1. 在mysql数据库中，unsigned表面含义是 '无符号’的意思,unsigned既为非负数，用此类型可以增加数据长度.
    例如，如果 int最大是2147683647，那 tint unsigned 最大 就可以到 2147683647 * 2

  2. 与unsigned类似的还有binary,unsigned 属性只针对整型，而binary属性只用于char 和varchar。

  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
    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

-- 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);

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