Du vill förmodligen titta på MySQL:s användardefinierade variabler , och då vill du förmodligen göra något sånt här:
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)
Jag har inte kollat syntax så det kan vara lite fel men det är den allmänna idén. Observera att jag har lagt till datum så att du kan beställa efter det, annars kan resultaten bli meningslösa.
EDIT:
Körde precis det här på min maskin:
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)