Access SQL - Count days in each month between two dates - sql

Question

I have two tables - [Students] Which holds a list of students (and the name of their teacher), and [Absence Extract] Which holds a record of each instance of absence for any student (along with the start and end dates and total days).
I am trying to write a query that will group the days of absence by teacher and then show how many days of absence they have had in each month. I've started by writing the below query where I have shown the calculation I've come up with so far for days lost in January and February (I would then add the calculations for the other months), however this example isn't working as if they had e.g 15 days absence spread across January and February this is returning 15 days for both months.
Could someone point me in the right direction with this please?
SELECT
[Students].[Teacher Name] AS [Teacher],
SUM(IIF(ae.[Absence End Date] >= #1/1/18# AND ae.[Absence Start Date] <= #1/31/18#,[Total Days],0)) AS [Jan Days],
SUM(IIF(ae.[Absence End Date] >= #2/1/18# AND ae.[Absence Start Date] <= #2/28/18#,[Total Days],0)) AS [Feb Days]
FROM
[Students]
INNER JOIN
[Absence Extract] ae ON [Students].[ID] = [ae].[Student ID]
GROUP BY [Students].[Teacher Name];

Answer 1

Do not use the calculated column, Total Days, as it sums the difference between start and end dates across months and even years. Consider calculating separate durations:
absence duration from start date to end of start date month
absence duration from first of end date month to end date
Then join the aggregations together. So for example, the absence range Jan 30, 2018 - Feb 2, 2018:
first query calculates days from Jan 30 to Jan 31 (end of month), grouped to January month
second query calculates from Feb 1 (start of month) to Feb 2, grouped to February month
join query aligns to Teacher, Year, and Month, and calculates total duration with arithmetic addition, +, where 2 days result for Jan and 2 days result for Feb on different rows
Start Date Query
SELECT
s.[Teacher Name] AS [Teacher],
Year(ae.[Absence End Date]) As Year_Absence,
MonthName(Month(ae.[Absence Start Date]), TRUE) As Month_Absence,
SUM(DateDiff('d', ae.[Absence Start Date],
DateAdd('m', 1, ae.[Absence Start Date]) -
Day(ae.[Absence Start Date]))) As StartDuration
FROM
[Students] s
INNER JOIN
[Absence Extract] ae ON s[ID] = ae.[Student ID]
GROUP BY s.[Teacher Name],
Year(ae.[Absence Start Date])
MonthName(Month(ae.[Absence Start Date]), TRUE);
End Date Query
SELECT
s.[Teacher Name] AS [Teacher],
Year(ae.[Absence End Date]) As Year_Absence,
MonthName(Month(ae.[Absence End Date]), TRUE) As Month_Absence,
SUM(DateDiff('d', ae.[Absence End Date],
ae.[Absence End Date] -
(Day(ae.[Absence End Date])-1))) As EndDateDuration
FROM
[Students] s
INNER JOIN
[Absence Extract] ae ON s[ID] = ae.[Student ID]
GROUP BY s.[Teacher Name],
Year(ae.[Absence End Date])
MonthName(Month(ae.[Absence End Date]), TRUE);
Join Query (Long Format)
SELECT s.[Teacher Name],
s.Year_Absence,
s.Month_Absence,
NZ(s.StartDuration) + NZ(e.EndDateDuration) As TotalDuration
FROM startdate_query s
LEFT JOIN enddate_query e
ON s.[Teacher Name] = e.[Teacher Name]
AND s.Year_Absence = e.Year_Absence
AND s.Month_Absence = e.Month_Absence
And since you are looking for a wide report with month columns maintaining aggregate sums of absence duration, consider MS Access's own crosstab query.
Crosstab Query (Wide Format)
TRANSFORM SUM(NZ(s.StartDuration) + NZ(e.EndDateDuration)) AS [SumDays]
SELECT s.[Teacher Name],
s.Year_Absence
FROM startdate_query s
LEFT JOIN enddate_query e
ON s.[Teacher Name] = e.[Teacher Name]
AND s.Year_Absence = e.Year_Absence
AND s.Month_Absence = e.Month_Absence
GROUP BY s.[Teacher Name],
s.Year_Absence
PIVOT s.Month_Absence IN ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec')
Of course, all this is untested without actual data. So, various adjustments may be needed.