MySQL trigger working on one table but not the other - mysql

Question

I have created a mysql trigger to update on insert a value from another table:
DROP TRIGGER IF EXISTS `CourseCode`//
CREATE TRIGGER `CourseCode` BEFORE INSERT ON `race`
FOR EACH ROW BEGIN
SET NEW.Course_Code = (
SELECT
course_code
from
tb_course
where
tb_course.course_name = NEW.course_name
)
END
//
This works perfectly. It returns the 4 character code for the course_name based on the tb_course table.
What I'm trying to do is the exact same thing in another table, I copy and paste the trigger and rename the trigger and the table (field names and types are also identical) it but it won't work:
DROP TRIGGER IF EXISTS `CourseCode2`//
CREATE TRIGGER `CourseCode2` BEFORE INSERT ON `fields`
FOR EACH ROW
BEGIN
SET NEW.Course_Code = (
SELECT
course_code
from
tb_course
where
tb_course.course_name = NEW.course_name
)
END
//
However this results in null values. I've tried replacing New.Course_name with a string (i.e. tb_course.course_name="String") and that updates that static value fine so the trigger appears to be working but either it isn't matching the select statement in this table for or it's not setting the Course_Code field...
Is there any sort of debugging you can suggest to figure this out? It's driving me nuts and I don't know what to do to diagnose the problem.
Cheers

Answer 1

I have no idea what is wrong with the second trigger.
However, the 'debugging' part i can maybe help with.
A search of the 'net' will return various software (paid for) that will allow single step debugging. Rather than pay for stuff, all i want is 'print' statements from inside stored function procedures and triggers.
a quick search lead me to someone who has done all the work already.
Debugging Stored Procedures in MySql
I have just set it up in MySql and tried it and it works fine.
Here is some sample code, based on your trigger and the output i got.
DELIMITER $$
USE `testmysql`$$
DROP TRIGGER /*!50032 IF EXISTS */ `CourseCode`$$
CREATE
/*!50017 DEFINER = 'test'#'localhost' */
TRIGGER `CourseCode` BEFORE INSERT ON `race`
FOR EACH ROW BEGIN
DECLARE v_course_code INT;
DECLARE v_course_title VARCHAR(255);
CALL procLog("line 12: Entering CourseCode");
SELECT
course_code, course_title
INTO
v_course_code, v_course_title
FROM
tb_course
WHERE
tb_course.course_name = NEW.course_name;
SET NEW.course_code = v_course_code;
CALL procLog(CONCAT("line_22: ", v_course_title, " code: ", NEW.course_code));
CALL procLog("line 24: Exiting CourseCode");
END;
$$
DELIMITER ;
Query:
CALL setupProcLog();
INSERT INTO `race` (course_name, race_info)
VALUES ('learn_it_02', 'this is another test');
CALL cleanup('query end');
procLog output:
entrytime connection_id msg
2014-02-23 12:32:21 5 line 12: Entering CourseCode
2014-02-23 12:32:21 5 line_22: Learn It Course 02 code: 2
2014-02-23 12:32:21 5 line 24: Exiting CourseCode
2014-02-23 12:32:21 5 cleanup() query end
Ok, a little clumsy maybe but it will help with debugging 'stored' programs.
I will be using it from now on.