CREATE TABLE `helei` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`num1` double DEFAULT NULL,
`num2` double DEFAULT NULL,
`num3` decimal(5,3) DEFAULT NULL,
`num4` decimal(5,3) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=13 DEFAULT CHARSET=latin1;
mysql> select * from helei;
+----+------+------+-------+-------+
| id | num1 | num2 | num3 | num4 |
+----+------+------+-------+-------+
| 1 | 1 | 1 | 1.000 | 1.000 |
| 2 | 2 | 2 | 2.000 | 2.000 |
| 3 | 3 | 4 | 3.000 | 4.000 |
| 4 | 6 | 5 | 6.000 | 5.000 |
| 5 | 3.3 | 4.4 | 3.300 | 4.400 |
| 6 | 6.6 | 5.5 | 6.600 | 5.500 |
+----+------+------+-------+-------+
6 rows in set (0.00 sec)
mysql> select (num2-num1)num5,(num4-num3)num6 from helei;
+---------------------+--------+
| num5 | num6 |
+---------------------+--------+
| 0 | 0.000 |
| 0 | 0.000 |
| 1 | 1.000 |
| -1 | -1.000 |
| 1.1000000000000005 | 1.100 |
| -1.0999999999999996 | -1.100 |
+---------------------+--------+
6 rows in set (0.00 sec)
可以看到double列的减法出现了问题，我们可用demical替换即可