计算两个日期之间的天数
(1)MySQL解决方案：
select
datediff(xj_date,xz_date)
from(
select rdr_date as xj_date
from readrinfo
where rdr_name = 'xiaojie'
) x,
(
select rdr_date as xz_date
from readrinfo
where rdr_name = 'xiaozhao'
) y
;
(2)SQL Server解决方案：就是交换datediff函数中的参数的位置且增加‘day’，
例：
datediff(day
,xz_date,xj_date).