Probablemente desee examinar las variables definidas por el usuario , y luego probablemente quieras hacer algo como esto:
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)
No he comprobado la sintaxis, por lo que podría estar un poco mal, pero esa es la idea general. Tenga en cuenta que agregué la fecha para que pueda ordenar por ella, de lo contrario, los resultados pueden no tener sentido.
EDITAR:
Acabo de ejecutar esto en mi máquina:
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)
