Questions[SQL]：Calculate the Number of Days in a Month

Q：Can you use a batch SQL or store procedure to calculating the Number of Days in a Month?

A:

解法一：使用CTE，结构清晰

     with  arg0  as
    (
         -- 计算当月第一天
         select   cast (
             cast ( year ( getdate ())  as   varchar ) + ' - ' + cast ( month ( getdate ())  as   varchar ) + ' -01 '  
             as   datetime )  as  nowdt
    ),
    arg1  as
    (
         -- 计算下月第一天
         select   dateadd (mm, 1 ,nowdt)  as  dt  from  arg0
    ),
    arg2  as
    (
         -- 计算下月减1，即本月最后一天
         select   dateadd (dd, - 1 , dt)  as  dt  from  arg1
    ),
    arg3  as
    (
         -- 取天数部分
         select   datepart (dd, dt)  as  daies  from  arg2
    )
     select   *   from  arg3

 

解法二：使用变量

declare   @now    datetime
set   @now =   cast (
             cast ( year ( getdate ())  as   varchar ) + ' - ' + cast ( month ( getdate ())  as   varchar ) + ' -01 '  
             as   datetime ) 
declare   @day   int
set   @day   =   datepart (dd,  dateadd (dd, - 1 ,  dateadd (mm, 1 , @now )))
select   @day

解法三：单一语句

select   datepart (dd, dateadd (dd, - 1 , dateadd (mm, 1 , cast ( cast ( year ( getdate ())  as   varchar ) + ' - ' + cast ( month ( getdate ())  as   varchar ) + ' -01 '   as   datetime ))));