构建多个子查询结果的总和

Question

我正在尝试构建来自多个子查询的结果的总和。 我用两个变量定义了许多时期：

declare 
@left1 date = '2014-06-25',
@return1 date ='2014-08-21',

 @left2 date = '2014-10-29',
 @return2 date = '2015-03-15',

@left3 date = '2015-04-21',
@return3 date = '2015-09-19'

然后我正在计算这两个日期之间的天数，但最终我想将它们相加并得到一个总数：

select  datediff(day, @left1, @return1 )

select  datediff(day, @left2, @return2 )

select  datediff(day, @left3, @return3 )

我想到了 CTE，但没有成功。这样做的正确方法是什么？

Answer 1

很确定你只需要一些基本的补充。

select datediff(day, @left1, @return1 )
     + datediff(day, @left2, @return2 )
     + datediff(day, @left3, @return3 )

Answer 2

select  
   datediff(day, @left1, @return1 ) + 
   datediff(day, @left2, @return2 ) + 
   datediff(day, @left3, @return3 )

Answer 3

这个查询可以帮助你：

Select 
    sum(NbDay) as totaldiffday
From (
        select  datediff(day, @left1, @return1 ) As NbDay
        Union
        select  datediff(day, @left2, @return2 ) As NbDay 
        Union 
        select  datediff(day, @left3, @return3 ) As NbDay
) as SubQuery

Answer 4

SELECT SUM(DATEDIFF(DAY,[LEFT],[Return])) AS TOTALDAYS
FROM
(
  select @left1 AS [Left],@return1 AS [Return]
  UNION ALL
  select @left2 AS [Left],@return2 AS [Return]
  UNION ALL
  select @left3 AS [Left],@return3 AS [Return]
) X