SQL Server: Finding date given EndDate and # Days, excluding days from specific date ranges - sql

Question

I have a TableA in a database similar to the following:
Id | Status | Start | End
1 | Illness | 2013-04-02 | 2013-04-23
2 | Illness | 2013-05-05 | 2014-01-01
3 | Vacation | 2014-02-01 | 2014-03-01
4 | Illness | 2014-03-08 | 2014-03-09
5 | Vacation | 2014-05-05 | NULL
Imagine it's keeping track of a specific user's "Away" days. Given the following Inputs:
SomeEndDate (Date),
NumDays (Integer)
I want to find the SomeStartDate (Date) that is Numdays non-illness days from EndDate. In other words, say I am given a SomeEndDate value '2014-03-10' and a NumDays value of 60; the matching SomeStartDate would be:
2014-03-10 to 2014-03-09 = 1
2014-03-08 to 2014-01-01 = 57
2013-05-05 to 2013-05-03 = 2
So, at 60 non-illness days, we get a SomeStartDate of '2013-05-03'. IS there any easy way to accomplish this in SQL? I imagine I could loop each day, check whether or not it falls into one of the illness ranges, and increment a counter if not (exiting the loop after counter = #numdays)... but that seems wildly inefficient. Appreciate any help.

Answer 1

Make a Calendar table that has a list of all the dates you will ever care about.
SELECT MIN([date])
FROM (
SELECT TOP(#NumDays) [date]
FROM Calendar c
WHERE c.Date < #SomeEndDate
AND NOT EXISTS (
SELECT 1
FROM TableA a
WHERE c.Date BETWEEN a.Start AND a.END
AND Status = 'Illness'
)
ORDER BY c.Date
) t
The Calendar table method lets you also easily exclude holidays, weekends, etc.

Answer 2

SQL Server 2012:
Try this solution:
DECLARE #NumDays INT = 70, #SomeEndDate DATE = '2014-03-10';
SELECT
[RangeStop],
CASE
WHEN RunningTotal_NumOfDays <= #NumDays THEN [RangeStart]
WHEN RunningTotal_NumOfDays - Current_NumOfDays <= #NumDays THEN DATEADD(DAY, -(#NumDays - (RunningTotal_NumOfDays - Current_NumOfDays))+1, [RangeStop])
END AS [RangeStart]
FROM (
SELECT
y.*,
DATEDIFF(DAY, y.RangeStart, y.RangeStop) AS Current_NumOfDays,
SUM( DATEDIFF(DAY, y.RangeStart, y.RangeStop) ) OVER(ORDER BY y.RangeStart DESC) AS RunningTotal_NumOfDays
FROM (
SELECT LEAD(x.[End]) OVER(ORDER BY x.[End] DESC) AS RangeStart, -- It's previous date because of "ORDER BY x.[End] DESC"
x.[Start] AS RangeStop
FROM (
SELECT #SomeEndDate AS [Start], '9999-12-31' AS [End]
UNION ALL
SELECT x.[Start], x.[End]
FROM #MyTable AS x
WHERE x.[Status] = 'Illness'
AND x.[End] <= #SomeEndDate
) x
) y
) z
WHERE RunningTotal_NumOfDays - Current_NumOfDays <= #NumDays;
/*
Output:
RangeStop RangeStart
---------- ----------
2014-03-10 2014-03-09
2014-03-08 2014-01-01
2013-05-05 2013-05-03
*/
Note #1: LEAD(End) will return the previous End date (previous because of ORDER BY End DESC)
Note #2: DATEDIFF(DAY, RangeStart, RangeStop) computes the num. of days between current start (alias x.RangeStop) and "previous" end (alias x.RangeStar) => Current_NumOfDays
Note #3: SUM( Current_NumOfDays ) computes a running total thus: 1 + 66 + (3)
Note #4: I've used #NumOfDays = 70 (not 60)