T-SQL - Get start and end date based on week number. - sql

Question

Our business considers a week from (Monday - Sunday). I need to write a T-SQL function, which passes in year, week no as parameters and it will return the start and end date of that week. However I've seen many examples but the problem lies within the year overlapping.
e.g December 26, 2011 (Monday) - January 01, 2012 (Sunday)... << Would want to consider this as the last week of 2011.
And also in T-SQL the datepart(ww,DATE) considers Sunday as the start of the week??
Or Am I better creating my own table with the week no and storing its start and end date?

Answer 1

DECLARE
#Year INT,
#Week INT,
#FirstDayOfYear DATETIME,
#FirstMondayOfYear DATETIME,
#StartDate DATETIME,
#EndDate DATETIME
SET #Year = 2011
SET #Week = 52
-- Get the first day of the provided year.
SET #FirstDayOfYear = CAST('1/1/' + CAST(#YEAR AS VARCHAR) AS DATETIME)
-- Get the first monday of the year, then add the number of weeks.
SET #FirstMondayOfYear = DATEADD(WEEK, DATEDIFF(WEEK, 0, DATEADD(DAY, 6 - DATEPART(DAY, #FirstDayOfYear), #FirstDayOfYear)), 0)
SET #StartDate = DATEADD(WEEK, #Week - 1, #FirstMondayOfYear)
-- Set the end date to one week past the start date.
SET #EndDate = DATEADD(WEEK, 1, #StartDate)
SELECT #StartDate AS StartDate, DATEADD(SECOND, -1, #EndDate) AS EndDate

Answer 2

You should create a table with the holidays and days you don´t wnat to consider in your counts.
After this count the days between the initial date and final date. (Step1)
Select in your table to check how many days are between your ini and final dates. (Step2)
Finally subtract the result of step 2 with result of step 1;

Answer 3

This is the sort of thing where you're better off creating a calendar table: the main issue is knowing how far into the past/future to populate it, but beyond that, have the table schema include week number and date. Depending on what other date-based queries you want to do, you might want to have additional column to break the date down into its constituant parts (e.g., have three separate columns for day/month/year/name of day, etc).

Answer 4

How about this one:
--DROP FUNCTION dbo.GetBusinessWeekStart
CREATE FUNCTION dbo.GetBusinessWeekStart(
#Year SMALLINT,
#Week TINYINT
)
RETURNS DATETIME
AS
BEGIN
DECLARE #FirstMonday TINYINT
DECLARE #Result DATETIME
IF ISNULL(#Week,0)<1 OR ISNULL(#Year,0)<1900
BEGIN
SET #Result= NULL;
END
ELSE
BEGIN
SET #FirstMonday=1
WHILE DATEPART(dw,CONVERT(DATETIME, '01/0' + CONVERT(VARCHAR,#FirstMonday) + '/' + CONVERT(VARCHAR,#Year)))<>2
BEGIN
SET #FirstMonday=#FirstMonday+1
END
SET #Result=CONVERT(DATETIME, '01/0' + CONVERT(VARCHAR,#FirstMonday) + '/' + CONVERT(VARCHAR,#Year))
SET #Result=DATEADD(d,(#Week-1)*7,#Result)
IF DATEPART(yyyy,#Result)<>#Year
BEGIN
SET #Result= NULL;
END
END
RETURN #Result
END
GO
--Example
SELECT dbo.GetBusinessWeekStart(2011,15) [Start],dbo.GetBusinessWeekStart(2011,15)+6 [End]