Get SQL to process each row 1 by 1 - sql

Question

I have been going around for while trying to get an anwswer to my issue, I think it revolves around cursors in SQL but I am not sure. I think I know how to write the loop for a single row of data but I don't know how to run it for all the records:
Hopefully there is an easy answer:
I have a table, let's call it A, that has Product_Code, Start_Date, End_Date and Value
I would need an output table B that has column: Product_Code, Month, Year, Value when Month * Year is in between Start_Date and End_date
Each record of A should then create several record into B. Hope that's fairly clear, I'm happy to elaborate if not! :)

Answer 1

CREATE TABLE YearMonth(
Year int not null,
Month int not null,
FirstDay date not null,
LastDay date not null
);
Fill this table with as many years and months that your range of data is covered (no problem if you have too much).
You could do this with a statement like this:
WITH y(year) AS (
SELECT 2007
union all
SELECT 2008
union all
SELECT 2009
union all
SELECT 2010
union all
SELECT 2011
union all
SELECT 2012
union all
SELECT 2013
union all
SELECT 2014
union all
SELECT 2015
union all
SELECT 2016
),
m(month) AS (
SELECT 1
union all
SELECT 2
union all
SELECT 3
union all
SELECT 4
union all
SELECT 5
union all
SELECT 6
union all
SELECT 7
union all
SELECT 8
union all
SELECT 9
union all
SELECT 10
union all
SELECT 11
union all
SELECT 12
)
INSERT INTO YearMonth(Year, Month, FirstDay, LastDay)
SELECT y.year
,m.month
,convert(date, convert(nvarchar(4), y.year) + '.' + convert(nvarchar(2), m.month) + '.01', 102)
,DateAdd(day, - 1,
CASE WHEN m.month = 12 THEN
convert(date, convert(nvarchar(4), y.year + 1) + '.01.01', 102)
ELSE
convert(date, convert(nvarchar(4), y.year) + '.' + convert(nvarchar(2), m.month + 1) + '.01', 102)
END)
FROM y CROSS JOIN m
The tricky part to calculate the LastDay works like this: create a date that is the first of the following month, then subtract one day from it. This handles the problem that the last day of the month can be 28, 29, 30, or 31.
Then just use a join:
INSERT INTO B(Product_Code, Month, Year, Value)
SELECT A.Product_Code
,YearMonth.Month
,YearMonth.Year
,A.Value
FROM A
JOIN YearMonth ON YearMonth.LastDay <= A.StartDate
AND YearMonth.FirstDay <= A.EndDate
Depending on the exact interpretation of "Month*Year is in between Start_Date and End_date", you might have to switch one or both of the <=s to <.