SQL查询：集团按月份及年份针对这种情况

Question

我有一个表产品有以下的列

ProductId Name RegistrationDate UnregistrationDate 
1   AB 2013-01-01  2013-03-01 
2   CD 2013-01-10  2013-03-13 

等

我想获得注册产品每一个列表一年中的每个月。

示例：年份，月份以及注册和未注销经销商的数量。

Year Month RegisteredProucts 
2013 2   35 
2013 3   45(includes products even registered before March 2013) 

我写的follwing存储过程来找到注册的产品为一个月： &它的工作原理：

@Begin Time = First Day of the Month 
@End Time = Last Day of the Month 


select COUNT(DISTINCT P.ProductId) as RegisteredProducts from Product P  
where ((P.RegisteredDate < @EndTime) 
AND (P.UnregisteredDate > @EndTime)) 

然后我写了下面的查询，但它似乎组的结果由RegisteredDate 。 我想知道我可以在每个月末 将注册产品（非注册产品）分组一年吗？

select YEAR(P.RegisteredDate) AS [YEAR],MONTH(P.RegisteredDate) AS [MONTH],  COUNT(DISTINCT P.ProductId) as RegisteredProducts from Product P  
where ((P.RegisteredDate < @EndTime) 
AND (P.UnregisteredDate > @EndTime)) 
group by YEAR(D.RegisteredDate), MONTH(D.RegisteredDate) 

Answer 1

WITH months (mon) AS 
     (
     SELECT CAST('2013-01-01' AS DATE) AS mon 
     UNION ALL 
     SELECT DATEADD(month, 1, mon) 
     FROM months 
     WHERE mon < DATEADD(month, -1, GETDATE()) 
     ) 
SELECT mon, COUNT(productId) 
FROM months 
LEFT JOIN 
     registeredProducts 
ON  registrationDate < DATEADD(month, 1, mon) 
     AND (unregistrationDate >= mon OR unregistrationDate IS NULL) 
GROUP BY 
     mon 

Answer 2

; with months as 
     (
     select cast('2013-01-01' as date) as dt 
     union all 
     select dateadd(month, 1, dt) 
     from months 
     where dt < '2014-01-01' 
     ) 
select * 
from months m 
cross apply 
     (
     select count(*) as ProductCount 
     from Product p 
     where p.RegistrationDate < dateadd(month, 1, m.dt) and 
       (
        UnregistrationDate is null 
        or UnregistrationDate >= m.dt 
       ) 
     ) p 

Example at SQL Fiddle.