Perform logic using a CTE instead of loop - sql-server

Question

Suppose I have a table
Declare #table table
(
id int,
month_col varchar(10),
present_col bit,
absent_col bit,
leave_col bit
);
insert into #table
values (1, 'Jan', 1, 0, 0),
(2, 'Jan', 1, 0, 0),
(3, 'Jan', 0, 1, 0),
(4, 'Jan', 1, 0, 0),
(5, 'Jan', 0, 1, 0),
(6, 'Jan', 1, 0, 0),
(7, 'Jan', 1, 0, 0);
While updating some rows, I need to check all previous rows leave col to take decision for current row, this need to be done for all rows, I had achieve this with while loop like this
while(for all rows)
begin
update #table
set l = mylogic_function((select count(leave_col)
from #table
where id<currentrow.ID))
where id = currentrow.ID
end
but I want to do this logic by using a CTE.

Answer 1

I don't know what your function does so, for my example I created a function that takes an input and returns a 1 when that number is divisible by three. Obviously you can change the logic to what you need.
CREATE FUNCTION dbo.someFunction(#x INT) RETURNS BIT AS BEGIN RETURN (SIGN(#x%3)-1) END;
It's good that you don't want to do this using a loop BUT a solution that uses a recursive CTE (if that's what you are looking for) will likely perform poorly and will be overly complicated for what you are trying to do. Set-based is always the way to go.
To get a count of all rows before your "current" row you could simply use this syntax:
COUNT(t.leave_col) OVER (ORDER BY t.Id)
This will return the same thing as above but will be faster:
ROW_NUMBER() OVER (ORDER BY t.Id)
This query will do what you're looking for and will be faster than using a loop or recursive CTE:
-- Sample data
Declare #table table(id int,month_col varchar(10),present_col bit, absent_col bit,leave_col bit);
insert into #table values
(1,'Jan',1,0,0),
(5,'Jan',1,0,0),
(13,'Jan',0,1,0),
(14,'Jan',1,0,0),
(25,'Jan',0,1,0),
(26,'Jan',1,0,0),
(37,'Jan',1,0,0);
-- Solution
WITH Prep(leave_col_count, leave_col) AS
(
SELECT ROW_NUMBER() OVER (ORDER BY t.Id), t.leave_col
FROM #table AS t
)
UPDATE Prep
SET leave_col = dbo.someFunction(Prep.leave_col_count)
FROM Prep;