Week to date

Question

Dear All,
It is no problem to convert a date to get the week number:
TO_CHAR(sysdate, 'YYYYIW')
But what I need to do is to go the other way and generate the date of the Monday for that particular week.
With a month and the TO_DATE command this would be no problem. And from my googling I see that there are procedures available that will do what I need.
But my question is whether I am able to do this conversion with an in-built Oracle function? Or do I need to incorporate PL/SQL?
Any tips would be gratefully received.
Regards and thanks,
Alan

Answer 1

Hi, Alan,
Alan Searle wrote:
Dear All,
It is no problem to convert a date to get the week number:
TO_CHAR(sysdate, 'YYYYIW')You mean '<b>I</b>YYYIW', don't you?
But what I need to do is to go the other way and generate the date of the Monday for that particular week.
With a month and the TO_DATE command this would be no problem. And from my googling I see that there are procedures available that will do what I need.
But my question is whether I am able to do this conversion with an in-built Oracle function? Or do I need to incorporate PL/SQL?You don't need PL/SQL, but when you see how many functions you have to call to do what should be a simple task, perhaps you'll want it.
Assuming you have a string like '201002' meaning 'week 2 of ISO year 2010', the following query tells you when that week started:
SELECT     TRUNC ( TO_DATE ( SUBSTR (:iyw, 1, 4) || '0131'
               , 'YYYY'            || 'MMDD'
           , 'IYYY'
     + ( 7
       * ( TO_NUMBER ( SUBSTR (:iyw, 5))
         - 1
       )     AS iyw_monday
FROM     dual
;The following is one way to get such a string, which you can use for testing:
VARIABLE  iyw     VARCHAR2 (6)
EXEC     :iyw := '201002';The ISO year may begin a few days into January, but never as late as January 31. The query above uses January 31 as a date that is guaranteed to be within the same calendar year and ISO year, so we can use TO_CHAR (..., 'YYYY') to turn the string into a DATE, and then TRUNC (..., 'IYYY') to find the beginning of the ISO year.