MySQL：十进制数据类型的大小

来自 MySQL：

DECIMAL 列的声明语法是 DECIMAL（M，D）。MySQL 5.1中参数的值范围如下：

M 是最大位数（精度）。它的范围为 1 到 65。（旧版本的MySQL允许1到254的范围。

D 是小数点（小数位数）右侧的位数。它的范围为 0 到 30，并且不能大于 M。

考虑这个数字：123456789.12345在这里是，然后基于这个原则，你可以根据它们的预期最大值为每列设置小数（M，D）。M14D5

由于接受的答案让我感到匮乏，因此我在一个表上运行，该表包含定义为的几列（请注意缺少M和D的值。SHOW CREATE TABLEDECIMAL DEFAULT NULL

结果如下。

data_warehouse as davidg Wed Dec 05 12:10:36 2018 >SHOW CREATE TABLE erth_calendarmonths_historic_usage_preload;
+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table                                      | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |
+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| erth_calendarmonths_historic_usage_preload | CREATE TABLE `erth_calendarmonths_historic_usage_preload` (
  `market` varchar(100) NOT NULL,
  `commodity` varchar(100) NOT NULL,
  `account_number` varchar(100) NOT NULL,
  `meter_number` varchar(100) NOT NULL,
  `period_year_month` int(11) NOT NULL,
  `estimated_usage_amount` decimal(18,7) DEFAULT NULL,
  `unit` varchar(100) DEFAULT NULL,
  `meter_read_start_date_part_1` datetime DEFAULT NULL,
  `meter_read_end_date_part_1` datetime DEFAULT NULL,
  `gross_nonadjusted_usage_amount_part_1` decimal(10,0) DEFAULT NULL,
  `applied_nonadjusted_usage_amount_part_1` decimal(10,0) DEFAULT NULL,
  `meter_read_start_date_part_2` datetime DEFAULT NULL,
  `meter_read_end_date_part_2` datetime DEFAULT NULL,
  `gross_nonadjusted_usage_amount_part_2` decimal(10,0) DEFAULT NULL,
  `applied_nonadjusted_usage_amount_part_2` decimal(10,0) DEFAULT NULL,
  `utility_rate_class` varchar(100) DEFAULT NULL,
  `utility_rate_subclass` varchar(100) DEFAULT NULL,
  `load_profile` varchar(100) DEFAULT NULL,
  `hu_type` varchar(100) DEFAULT NULL,
  `type` varchar(100) DEFAULT NULL,
  `utility_duns` varchar(100) DEFAULT NULL,
  `create_date` datetime DEFAULT NULL,
  `update_date` datetime DEFAULT NULL,
  `UsedBuckets` tinyint(4) NOT NULL DEFAULT '0',
  PRIMARY KEY (`market`,`commodity`,`account_number`,`meter_number`,`period_year_month`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+--------------------------------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

上述情况使M和D的默认值不言而喻;M = 10，而 D = 0。毋庸置疑，这几乎肯定不是预期的结果。

你总是要指定你的M和D值。