MySQL Trigger for Aging - mysql

Question

I'm trying to create a trigger that will automatically increase the year column by 1 when the month column is 12, and then empty the month column.
But I don't understand triggers that well and I'm confusing myself in my attempts, so I was hoping someone could look over this code to see if it would work or suggest improvements:
CREATE TRIGGER aging BEFORE UPDATE ON dogs
FOR EACH ROW
DELETE FROM dogs WHERE month = 12;
UPDATE year SET year = year+1;
Thanks!

Answer 1

Two things to point out...
First, a trigger is clearly the wrong choice for what you want to do. Not only because the trigger can be avoided by calculating the age on the fly based on birth of date. But also, because a trigger only affects the rows that are affected by the corresponding statement. Don't be fooled by the FOR EACH ROW. You write something like
UPDATE table SET whatever = whatever WHERE month = 12;
and all the rows where month = 12 is true are affected. What you seem to want to do, is to update the whole table to check if in some rows your condition is true. Not only is this a performance nightmare, you are also restricted to one trigger (per event) per table. Meaning you can have i.e. one BEFORE UPDATE and 1 AFTER UPDATE but not 2 BEFORE UPDATE.
The second thing to note is, that you can't issue an UPDATE statement in an UPDATE trigger. This would cause an infinite loop :)
Instead you use the aliases NEW and OLD. NEW refers to the value of a column given by the UPDATE statement and OLD refers to the value of the column before the UPDATE statement was executed.
In your case (althoug it's hopefully clear by now, that you shouldn't use the trigger in this case), one would write the trigger like this:
Given this initial statement
UPDATE myTable SET whatever = whatever;
which updates the whole table, this trigger
DROP TRIGGER IF EXISTS [whatever you call this thing];
DELIMITER ##
CREATE TRIGGER aging BEFORE UPDATE ON dogs
FOR EACH ROW
IF (month = 12) THEN /*month refers to the column here, not a variable like in Gordon's answer*/
SET NEW.year = OLD.year+1;
SET NEW.month = 1;
END IF;
END ##
would update the columns correctly.

Answer 2

I've never created a trigger before, but I just read through the docs. This is my attempt...
DELIMITER ||
DROP TRIGGER ID EXISTS [whatever you call this thing]
||
DELIMITER ##
CREATE TRIGGER aging BEFORE UPDATE ON dogs
FOR EACH ROW
DECLARE month integer;
IF (month = 12) THEN
-- DELETE FROM dogs WHERE month = 12;
-- this will delete the record; probably not what you want
UPDATE year SET year = year+1;
UPDATE month SET month = NULL;
END IF;
END;
##