Question

I’m trying to determine the day of a given a week date given (i.e. YYYY-W##) in the ISO 8601 format. My end goal is to convert an ISO 8601 Week Date to an ISO 8601 Calendar Date. I need to do this in TSQL (I’m working with SQL Server 2005), I’m not sure if there’s anything already built-in to SQL Server 05 that allows this, but it would help to see an example in another language or in generic pseudocode

UPDATE:

Sorry if the structure of my question is confusing. Basically, I have a ISO8601 week date that I’m trying to convert to ISO8601 Calendar Date.

Examples (from Wikipedia)

ISO 8601 Week Date: 2012-W02 (YYYY-W##)

Convert to…

ISO 8601 Calendar Date: 2012-01-09 (YYYY-MM-DD)

Since the day component isn’t given in my week date example, the first day of the week can be assumed.

Answer 1

Try the following solution:

declare 
    @date varchar(10)
    ,@convertedDate datetime
    ,@wk int
    ,@yr int

set @date = '2012-W02';

set @yr = parsename(replace(@date, '-W', '.'), 2)
set @wk = parsename(replace(@date, '-W', '.'), 1)

set @convertedDate = convert(varchar(10), dateadd(week, @wk, dateadd (year, @yr-1900, 0)) - 5 - datepart(dw, dateadd (week, @wk, dateadd (year, @yr-1900, 0))), 121)

select 
    'Year' = @yr
    ,'Week' = @wk
    ,'Date' = @convertedDate

Output:

-----------------------------------------
| Year | Week |         Date            |
-----------------------------------------
| 2012 |   2  | 2012-01-09 00:00:00.000 |
-----------------------------------------