Iam quite new to functions in SQL and I would like to create a function to compare values in a MySQL table against previous and I am not sure how to do this.

For example (iId is the input value)

DECLARE pVal INT(20);
DECLARE val INT(20);

SELECT price INTO pVal FROM products WHERE Id=iId;

SELECT price FROM products;

IF price == pVal THEN
SET val = price;
END IF;

Thanks