SQL how to subtract result row 1 from row 2, row 2 from row 3

Question

How do I subtract row 1 from row 2 and row 2 from row 3, etc. in MySQL? The table i am pulling my data from contains multiple products and all products have multiple prices (on different dates)

The code i am working with:

 SELECT 
        orderline_sales.product_name,
        orderline_sales.price
    FROM
        orderline_sales         
    GROUP BY price
    HAVING orderline_sales.product_name = 'Ibuprofen';

The result I am getting:

|---------------------|------------------|
|      product_name   |     price        |
|---------------------|------------------|
|       Ibuprofen     |      30.20       |
|---------------------|------------------|
|       Ibuprofen     |      32.20       |
|---------------------|------------------|
|       Ibuprofen     |      35.20       |
|---------------------|------------------|

The result I want:

|---------------------|------------------|------------------|
|      product_name   |     price        |   price_change   |
|---------------------|------------------|------------------|
|       Ibuprofen     |      30.20       |         0        |
|---------------------|------------------|------------------|
|       Ibuprofen     |      32.20       |         2        |
|---------------------|------------------|------------------|
|       Ibuprofen     |      35.20       |         3        |
|---------------------|------------------|------------------|

Answer 1

You probably want to look into MySQL's user defined variables, and then you probably want to do something like this:

SET @prev := NULL;
SELECT
    DATE(created_at),
    price - COALESCE(@prev, price) AS price_change,
    name,
    (@prev := price) AS price FROM (
        SELECT * FROM items ORDER BY DATE(created_at)
    ) t1
GROUP BY
    name, price, DATE(created_at)
HAVING name = 'Ibuprofen'
ORDER BY DATE(created_at);
Query OK, 0 rows affected (0.00 sec)

I haven't checked syntax so it might be a little off but that is the general idea. Note that I added date so that you can order by it, otherwise the results may be meaningless.

EDIT:

Just ran this on my machine:

SET @prev := NULL;
SELECT
    DATE(created_at),
    price - COALESCE(@prev, price) AS price_change,
    name,
    (@prev := price) AS price FROM (
        SELECT * FROM items ORDER BY DATE(created_at)
    ) t1
GROUP BY
    name, price, DATE(created_at)
HAVING name = 'Ibuprofen'
ORDER BY DATE(created_at);

Query OK, 0 rows affected (0.00 sec)

+------------------+--------------+-----------+-------+
| DATE(created_at) | price_change | name      | price |
+------------------+--------------+-----------+-------+
| 2018-12-10       |            0 | Ibuprofen |   110 |
| 2018-12-13       |          -10 | Ibuprofen |   100 |
| 2018-12-13       |           20 | Ibuprofen |   120 |
+------------------+--------------+-----------+-------+

3 rows in set, 1 warning (0.00 sec)

SELECT * FROM items;
+----+-------+----------------+---------------------+
| id | price | name           | created_at          |
+----+-------+----------------+---------------------+
|  8 |   100 | Ibuprofen      | 2018-12-13 12:52:35 |
|  9 |   110 | Ibuprofen      | 2018-12-10 12:12:12 |
| 10 |   120 | Ibuprofen      | 2018-12-13 12:52:35 |
| 11 |  1000 | Something else | 2018-12-13 13:01:19 |
+----+-------+----------------+---------------------+

4 rows in set (0.00 sec)