Date difference function that returns minutes between two dates and excludes weekends and holidays

Question

Is there a way to get this to work as a function? 
Currently returns error "Select statements included within a function cannot return data to a client"
CREATE FUNCTION [dbo].[WorkDays](@DateFrom datetime,@DateTo datetime)
RETURNS int
AS
BEGIN
--Working time
DECLARE @WTFrom TIME = '8:00AM';
DECLARE @WTTo TIME = '5:00PM';
DECLARE @minCount BIGINT
--Date ranges
IF (DATEDIFF(HOUR, @DateFrom, @DateTo) > 12)
BEGIN
WITH CTE AS
SELECT @DateFrom AS DateVal
UNION ALL
SELECT DATEADD(HOUR, 1, DateVal)
FROM CTE
WHERE DateVal < DATEADD(HOUR, -1,@DateTo)
SELECT DATEDIFF(minute, MIN(CTE.DateVal), MAX(CTE.DateVal))
FROM CTE
WHERE (CAST(CTE.DateVal AS time) > @WTFrom AND CAST(CTE.DateVal AS time) < @WTTo) AND DATEPART(dw, CTE.DateVal) NOT IN (1, 7) AND NOT EXISTS (SELECT * FROM Holiday AS H WHERE H.holiday = CTE.DateVal)
OPTION (MAXRECURSION 0);
END;
ELSE
BEGIN
WITH CTE AS
SELECT @DateFrom AS DateVal
UNION ALL
SELECT DATEADD(MINUTE, 1, DateVal)
FROM CTE
WHERE DateVal < DATEADD(MINUTE, -1,@DateTo)
SELECT DATEDIFF(minute, MIN(CTE.DateVal), MAX(CTE.DateVal))
FROM CTE
WHERE (CAST(CTE.DateVal AS time) > @WTFrom AND CAST(CTE.DateVal AS time) < @WTTo) AND DATEPART(dw, CTE.DateVal) NOT IN (1, 7) AND NOT EXISTS (SELECT * FROM Holiday AS H WHERE H.holiday = CTE.DateVal)
OPTION (MAXRECURSION 0);
END;
END
Thanks for your help.

Answer 1

Please post DDL, so that people do not have to guess what the keys, constraints, Declarative Referential Integrity, data types, etc. in your schema are. Learn how to follow ISO-11179 data element naming conventions and formatting rules (you do not). Temporal
data should use ISO-8601 formats (you do not!). Code should be in Standard SQL as much as possible and not local dialect. 
This is minimal polite behavior on SQL forums. 
We hate functions in SQL. This is a declarative language and you are using it like 1950's FORTRAN. We hate local variables (more FORTRAN!)
 The name of a function has to be either a known, common name, like “sine”, “cosine” etc. Or it is “<verb>_<object>”; you think a noun is a verb! Do you really need BIGINT? Why did you invite garbage data with it? Why do you think that SQL
uses AM/PM? Have you never seen the TIME data type? 
Think about “date_val” as a data element name. A date is a unit of temporal measurement on a calendar scale. This would be a “<something>_date” in a valid schema. 
>> Is there a way to get this to work as a function? <<
Probably, but why do it wrong?
Build a calendar table with one column for the calendar data and other columns to show whatever your business needs in the way of temporal information. Do not try to calculate holidays in SQL -- Easter alone requires too much math.
The julian_business_nbr is how SQL people do this. Here is the skeleton. 
CREATE TABLE Calendar
(cal_date DATE NOT NULL PRIMARY KEY, 
 julian_business_nbr INTEGER NOT NULL, 
Here is how it works:
INSERT INTO Calendar 
VALUES ('2007-04-05', 42), 
 ('2007-04-06', 43), -- Good Friday 
 ('2007-04-07', 43), 
 ('2007-04-08', 43), -- Easter Sunday 
 ('2007-04-09', 44), 
 ('2007-04-10', 45); --Tuesday
To compute the business days from Thursday of this sample week to next Tuesday:
SELECT (C2.julian_business_nbr - C1.julian_business_nbr)
  FROM Calendar AS C1, Calendar AS C2
 WHERE C1.cal_date = '2007-04-05',
   AND C2.cal_date = '2007-04-10'; 
See how simple it can be when you stop trying to write FORTRAN and think in sets? 
--CELKO-- Books in Celko Series for Morgan-Kaufmann Publishing: Analytics and OLAP in SQL / Data and Databases: Concepts in Practice Data / Measurements and Standards in SQL SQL for Smarties / SQL Programming Style / SQL Puzzles and Answers / Thinking
in Sets / Trees and Hierarchies in SQL