multiple row update on related table - mysql

Question

I would like to update multiple rows of column 'student_total' in Table 'Teacher' when a student is deleted, using triggers/procedures
Updating multiple rows in related tables, many-to-many realtionships
'n' teachers can have 'm' students
is it possible at all ? because its not possible to store a
result set (learnt from your site)
in Mysql, Postgress etc?
Thanks in Adv
Ritin
--------------- SQL
CREATE TABLE `Teacher` (
`id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
`student_total` int(11) DEFAULT '0',
PRIMARY KEY (`id`)
);
CREATE TABLE `Student` (
`id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(255) NOT NULL,
PRIMARY KEY (`id`)
);
CREATE TABLE `Teacher_has_Student` (
`teacher_id` smallint(5) unsigned NOT NULL,
`student_id` smallint(5) unsigned NOT NULL,
PRIMARY KEY (`teacher_id`,`student_id`),
KEY `fk_Teacher_has_Student_teacher` (`teacher_id`),
KEY `fk_breeder_has_breed_student` (`student_id`),
CONSTRAINT `fk_Teacher_has_Student_teacher` FOREIGN KEY (`teacher_id`) REFERENCES `Teacher` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
CONSTRAINT `fk_breeder_has_breed_student` FOREIGN KEY (`student_id`) REFERENCES `Student` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
);
i have triggers for INSERT/DELETE/UPDATE
ex:
---------------------- SQL
CREATE TRIGGER teacher__student_insert AFTER INSERT ON Teacher_has_Student
FOR EACH ROW
BEGIN
UPDATE Teacher SET student_total = student_total + 1 WHERE id = NEW.teacher_id;
END;
The trigger below updates just 1 row, whereas the aim is to update all the rows.
DELIMITER |
CREATE TRIGGER my_student__delete AFTER DELETE ON Student
FOR EACH ROW
BEGIN
set #std_id = old.id;
UPDATE teacher SET student_total = student_total - 1
WHERE id = #std_id;
END
|

Answer 1

I think your 2nd trigger doesn't make sense. On a delete of a student you take his id and decrease the student_total of the teacher, which has accidentally the same id as the as the studend.
Your definition plus the first trigger should already do the job. If you delete a student his relations are deleted from Teacher_has_Student by the CASCADE and on this DELETE your triggers should be fired to decrease the student_total of the affected teachers. Anyway a short Google research shows, that MySQL apparently doesn't fire triggers on a cascaded delete:
http://bugs.mysql.com/bug.php?id=13102
Possible Workarounds
You could try to fix your 2nd trigger to do the job manually:
DELIMITER |
CREATE TRIGGER my_student__delete AFTER DELETE ON Student
FOR EACH ROW
BEGIN
SET #std_id = old.`id`;
UPDATE
`Teacher` AS T
INNER JOIN `Teacher_has_Student` AS TS ON T.`id`=TS.`teacher_id`
SET `student_total` = `student_total` - 1
WHERE ST.`student_id` = #std_id;
END
|
But this will not work, if the CASCADE on the FOREIGN KEY is executed before your trigger.
Maybe a more reliable workaround is to drop the FOREIGN KEY definitions and to create triggers on Teacher and Student to cascade the deletes and updates to Teacher_has_Student.
Normalize it
With the workarounds you have to keep a lot of different scenarios in mind to keep student_total valid. If it‘s not absolutely necessary to store student_total directly at the Teachers-Table, I suggest strongly not to do it.
The information is absolutely redundant. Keep your constraints and cascades, but drop the student_total column and delete all triggers. Then use a JOIN to get your student numbers:
SELECT
T.`id`, T.`name`, COUNT(*) AS student_total
FROM `Teacher` AS T
INNER JOIN `Teacher_has_Student` AS TS ON T.`id`=TS.`teacher_id`
GROUP BY T.`id`