mysql|unsigned 与 signed 类型

通过mysql的数值类型设置，控制数值的正负

1，如何使用

在mysql的编辑器中，可以直接定义 bigint(20) unsigned

 2, 发挥的作用

一般默认定义的数据类型为signed（有符号类型），取值返回包含有负数范围，一般正负值的差依然等于无符号类型的范围的上限值。

当定义为unsigned（无符号类型）时， 取值范围仅为正数范围，下限值为0；

3，当设置为unsigned时候，报错BIGINT UNSIGNED value is out of range....如何解决

使用unsigned限制数值范围为正数的时候，如果执行相减操作产生负数；就会报错；解决方法

-- 需要添加四个计算值计算每日变化数据
-- dayConfirmed: 每日确诊
-- dayRecovered: 每日康复
-- dayDeaths: 每日死亡
-- dayExisting: 当日现存
select n.id,
       n.region_id,
       n.region_parent_id,
       n.recovered,
       n.deaths,
       n.day_date,
       n.confirmed,
       (cast(n.confirmed as signed) - cast(m.confirmed as signed)) as dayConfirmed,
       (cast(n.recovered as signed) - cast(m.recovered as signed)) as dayRecovered,
       (cast(n.deaths as signed) - cast(m.deaths as signed))       as dayDeaths,
       (cast(n.confirmed  as signed) - cast(n.recovered  as signed) - cast(n.deaths  as signed))    as dayExisting
from region_data_total as n
         join region_data_total as m on date_sub(n.day_date, interval 1 day) = m.day_date
where n.region_id = m.region_id and n.day_date = (select max(day_date) from region_data_total);

核心： 使用 cast(targetCol as signed) 将所有涉及到的unsigned字段先转化为signed类型后，再进行运算

相关内容补充：

MYSQL的数据类型（菜鸟教程）：

• https://www.runoob.com/mysql/mysql-data-types.html