如何从MySQL表中删除以相反顺序存储的重复值？

mysql> Select * from details_city;
+--------+--------+
| City1  | City2  |
+--------+--------+
| Delhi  | Nagpur |
| Delhi  | Mumbai |
| Nagpur | Delhi  |
| Katak  | Delhi  |
| Delhi  | Katak  |
+--------+--------+
5 rows in set (0.00 sec)

mysql> Select a.city1,a.city2 from details_city a WHERE a.city1 <= a.city2;
+-------+--------+
| city1 | city2  |
+-------+--------+
| Delhi | Nagpur |
| Delhi | Mumbai |
| Delhi | Katak  |
+-------+--------+
3 rows in set (0.06 sec)