double减法不准确的那些事儿

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

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