Calculate off days between 2 dates in sql - sql-server

Question

I have a function that calculate the number of working days between 2 dates. What I want is to convert this existing function into something like it will calculate the number of holidays in month or let's say in between 2 dates.
My existing function is something like this.
CREATE FUNCTION [dbo].[fnGetDaysWorked] (#StartDate datetime, #EndDate datetime)
RETURNS int
AS
BEGIN
DECLARE #dateFrom datetime = '2018/01/01'
DECLARE #dateTo datetime = '2018/01/31'
SET #StartDate = #dateFrom
SET #EndDate = #dateTo
DECLARE #WORKDAYS INT
SELECT #WORKDAYS = (DATEDIFF(dd, #StartDate, #EndDate) + 1)
-(DATEDIFF(wk, #StartDate, #EndDate) * 2)
-(CASE WHEN DATENAME(dw, #StartDate) = 'Sunday' THEN 1 ELSE 0 END)
-(CASE WHEN DATENAME(dw, #EndDate) = 'Saturday' THEN 1 ELSE 0 END)
RETURN #WORKDAYS
END
One more thing, I am unable to handle the situation in which there is some sort of public holiday in the month like Independence day or something. I have a table named calendar that handles the situation but I'm not able to integrate that here. Table structure is provided below.
CREATE TABLE Calendar
(
ID Int,
[DayName] Varchar(25),
Holiday datetime,
PublicHoliday Binary,
OffDay Binary,
Active Binary
);
Any help would be appreciated.

Answer 1

This will count the number of saturdays and sundays between 2 dates:
DECLARE #from date = '2018-02-03'
DECLARE #to date = '2018-02-13'
SELECT
datediff(day, -2, #to)/7-datediff(day, -1, #from)/7 -- saturdays
+ datediff(day, -1, #to)/7-datediff(day, 0, #from)/7 -- sundays
Holidays are different from country to country.
This function will determine when Easter starts for a specific year:
CREATE FUNCTION [dbo].[f_EasterByYear] (#Year SMALLINT) RETURNS DATE AS
BEGIN
DECLARE
#c INT,
#n INT,
#k INT,
#i INT,
#j INT,
#l INT,
#m INT,
#d INT,
#Easter DATE
SET #c = (#Year / 100)
SET #n = #Year - 19 * (#Year / 19)
SET #k = (#c - 17) / 25
SET #i = #c - #c / 4 - ( #c - #k) / 3 + 19 * #n + 15
SET #i = #i - 30 * ( #i / 30 )
SET #i = #i - (#i / 28) * (1 - (#i / 28) *
(29 / (#i + 1)) * ((21 - #n) / 11))
SET #j = #Year + #Year / 4 + #i + 2 - #c + #c / 4
SET #j = #j - 7 * (#j / 7)
SET #l = #i - #j
SET #m = 3 + (#l + 40) / 44
SET #d = #l + 28 - 31 * ( #m / 4 )
SET #Easter = DATEADD(month, #m, DATEADD(year, #Year- 1900,
DATEADD(d, #d, '1899-11-30')))
RETURN #Easter
END
This function will return 1 for workday otherwise 0:
CREATE FUNCTION [dbo].[f_IsWorkDay] (#Date DATE) RETURNS BIT AS
BEGIN
DECLARE #IsWorkDay BIT = 1
SELECT #IsWorkDay =
CASE
--Exclude Saturday and Sunday (Datefirst-independent)
WHEN DATEDIFF(d, 0, #Date) % 7 IN (5,6) THEN 0
WHEN MONTH(#Date)=12 AND DAY(#Date) IN (24,25,26,31) THEN 0
--Exclude New Years
WHEN MONTH(#Date)=1 AND DAY(#Date)=1 THEN 0
--Exclude Easter, Pentecost and Ascension of Jesus
WHEN #Date IN (
DATEADD(DAY,-3,dbo.f_EasterByYear(YEAR(#Date))),
DATEADD(DAY,-2,dbo.f_EasterByYear(YEAR(#Date))),
DATEADD(DAY,1, dbo.f_EasterByYear(YEAR(#Date))),
DATEADD(DAY,26,dbo.f_EasterByYear(YEAR(#Date))),
DATEADD(DAY,39,dbo.f_EasterByYear(YEAR(#Date))),
DATEADD(DAY,50,dbo.f_EasterByYear(YEAR(#Date)))
) THEN 0
ELSE 1
END
RETURN #IsWorkDay
END
You can test if a day is a workday this way:
SELECT dbo.f_Isworkday(date) isWorkDay, date
FROM (values('2018-02-09'),('2018-02-10'),('2018-02-11'),('2018-02-12')) x(date)
Advanced test giving an interval as input:
DECLARE
#from DATE = '2018-02-09',
#to DATE = '2018-02-13'
;WITH N(N)AS
(SELECT 1 FROM(VALUES(1),(1),(1),(1),(1),(1),(1),(1),(1),(1))M(N)),
tally(N)AS(SELECT ROW_NUMBER()OVER(ORDER BY N.N)FROM N,N a,N b,N c,N d,N e)
SELECT
dbo.f_Isworkday(dateadd(d, N-1, #from)) isworkday,
dateadd(d, N-1, #from) date
FROM tally
WHERE N <= datediff(d, #from, #to) + 1

Answer 2

Use this Function, It will work with your Calendar table too. All Active entries in the Calendar table will not be counted as working days
CREATE FUNCTION [dbo].[fnGetDaysWorked2] (#StartDate datetime, #EndDate datetime)
RETURNS int
BEGIN
DECLARE #tblNonWorking TABLE(dt DATETIME,dtw varchar(30))
While (#StartDate<=#EndDate)
BEGIN
INSERT INTO #tblNonWorking(dt,dtw) VALUES (#StartDate,Lower(DATENAME(dw, #StartDate)))
SET #StartDate=Dateadd(day,1,#StartDate)
END
DECLARE #WORKDAYS INT
SET #WORKDAYS=0
SELECT #WORKDAYS=COUNT(1)
FROM #tblNonWorking a
LEFT JOIN Calendar c ON a.dt=c.Holiday AND c.Active=1
WHERE a.dtw NOT IN ('saturday','sunday')
AND c.ID IS NULL
RETURN #WORKDAYS
END