Calculation of date in function - sql

Question

1 28/11/2011 ...
How do I write a function in SQL to implement the above pattern?

Answer 1

You could do something like this in SQL Server:
DECLARE #BaseDate DATE = '20111107';
DECLARE #EndDate DATE = GETDATE(); --Or the "end of dates in the database"
WITH RecursiveCTE AS (
SELECT
1 AS [Counter],
#BaseDate AS [MyDate]
UNION ALL
SELECT
[Counter] + 1,
DATEADD(DAY, 7, MyDate)
FROM
RecursiveCTE
WHERE
MyDate < #EndDate)
SELECT * FROM RecursiveCTE OPTION (MAXRECURSION 0);
To handle dates that aren't exact and make this into a function you would do this:
--Function definition
CREATE FUNCTION SuperDuperDataCalculator (
#BaseDate DATE = '20131016',
#EndDate DATE = '20131020')
RETURNS #Results TABLE (
[Counter] INT,
[Date] DATE)
AS
BEGIN
WITH RecursiveCTE AS (
SELECT
1 AS [Counter],
#BaseDate AS [MyDate]
UNION ALL
SELECT
[Counter] + 1,
CASE WHEN DATEADD(DAY, 7, MyDate) > #EndDate THEN #EndDate ELSE DATEADD(DAY, 7, MyDate) END
FROM
RecursiveCTE
WHERE
MyDate < #EndDate)
INSERT INTO
#Results
SELECT * FROM RecursiveCTE OPTION (MAXRECURSION 0);
RETURN;
END;
GO
--Usage
SELECT * FROM SuperDuperDataCalculator('20131016', '20131020');
--Results
Counter Date
1 2013-10-16
2 2013-10-20
Note that we have to use a multi-statement table-valued function as there is a bug in SQL Server where it won't let you use OPTIONs in a simple table-valued function. The alternative would be to remove the OPTION (MAXRECURSION 0) from the function and remember to use this every time you reference it (i.e. a pretty poor alternative).
...and finally, if you wanted to just return the maximum counter value you could rewrite this as a scalar-valued function, i.e.:
--Function definition
CREATE FUNCTION SuperDuperDataCalculator (
#BaseDate DATE = '20131016',
#EndDate DATE = '20131020')
RETURNS INT
AS
BEGIN
DECLARE #Results TABLE (
[Counter] INT,
[Date] DATE);
DECLARE #ReturnValue INT;
WITH RecursiveCTE AS (
SELECT
1 AS [Counter],
#BaseDate AS [MyDate]
UNION ALL
SELECT
[Counter] + 1,
CASE WHEN DATEADD(DAY, 7, MyDate) > #EndDate THEN #EndDate ELSE DATEADD(DAY, 7, MyDate) END
FROM
RecursiveCTE
WHERE
MyDate < #EndDate)
INSERT INTO
#Results
SELECT * FROM RecursiveCTE OPTION (MAXRECURSION 0);
SELECT #ReturnValue = MAX([Counter]) FROM #Results;
RETURN #ReturnValue;
END;
GO
SELECT dbo.SuperDuperDataCalculator('20131016', '20131020');

Answer 2

Try this - It will get all the weeks and assign a rownumber in the subquery. Then only select the records where row number = 1 because there might be more results for that week. So hence RowNo = 1
SELECT ROW_NUMBER() OVER(ORDER BY RowNo) AS IncrementalWeek,dte
FROM
(
SELECT DISTINCT DATEPART(ww,CONVERT(VARCHAR(20),createdDate,111)) AS [week],
CONVERT(VARCHAR(20),createdDate,111) AS dte,
ROW_NUMBER() OVER(PARTITION BY DATEPART(ww,Convert(VARCHAR(20),createdDate,111)) ORDER BY DATEPART(ww,CONVERT(VARCHAR(20),createdDate,111))) AS RowNo
FROM YourTable
) AS tble
WHERE RowNo = 1
ORDER BY [week]