mysql> select u, u-1 from t;
+-------+-------+
| u     | u-1   |
+-------+-------+
|  8946 |  8945 |
| 29972 | 29971 |
| 46971 | 46970 |
| 47224 | 47223 |
| 54812 | 54811 |
| 71681 | 71680 |
+-------+-------+