今天在做一个功能的时候，用到两个时间戳相减来做查询条件，由于其两个字段都是unsigned的，并两个的大小是不一样。所以直接相减查询的时候，
就出现ERROR 1690 (22003): BIGINT UNSIGNED value is out of range in..的错误，因为结果可能会出现负数。。。。
过后查资料，发现可以通过下面的方法来解决：
mysql> select lastvisit, lastactivity from pre_common_member_status limit 1;
+------------+--------------+
| lastvisit | lastactivity |
+------------+--------------+
| 1199200260 | 1198336989 |
+------------+--------------+
1 row in set (0.01 sec)
mysql> select lastvisit-lastactivity from pre_common_member_status limit 1;
+------------------------+
| lastvisit-lastactivity |
+------------------------+
| 863271 |
+------------------------+
1 row in set (0.05 sec)
mysql> select abs(lastvisit-lastactivity) from pre_common_member_status limit 1;
+-----------------------------+
| abs(lastvisit-lastactivity) |
+-----------------------------+
| 863271 |
+-----------------------------+
1 row in set (0.03 sec)
mysql> select lastactivity-lastvisit from pre_common_member_status limit 1;
ERROR 1690 (22003): BIGINT UNSIGNED value is out of range in '(`discuz`.`pre_c
ommon_member_status`.`lastactivity` - `discuz`.`pre_common_member_status`.`las
tvisit`)'
mysql> select cast(lastactivity as signed)-cast(lastvisit as signed) from pre_co
mmon_member_status limit 1;
+--------------------------------------------------------+
| cast(lastactivity as signed)-cast(lastvisit as signed) |
+--------------------------------------------------------+
| -863271 |
+--------------------------------------------------------+
1 row in set (0.02 sec)
mysql> select abs(cast(lastactivity as signed)-cast(lastvisit as signed)) from p
re_common_member_status limit 1;
+-------------------------------------------------------------+
| abs(cast(lastactivity as signed)-cast(lastvisit as signed)) |
+-------------------------------------------------------------+
| 863271 |
+-------------------------------------------------------------+
1 row in set (0.00 sec)
mysql>