Du vil sandsynligvis kigge nærmere på MySQL's brugerdefinerede variabler , og så vil du sikkert gøre sådan noget her:
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)
Jeg har ikke tjekket syntaks, så det kan være en smule off, men det er den generelle idé. Bemærk, at jeg tilføjede dato, så du kan bestille efter den, ellers kan resultaterne være meningsløse.
EDIT:
Har lige kørt dette på min maskine:
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)