Working days on SQL Server using CTE in a function - sql-server

Question

I'm trying to write a function to return the number of working days between 2 dates. I have a table with all the bank holidays in which has two fields: BHID and BHDate.
I've written the following SELECT query to test out my theory and it works nicely:
DECLARE #StartDate AS Date, #EndDate AS Date
SET #StartDate = '2015-01-01'
SET #EndDate = '2015-07-01'
;WITH CTE AS (
SELECT #StartDate AS StartDate, #EndDate AS EndDate, #StartDate AS DateCalc
UNION ALL
SELECT #StartDate AS StartDate, #EndDate AS EndDate, DATEADD(dd, 1, DateCalc) AS DateCalc FROM CTE
WHERE DATEADD(dd, 1, DateCalc) <= EndDate)
SELECT COUNT(*) AS Days
FROM CTE
LEFT JOIN psd.dbo.LUBankHolidays BH
ON CTE.DateCalc = BH.BHDate
WHERE BHID IS NULL
AND LEFT(DATENAME(dw, DateCalc) ,1) <> 'S'
option (Maxrecursion 0);
But when I try and put it in a function I get some errors around the SET line, it could be me just not being quite as good on functions as the rest of my knowledge or I was wondering whether it was to do with the CTE not having been used ahead of the SET line? Here's my attempt:
CREATE FUNCTION [dbo].[WorkingDays]
(#StartDate AS Date, #EndDate AS Date
)
RETURNS INT
AS
BEGIN
DECLARE #Days AS INT
;WITH CTE AS (
SELECT #StartDate AS StartDate, #EndDate AS EndDate, #StartDate AS DateCalc
UNION ALL
SELECT #StartDate AS StartDate, #EndDate AS EndDate, DATEADD(dd, 1, DateCalc) AS DateCalc FROM CTE
WHERE DATEADD(dd, 1, DateCalc) <= EndDate)
SET #Days =
(SELECT COUNT(*) AS Days
FROM CTE
LEFT JOIN psd.dbo.LUBankHolidays BH
ON CTE.DateCalc = BH.BHDate
WHERE BHID IS NULL
AND LEFT(DATENAME(dw, DateCalc) ,1) <> 'S'
option (Maxrecursion 0);)
RETURN #Days
END
I even tried inserting the result of the INT into a temporary table and then dropping this after setting #Days before then returning #Days but this then results in an error regarding not being allowed to produce temporary tables as part of a function.
Any help would be great, I'm sure it's a small thing but just escaping me currently.

Answer 1

Because a CTE can only be used in a query, rather than using SET to set the variable, use SELECT instead:
SELECT #Days =
(SELECT COUNT(*) AS Days
FROM CTE …
From MSDN:
A CTE must be followed by a single SELECT, INSERT, UPDATE, or DELETE
statement that references some or all the CTE columns.

Answer 2

Worked it out with stuartd's help:
CREATE FUNCTION [dbo].[WorkingDays]
(#StartDate AS Date, #EndDate AS Date
)
RETURNS INT
AS
BEGIN
DECLARE #Days AS INT
;WITH CTE AS (
SELECT #StartDate AS StartDate, #EndDate AS EndDate, #StartDate AS DateCalc
UNION ALL
SELECT #StartDate AS StartDate, #EndDate AS EndDate, DATEADD(dd, 1, DateCalc) AS DateCalc FROM CTE
WHERE DATEADD(dd, 1, DateCalc) <= EndDate)
SELECT #Days =
(SELECT COUNT(*) AS Days
FROM CTE
LEFT JOIN psd.dbo.LUBankHolidays BH
ON CTE.DateCalc = BH.BHDate
WHERE BHID IS NULL
AND LEFT(DATENAME(dw, DateCalc) ,1) <> 'S')
option (Maxrecursion 0);
RETURN #Days
END
Thanks All

Answer 3

Use this function
CREATE FUNCTION [dbo].[WorkingDays]
(#Start_Date AS Date, #end_Date AS Date
)
RETURNS INT
AS
BEGIN
DECLARE #Days AS INT
WITH AllDays
AS ( SELECT #start_date AS [Date], 1 AS [level]
UNION ALL
SELECT DATEADD(DAY, 1, [Date]), [level] + 1
FROM AllDays
WHERE [Date] < #end_date )
SELECT #Days =COUNT(*) AS Days
FROM AllDays
LEFT JOIN psd.dbo.LUBankHolidays BH
ON AllDays.Date= BH.BHDate
WHERE BHID IS NULL
AND LEFT(DATENAME(dw, AllDays.Date) ,1) <> 'S'
RETURN #Days
END