Converting SQL Server UDF to inline table-valued function - sql

Question

I am new here and new to SQL. I got this tip to create a scalar function that extends the functionality of the built-in DateAdd function (namely to exclude weekends and holidays). It is working fine for a single date but when I use it on a table, it is extremely slow.
I have seen some recommendation to use inline table-valued function instead. Would anyone be so kind to point me in the direction, how I would go about converting the below to inline table-valued function? I greatly appreciate it.
ALTER FUNCTION [dbo].[CalcWorkDaysAddDays]
(#StartDate AS DATETIME, #Days AS INT)
RETURNS DATE
AS
BEGIN
DECLARE #Count INT = 0
DECLARE #WorkDay INT = 0
DECLARE #Date DATE = #StartDate
WHILE #WorkDay < #Days
BEGIN
SET #Count = #Count - 1
SET #Date = DATEADD(DAY, #Count, #StartDate)
IF NOT (DATEPART(WEEKDAY, #Date) IN (1,7) OR
EXISTS (SELECT * FROM RRCP_Calendar WHERE Is_Holiday = 1 AND Calendar_Date = #Date))
BEGIN
SET #WorkDay = #WorkDay + 1
END
END
RETURN #Date
END

Answer 1

This should do the trick...
CREATE FUNCTION dbo.tfn_CalcWorkDaysAddDays
(
#StartDate DATETIME,
#Days INT
)
RETURNS TABLE WITH SCHEMABINDING AS
RETURN
SELECT
TheDate = MIN(x.Calendar_Date)
FROM (
SELECT TOP (#Days)
c.Calendar_Date
FROM
dbo.RRCP_Calendar c
WHERE
c.Calendar_Date < #StartDate
AND c.Is_Holiday = 0
AND c.is_Weekday = 1 -- this should be part of your calendar table. do not calculate on the fly.
ORDER BY
c.Calendar_Date DESC
) x;
GO
Note: for best performance, you'll want a unique, filtered, nonclustered index on on your calendar table...
CREATE UNIQUE NONCLUSTERED INDEX uix_RRCPCalendar_CalendarDate_IsHoliday_isWeekday ON dbo.RRCP_Calendar (
Calendar_Date, Is_Holiday, is_Weekday)
WHERE Is_Holiday = 0 AND is_Weekday = 1;

Answer 2

Try this and see if it returns the same values as your function, just without the loop:
SELECT WorkDays =
DATEADD(WEEKDAY, #Days, #StartDate) -
(SELECT COUNT(*)
FROM RRCP_Calendar
WHERE Is_Holiday = 1
AND Calendar_Date >= #StartDate
AND Calendar_Date <= DATEADD(DAY, #Days, #StartDate)
)
And yes, you can sometimes get substantially better performance with a non-procedural table-valued-function, but you have to set it up right. Look up SARGability and non-procedural table-valued-functions for more info, but if the above query works, this should do the trick:
CREATE FUNCTION dbo.SelectWorkDaysAddDays(#StartDate DATE, #Days INT)
RETURNS TABLE
AS
RETURN
SELECT WorkDays =
DATEADD(WEEKDAY, #Days, #StartDate) -
(SELECT COUNT(*)
FROM RRCP_Calendar
WHERE Is_Holiday = 1
AND Calendar_Date >= #StartDate
AND Calendar_Date <= DATEADD(DAY, #Days, #StartDate)
)
GO
And then you call the function by using an OUTER APPLY join:
SELECT y.foo
, y.bar
, dt.WorkDays
FROM dbo.YourTable y
OUTER APPLY dbo.SelectWorkDaysAddDays(#StartDate, #Days) dt

Answer 3

Say [dbo].[CalcWorkDaysAddDays], getdate(), 2 would return Sept 8,
2017 since it is adding two days. This function is similar to DateAdd
but it is excluding weekends and holidays
The code you've posted doesn't do this.
But if you want the result described, the function can be smth like this:
alter FUNCTION [dbo].[CalcWorkDaysAddDays_inline](#StartDate As DateTime,#Days AS INT)
returns table
as return
with cte as
(
select *,
ROW_NUMBER() over(order by Calendar_Date) as rn
from RRCP_Calendar
where Calendar_Date > #StartDate and #Days > 0
and not (DATEPART(WEEKDAY,Calendar_Date) IN (1,7) or Is_Holiday = 1)
union ALL
select *,
ROW_NUMBER() over(order by Calendar_Date desc) as rn
from RRCP_Calendar
where Calendar_Date < #StartDate and #Days < 0
and not (DATEPART(WEEKDAY,Calendar_Date) IN (1,7) or Is_Holiday = 1)
)
select cast(Calendar_Date as date) as dt
from cte
where rn = abs(#Days);