Tracking Increasing or decreasing prices in MySQL - mysql

Question

I tried to write a query that returns the id, product, price, and change columns. The change column should follow this logic. If the item price has increased it should write positive and if it has decreased negative depending on the product and excluding the first initial product price. The last product price should be taken into consideration.
This is how the result should look like.
id product price change
1 apple 1
2 apple 1.5 positive
3 apple 3 positive
4 melon 4
5 melon 3 negative
6 apple 2 negative
I have tried to use Case When statement but failed.
select
p.id,
p.product,
p.price,
CASE
WHEN p.product = p.product AND p.price > p.price THEN 'Positive'
WHEN p.product = p.product AND p.price > p.price THEN 'Negative'
END AS 'Change'
from products p
Create and insert statements
CREATE TABLE `products` (
`id` INT(11) NOT NULL,
`product` VARCHAR(50) NOT NULL COLLATE 'utf8_unicode_ci',
`price` DECIMAL(10,2) NOT NULL DEFAULT '0.00'
)
COLLATE='utf8_unicode_ci'
ENGINE=InnoDB
;
INSERT INTO products (id, product, price)
VALUES (1, 'apple', 1);
INSERT INTO products (id, product, price)
VALUES (2, 'apple', 1.5);
INSERT INTO products (id, product, price)
VALUES (3, 'apple', 3);
INSERT INTO products (id, product, price)
VALUES (4, 'melon', 4);
INSERT INTO products (id, product, price)
VALUES (5, 'melon', 3);
INSERT INTO products (id, product, price)
VALUES (6, 'apple', 2);

Answer 1

Use lag():
select p.*,
(case when lag(price) over (partition by product order by id) < price
then 'negative'
when lag(price) over (partition by product order by id) > price
then 'positive'
end)
from products p;
In archaic versions of MySQL, you can use a correlated subquery:
select p.*
(case when prev_price < price
then 'negative'
when prev_price > price
then 'positive'
end)
from (select p.*,
(select p2.price
from product p2
where p2.product = p.product and p2.id < p.id
order by p2.id desc
limit 1
) as prev_price
from product p
) p;