Count Business Days Between Start Date and Current Date - sql

Question

I am trying to use a predefined INT value giving 1 or 0 as being a business day or not to count the rolling business days sequence. I have tried a lot of different code and checked out different posts here but none are specific enough to work with mine.
The predefined INT value is "business_day_flag_int". This comes from converting the calculations giving me "day_business_day_flag" which is a bit value. Currently Saturday/Sunday and Banking holidays trigger a 0 for "day_business_day_flag" and Mon-Fri being non holiday give it a 1 value.
How can I get this to work in an Update table that I can add to the rest of my table generation file? If more information is needed let me know.
I've tried a bunch of different alterations and variations of what I have here. If I remove date values it gives me an int value for all rows in the column of roughly 12600 business days. If I use with the date range it gives me
DECLARE #StartDate DATETIME = '01/01/2000' --Starting value of Date Range
DECLARE #EndDate DATETIME = '01/01/2050' --End Value of Date Range
DECLARE
#DayOfWeekInMonth INT,
#DayOfWeekInYear INT,
#DayOfQuarter INT,
#WeekOfMonth INT,
#CurrentYear INT,
#CurrentMonth INT,
#CurrentQuarter INT
DECLARE #CurrentDate AS DATETIME = #startDate
SET #CurrentMonth = DATEPART(MM, #CurrentDate)
SET #CurrentYear = DATEPART(YY, #CurrentDate)
SET #CurrentQuarter = DATEPART(QQ, #CurrentDate)
UPDATE [EDW_MDM].[dbo].[CALENDAR_DIM] SET
business_day_flag_int = Convert(INT, day_business_day_flag)
UPDATE [EDW_MDM].[dbo].[CALENDAR_DIM] SET
rolling_business_day_sequence = (SELECT count(business_day_flag_int) FROM [EDW_MDM].[dbo].[CALENDAR_DIM]
WHERE business_day_flag_int = 1 and
day_date between #StartDate and #CurrentDate)
I want the column "rolling_business_day_sequence" to count sequentially business days past. For example row 1 = 1, row 2 = 2, etc. Until the end of my calendar.
Update 1: Edited the line of code to
UPDATE [EDW_MDM].[dbo].[FCFCU_CALENDAR_DIM] SET
rolling_business_day_sequence = datediff(day,#StartDate,day_date) WHERE day_business_day_flag = 1
This gave me counting days and set rolling_business_day_sequence row values to null where not having day_busienss_day_flag = 1 and still counting them instead of not counting them. How can I make it not add the day?

Answer 1

Consider a window function sum on your WHERE conditions to cumulatively count all instances of business_day_flag_int = 1 within specified date range. However, to use window functions in UPDATE, a CTE or subquery is required.
WITH CTE AS
(
SELECT ID, SUM(CASE WHEN day_date BETWEEN #StartDate AND #CurrentDate
AND business_day_flag_int = 1
THEN 1
ELSE 0
END) OVER (ORDER BY day_date) AS running_sequence
FROM [EDW_MDM].[dbo].[CALENDAR_DIM]
)
UPDATE t
FROM [EDW_MDM].[dbo].[CALENDAR_DIM] t
JOIN CTE ON t.ID = CTE.ID
SET t.rolling_business_day_sequence = CTE.running_sequence