请问我在C#中写了一段SQL语句，希望计算两个日期之间的天数，请问应该如何修改SQL语句？

已经写完的如下
select a.code,
a.work_start_date,
a.work_end_date,
a.trial_start_date,
a.trial_end_date,
b.salary,
c.name
from employee as b inner join labor_contract as a on b.id = a.id inner join post as c on a.id = c.id

我想计算 a.trial_start_date, 与a.trial_end_date,之间的时间差
希望能把您修改完的发到回答里边，会有回馈分的，万分感谢

select a.code,
a.work_start_date,
a.work_end_date,
DateDiff(d,a.work_start_date,a.work_end_date) AS wd,
a.trial_start_date,
a.trial_end_date,
DateDiff(d,a.trial_start_date,a.trial_end_date) AS td,
b.salary,
c.name
from employee as b inner join labor_contract as a on b.id = a.id inner join post as c on a.id = c.id

在表中再加一列 例如state 列。将这三列的state的值设为1.其他的为0.1就代表需要进货的那三个产品。 用select * from 表 where 单号=1001 and state=1 即可