SQL本周本月等问题

---求相差天数   
select   datediff(day,'2004-01-01',getdate())       
     
--1.一个月第一天的   
SELECT   DATEADD(mm,   DATEDIFF(mm,0,getdate()),   0)   
     
--2.本周的星期一   
SELECT   DATEADD(wk,   DATEDIFF(wk,0,getdate()),   0)   
     
select   dateadd(wk,datediff(wk,0,getdate()),6)  

--3.一年的第一天   
SELECT   DATEADD(yy,   DATEDIFF(yy,0,getdate()),   0)   
     
--4.季度的第一天   
SELECT   DATEADD(qq,   DATEDIFF(qq,0,getdate()),   0)   
     
--5.当天的半夜   
SELECT   DATEADD(dd,   DATEDIFF(dd,0,getdate()),   0)   
     
--6.上个月的最后一天   
SELECT   dateadd(ms,-3,DATEADD(mm,   DATEDIFF(mm,0,getdate()),   0))   
     
--7.去年的最后一天   
SELECT   dateadd(ms,-3,DATEADD(yy,   DATEDIFF(yy,0,getdate()),   0))   
     
--8.本月的最后一天   
SELECT   dateadd(ms,-3,DATEADD(mm,   DATEDIFF(m,0,getdate())+1,   0))   
     
--9.本年的最后一天   
SELECT   dateadd(ms,-3,DATEADD(yy,   DATEDIFF(yy,0,getdate())+1,   0))   
     
--10.本月的第一个星期一   
select   DATEADD(wk,   
DATEDIFF(wk,0,dateadd(dd,6-datepart(day,getdate()),getdate())),   0)

--查询本周注册人数   
select   count(*)   from   [user]   
where   datediff(week,create_day-1,getdate())=0   
     
--上周注册人数   
select   count(*)   from   [user]   
where   datediff(week,create_day-1,getdate())=1   
     
     
--本月注册人数   
select   count(*)   from   [user]   
where   datediff(month,create_day,getdate())=0   
     
--上月注册人数   
select   count(*)   from   [user]   
where   datediff(month,create_day,getdate())=1   
   
--如果要效率,这样写查询   
     
--查询本周注册人数   
select   count(*)   from   [user]   
where   create_day>=dateadd(day,2-datepart(weekday,getdate()),convert(varchar,getdate(),112))   
and   create_day





=dateadd(day,-5-datepart(weekday,getdate()),convert(varchar,getdate(),112))   
and   create_day=dateadd(day,1-day(getdate()),convert(varchar,getdate(),112))   
and   create_day=dateadd(month,-1,dateadd(day,1-day(getdate()),convert(varchar,getdate(),112)))   
and   create_day