Charges using CROSS JOIN

gistfile1.txt

SELECT *, ROUND(amount / total * 100) AS percent
FROM charges
CROSS JOIN (SELECT SUM(amount) AS total FROM charges) t;

+----+--------+-------+---------+
| id | amount | total | percent |
+----+--------+-------+---------+
|  1 | 18.00  | 60.00 |      30 |
|  2 | 15.00  | 60.00 |      25 |
|  3 | 27.00  | 60.00 |      45 |
+----+--------+-------+---------+