mysql> create table t (Total decimal(9,2), Paid decimal(9,2), Refund decimal(9,2));
Query OK, 0 rows affected (0.15 sec)

mysql> insert into t values(23.50, 13.50, 10.00);
Query OK, 1 row affected (0.07 sec)

mysql> select Total-Paid-Refund from t;
+-------------------+
| Total-Paid-Refund |
+-------------------+
| 0.00 |
+-------------------+
1 row in set (0.04 sec)

mysql> select Total-Paid-Refund from t where Total-Paid-Refund > 0;
Empty set (0.01 sec)

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