如何推日在SQL服务器计算的每日计数？

为未来的问题，不要发布图像的数据。 相反，使用服务，如 dbfiddle. 我无论如何增加一个素描为一个答案，与一个更好的准备的问题，你可以得到完整的答案。 无论如何，这就是:

-- extrema is the least and the greatest date in staff table
with extrema(mn, mx) as (
    select least(min(hired),min(retired)) as mn
         , greatest(max(hired),max(retired)) as mx
    from staff
), calendar (dt) as (
    -- we construct a calendar with every date between extreme values
    select mn from extrema
    union all
    select dateadd(day, 1, d)
    from calendar
    where dt < (select mx from extrema)
)
-- finally we can count the number of employed people for each such date
select dt, count(1) 
from calendar c 
join staff s
    on c.dt between s.hired and s.retired
group by dt; 

如果你发现自己做这样的计算通常，这是一个很好的想法，以创建一个日历表。 你可以添加其他属性如，如果它是一个天在一周的中间等。

有约束：

CHECK(hired <= retired)

第一部分可以简化：

with extrema(mn, mx) as (
    select min(hired) as mn
         , max(retired) as mx
    from staff
),

假设目前的员工有一个空的退休日期

Declare @Date1 date = '2015-01-01'
Declare @Date2 date = getdate()

Select A.Date
      ,HeadCount = count(B.name)
 From ( Select Top (DateDiff(DAY,@Date1,@Date2)+1) 
               Date=DateAdd(DAY,-1+Row_Number() Over (Order By (Select Null)),@Date1) 
         From  master..spt_values n1,master..spt_values n2
      ) A
 Left Join YourTable B on A.Date >= B.Hired and A.Date <= coalesce(B.Retired,getdate())
 Group BY A.Date

你需要一个日历表。 你开始的日历，并留下加入其他的一切，使用 BETWEEN 逻辑。

你可以使用一个真正表。 或者你可以产生它在飞行中，像这样：

WITH
    L0 AS ( SELECT c = 1
            FROM (VALUES(1),(1),(1),(1),(1),(1),(1),(1),
                        (1),(1),(1),(1),(1),(1),(1),(1)) AS D(c) ),
    L1 AS ( SELECT c = 1 FROM L0 A, L0 B, L0 C, L0 D ),
    Nums AS ( SELECT rownum = ROW_NUMBER() OVER(ORDER BY (SELECT 1))
              FROM L1 ),
    Dates AS (
      SELECT TOP (DATEDIFF(day, '20141231', GETDATE()))
        Date = DATEADD(day, rownum, '20141231')
      FROM Nums
    )

SELECT
  d.Date,
  NumEmployed = COUNT(*)
FROM Dates d
JOIN YourTable t ON d.Date BETWEEN t.Hired AND t.Retired
GROUP BY
  d.Date;

如果你的日期有时间组成部分，那么你需要使用 >= AND < 逻辑

尝试限制范围内的日期表。 在这个例子我有一个表格的日期名为TallyStickDT.

SELECT dt, COUNT(name)
FROM (
    SELECT dt
    FROM tallystickdt
    WHERE dt >= (SELECT MIN(hired) FROM #employees)
    AND dt <= GETDATE()
) A
LEFT OUTER JOIN #employees E ON A.dt >= E.Hired AND A.dt <= e.retired
GROUP BY dt
ORDER BY dt