Question

I’m trying to create some triggers for my MySQL tables to keep track of changes. I thought of a table like

CREATE TABLE IF NOT EXISTS `contacts_changes` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `action` enum('insert','update','delete') NOT NULL,
  `contact_id` int(6) NOT NULL,
  `changes` text NOT NULL,
  PRIMARY KEY (`id`)
);

where contact_id is a foreign key to the table I want to monitor and in column changes I want to store the changes made using the JSON format.

So if I change the familyname and birthday of an entry in the contact table the changes column should contain {"familyname": "Smith", "birthday": "1982-06-24}.

I found a lot of examples for using triggers to track changes but none of them summarizes the changes made in a single column of a single row. What they do is to insert a new row for each change made, i.e. one row for changing the birthday and another for changing the familyname.

Due to my lack of knowledge of the (My)SQL language I failed to figure out if it’s possible to do something I had in mind and if it is how it can be done.

Any ideas or hints how to solve this?

Thanks!

Edit: Posted former edit as answer.

Answer 1

Alright, solved it myself. Wasn’t that hard … 😉

DELIMITER ;;
CREATE TRIGGER `contacts_bu` BEFORE UPDATE ON `contacts` FOR EACH ROW
BEGIN
SET @json = "{";
SET @first = true;
IF (OLD.name!=NEW.name) THEN
    SET @first = false;
    SET @json = CONCAT(@json, "\"name\"", ":", "\"", NEW.name, "\"");
END IF;
IF (OLD.birthdate!=NEW.birthdate) THEN
    IF (!@first) THEN
       SET @json = CONCAT(@json, ",");
    END IF;
    SET @first = false;
    SET @json = CONCAT(@json, "\"birthdate\"", ":", "\"", NEW.birthdate, "\"");
END IF;

SET @json = CONCAT(@json, "}");

INSERT INTO contacts_changes (`action`, `contact_id`, `changes`) 
VALUES ('update', NEW.id, @json);

END;;
DELIMITER ;