How to get Monday, Sunday based on a Week Number in SQL Server?

Question

I have a week number (e.g. 23) and I'd like to get the dates for Monday and Sunday of that week.
I am assuming Monday as the first day of the week (e.g. I have SET DATEFIRST 1 in my script).

Answer 1

DECLARE @startweek1 datetime
SET DATEFIRST 1

--first monday of year
SELECT @startweek1 = DATEADD(day, 8-DATEPART(weekday, '2011-01-01'), '2011-01-01')

--day based
SELECT
    DATEADD(day, 22 * 7, @startweek1) AS MondayWeek23,
    DATEADD(day, 23 * 7 -1 , @startweek1) AS SundayWeek23

--week based
SELECT
    DATEADD(week, 22, @startweek1) AS MondayWeek23,
    DATEADD(day, -1, DATEADD(week, 23 , @startweek1)) AS SundayWeek23

Edit:

This solution works if week 1 does not start on day 1 as Andomar said

Edit 2:

According to Wikipedia: 2008-12-29 is day 1 of week 1 of 2009 in ISO.

And week numbers vary as Andomar said

Cyberkiwi mentioned this code is wrong for 2007: it's wrong far more often than that. The same applies to his code too which matches 2007 but is equally wrong for the rest.

Answer 2

Declare @StartDate datetime;
Set @StartDate = '20110101';

With StartOfWeek As
    (
    Select DateAdd(
        week
        , 23
        , DateAdd(
            d 
            , -(DatePart(dw, @StartDate) - 1)
            , @StartDate
            ) ) As Sunday
    )
Select Sunday, DateAdd(d,1,Sunday) As Monday
From StartOfWeek