How to create a trigger the triggers a self-referencing trigger...yeah - mysql

Question

I have a MySQL database with...interesting architecture and a convoluted enrollment process. There are several program tables that need to insert rows on an historic enrollment table when they're updated, inserted or deleted. I've got that working using triggers on each of those tables (around 30 different tables) using the following iterated over all of the tables:
DROP TRIGGER IF EXISTS programTable_afterinsert;$$
CREATE TRIGGER programTable_afterinsert AFTER INSERT ON programTable
FOR EACH ROW
BEGIN
IF NEW.Enrolled = 1
THEN
INSERT INTO enrollment (ID, Action, Date_Updated, User, Program, Reason, Action_Date)
VALUES (NEW.ID, 'Enrolled', NOW(), 'programUser', 'programName', 'Enrolled in program', NOW());
ELSEIF NEW.Enrolled = 0
THEN
INSERT INTO enrollment (Member_ID, Action, Date_Updated, User, Program, Reason, Action_Date)
VALUES (NEW.ID, 'Disenrolled', NOW(), 'programUser', 'programName', 'Disenrolled from program', NOW());
END IF;
END;$$
DROP TRIGGER IF EXISTS programTable_afterupdate;$$
CREATE TRIGGER programTable_afterupdate AFTER UPDATE ON programTable
FOR EACH ROW
BEGIN
IF NEW.Enrolled = 1
THEN
INSERT INTO enrollment (ID, Action, Date_Updated, User, Program, Reason, Action_Date)
VALUES (NEW.Member_ID, 'Enrolled', NOW(), 'programUser', 'programName', 'Enrolled in program', NOW());
ELSEIF NEW.Enrolled = 0
THEN
INSERT INTO enrollment (ID, Action, Date_Updated, User, Program, Reason, Action_Date)
VALUES (NEW.ID, 'Disenrolled', NOW(), 'programUser', 'programName', 'Disenrolled from program', NOW());
END IF;
END;$$
DROP TRIGGER IF EXISTS programTable_afterdelete;$$
CREATE TRIGGER programTable_afterdelete AFTER DELETE ON programTable
FOR EACH ROW
BEGIN
IF OLD.Enrolled = 1
THEN
INSERT INTO enrollment (ID, Action, Date_Updated, User, Program, Reason, Action_Date)
VALUES (OLD.ID, 'Disenrolled', NOW(), 'programUser', 'programName', 'Removed from program', NOW());
END IF;
END;$$
A stripped down version of the enrollment and program tables can be created with the following:
delimiter $$
CREATE TABLE `programTable1` (
`ID` varchar(15) NOT NULL,
`Enrolled` tinyint(3) unsigned NOT NULL,
`Referral_Date` datetime NOT NULL,
`Referral_Source` varchar(255)
);$$
CREATE TABLE `programTable2` (
`ID` varchar(15) NOT NULL,
`Enrolled` tinyint(3) unsigned NOT NULL,
`Referral_Date` datetime NOT NULL,
`Referral_Source` varchar(255)
);$$
CREATE TABLE `enrollment` (
`ID` varchar(15) NOT NULL,
`Action` varchar(12) NOT NULL,
`Date_Updated` timestamp NOT NULL,
`User` varchar(12) default NULL,
`Program` varchar(25) NOT NULL,
`Notes` varchar(100) default NULL,
`Reason` varchar(45) default NULL,
`Action_Date` datetime NOT NULL
);$$
The hurdle I'm running into is the enrollment table needs to update the program tables' enrollment when it's modified or a row is added to it. Meaning if someone is enrolled on the program table, they need to have an entry on the enrollment table for that action; if someone is enrolled via the enrollment table, they need to be updated as enrolled or disenrolled on the program table that row applies to.
The main problem is that there are two different sources people are enrolling in the programs from.
Like I said, convoluted. I know the architecture of this application isn't the best, but it's not something that can be changed.
Any ideas would be welcome! Please let me know if there anyone has questions or if any clarification is needed. I've been working on this for awhile now, so I know I'm probably leaving some stuff out of the equation due to being so familiar with it.

Answer 1

There's no problem creating triggers that update each others' tables -- as long as the triggers don't continue to insert back and forth in an infinite loop.
You need to make sure that the INSERT performed by a trigger inserts a row to the other table that will not result in a reciprocal action.
I wrote an example in another recent question: Mirror tables: triggers, deadlock and implicit commits