Question

I’m working on a fairly simple ticket managment system. I want to keep a log for stuff that gets added, deleted, and changed.

I created three triggers, AFTER INSERT, AFTER DELETE, and AFTER UPDATE. The INSERT/DELETE triggers are straightforward, it’s theUPDATE trigger I’m having problems with.

I would like to add which columns has changed in the table with their old & new values, i.e. colname changed from X to Y

The trigger I have now “works”, except of course that it doesn’t insert the actual values I’d like.

How do I get the value from OLD and NEW using the col_name variable?

I’m also not sure if this is the best possible way of doing this … So if anyone has ideas on that, they’re welcome too … This trigger started out a lot simpler …

BEGIN
    DECLARE num_rows, i int default 1;
    DECLARE col_name CHAR(255);
    DECLARE updated TEXT;

    DECLARE col_names CURSOR FOR
        SELECT column_name FROM INFORMATION_SCHEMA.COLUMNS
        WHERE table_name = 'storing'
        ORDER BY ordinal_position;

    OPEN col_names;
    SELECT FOUND_ROWS() INTO num_rows;
    SET i = 1;
    SET @updated = 'Updated columns: ';

    the_loop: LOOP
        IF i > num_rows THEN
            LEAVE the_loop;
        END IF;

        FETCH col_names INTO col_name;

        /* So, how do I get the proper values? */
        /* IF NEW.@col_name != OLD.@col_name THEN */
        /*SET @updated = CONCAT(@updated, OLD.@col_name, ' changed into ', NEW.@col_name, ' ');*/
        SET @updated = CONCAT(@updated, 'OLD', ' changed into ', 'NEW', ' ');
        /* END IF;*/

        SET i = i + 1;
    END LOOP the_loop;

    CLOSE col_names;

    INSERT INTO `log` (`storing`, `medewerker`, `actie`, `data`)
    VALUES (NEW.`id`, NEW.`medewerker`, "Storing aangepast", @updated);
END

Answer 1

1. Since usage of prepared statements here is impossible, I would suggest you to call some INSERT statements, e.g. –

   IF NEW.column1 <> OLD.column1 THEN
   INSERT INTO…
   END IF;
   IF NEW.column2 <> OLD.column2 THEN
   INSERT INTO…
   END IF;
   …

2. Or try to copy all fields you need into another table.

In these cases you will avoid using cursor.