Cross Join

gistfile1.txt

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

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