mysql: timestamp only one column when update char - php

Question

I want to create a log with timestamp in mysql.
For this I created a column tm with datatype timestamp.
It works fine and is updated every time I change something.
The problem is that it should only update when I set the exist column to '1'. Not when I change any column.
Here you can view my table ..:
CREATE TABLE admins (
id int NOT NULL AUTO_INCREMENT,
bez varchar(55) DEFAULT 'unknown',
type varchar(7) NOT NULL DEFAULT 'unknown',
exist char NOT NULL DEFAULT '0',
tm TIMESTAMP
PRIMARY KEY ('id')
);
This is my UPDATE:
if ( isset($_GET["id"]) ) {
$id = $_GET["id"];
$sql = "UPDATE admins SET exist='0' WHERE id=$id";
$con->query($sql);
}
header("location: /index.php");
exit;
?>
So the UPDATE should change the timestamp and the exist.
Please dont tell me that i am open to sql injections, i know this and we have a security manager in this project who will change afterwards.
thanks for your help community!

Answer 1

The ON UPDATE CURRENT_TIMESTAMP feature that is implicitly enabled on the first TIMESTAMP on a table is an all-or-nothing feature, it does not allow for the fine grained control you are aiming for.
The behavior you are looking for can only be accomplished by either explicitly setting the TIMESTAMP column value in the UPDATE, or by adding a trigger to the table:
DELIMITER $$
CREATE TRIGGER trigger_name
BEFORE UPDATE
ON admins FOR EACH ROW
BEGIN
IF NEW.exists = 1 THEN
SET NEW.tm = NOW();
END IF;
END$$
DELIMITER ;
and chang the column definition from
tm TIMESTAMP;
to
tm TIMESTAMP default CURRENT_TIMESTAMP;
or maybe
tm TIMESTAMP DEFAULT 0;
or even
tm DATETIME;
to prevent the ON UPDATE CURRENT_TIMESTAMP behavior.