references EventTag table. But EventTag table is empty. Do you see the circular logic here?