Need help with my MySQL PROCEDURE #1193 - Unknown system variable I've this simple table:

CREATE TABLE IF NOT EXISTS `testing`
(
    `testing_ID` INT UNSIGNED NOT NULL AUTO_INCREMENT,
    `meter` INT NOT NULL,
    `price` DECIMAL(12,2) NOT NULL,
    `price_per_meter` DECIMAL(10,2) NOT NULL,
    PRIMARY KEY (`testing_ID`)
)


And I've written this MySQL PROCEDURE:

DELIMITER $$
CREATE PROCEDURE `price_per_meter_calc` (price DECIMAL(12,2))
BEGIN
    SET price_per_meter = price/meter;
END
$$
DELIMITER ;


but I've got an error #1193 - Unknown system variable 'price_per_meter'. I believe it's pretty straightforward code. What I want is simply MySQL to automatically calculate price_per_meter given the user will only insert price and meter values.
You need to declare variable first and then only you can set the value.

Something like:

DECLARE price_per_meter DECIMAL(6,2);


Then other part is just logics.