选择日期范围内的特定日期

Question

I have the query below which gives me the number of days that a student was absent. DATEDIFF and DATEPART calculate the weekdays and holidays should not be counted as an absent day. The absent days stored in studentTable in two fields which are fromDate and toDate. So the absent days are in a date range. If a student is one day absent, it is recorded 11/23/2015, 11/23/2015. If a student is two days absent ,then 11/23/2015, 11/24/2015.

DECLARE @startDate DATE SET @startDate = '20151121'
DECLARE @endDate DATE SET @endDate = '20151123'       

SELECT      
  a.studentName

  ,SUM(DATEDIFF(dd, fromDate, toDate) 
  - (DATEDIFF(wk, fromDate, toDate) * 2) 
  -CASE WHEN DATEPART(dw, fromDate) = 1 THEN 1 ELSE 0 END 
  +CASE WHEN DATEPART(dw, toDate) = 1 THEN 1 ELSE 0 END + 1 )- COUNT(h.holiday)
   AS totalAbsentDay        

  FROM studentTable a 
LEFT OUTER JOIN holiday h 
ON h.holiday < a.toDate and h.holiday > a.fromDate

WHERE a.fromDate = @startDate AND a.toDate = @endDate
GROUP BY a.studentName

The problem here is that when I try to declare a start and an end date, it does not give me the correct absent days. For example, if a student is absent between 11/23/2015 and 11/26/2015 which is 4 days absent , and I declare start date 11/22/2015 and end date 11/27/2015, the query doesn't give me the result of 3.

Answer 1

This query below will work for given database scheme, may not the best solution because use three level of queries

DECLARE @startDate DATE SET @startDate = '2016-02-05'
DECLARE @endDate DATE SET @endDate = '2016-02-20'       

SELECT
    studentName,
    SUM(AbsentDay) totalAbsentDay
FROM
(
    SELECT      
      a.studentName
      ,DATEDIFF(dd, fromDate, toDate) 
      - (DATEDIFF(wk, fromDate, toDate) * 2) 
      -CASE WHEN DATEPART(dw, fromDate) = 1 THEN 1 ELSE 0 END 
      +CASE WHEN DATEPART(dw, toDate) = 1 THEN 1 ELSE 0 END + 1 - COUNT(h.holiday)
       AS AbsentDay        
      FROM (
            SELECT
                studentName,-- Name,
                CASE WHEN fromDate<@startDate THEN @startDate ELSE fromDate END fromDate,
                CASE WHEN toDate>@endDate THEN @endDate ELSE toDate END toDate
            FROM 
                StudentTable S
            WHERE 
                S.toDate >= @startDate AND s.fromDate <= @endDate
        ) a 
        LEFT OUTER JOIN holiday h 
        ON h.holiday < a.toDate and h.holiday > a.fromDate

    GROUP BY studentName, fromDate, toDate
) B
GROUP BY studentName

For easier query and faster execution please consider to redesign studentTable to something like idStudent , AbsentDate ..just a suggestion..