How to get week of a year in American Standard

Question

Hi,
I know it might be a repetitive question, but did not find a convincing solution to it in any of the previous threads. I am looking for a function (user defined, if someone has already written it) to return me the week of the year, in American Standard. i.e,the week should start on Sunday and end on Saturday (and NOT from Monday to Sunday)
01/03/2010 - Should be the start of 1st week of 2010...... 01/10/2010 will be the start of second week and so on
01/04/2009 - Should be the start of 1st week of 2009...... 01/11/2009 will be the start of second week and so on.
Does any one have a function that takes a date as input and returns back the week of the year in this above format? Any help is greatly appreciated.
Thanks

Answer 1

It's a bit trickier because the ISO rule has some fine prints.
Take a look at the example below:
SQL> with t as (
  2  select to_date('01/02/2010', 'MM/DD/YYYY') dt from dual union all
  3  select to_date('01/03/2010', 'MM/DD/YYYY') dt from dual union all
  4  select to_date('01/03/2009', 'MM/DD/YYYY') dt from dual union all
  5  select to_date('01/04/2009', 'MM/DD/YYYY') from dual)
  6  --
  7  select dt,
  8         to_char(dt+1, 'iw') tweaked_week,
  9         to_char(dt, 'ww') nls_week
10    from t
11   order by dt;
DT          TWEAKED_WEEK NLS_WEEK
3/1/2009    01           01
4/1/2009    02           01
2/1/2010    53           01
3/1/2010    01           01
SQL> If may notice that 02/01/2010 is week 53 and and that at the same time 03/01/2009 is already week 1, being 04/01/2009 week 2.
The reason is in the docs:
An ISO week always starts on a Monday and ends on a Sunday.
* If January 1 falls on a Friday, Saturday, or Sunday, then the ISO week that includes January 1 is the last week of the previous year, because most of the days in the week belong to the previous year.
* If January 1 falls on a Monday, Tuesday, Wednesday, or Thursday, then the ISO week is the first week of the new year, because most of the days in the week belong to the new year.It depends on what your requirement asks.
If you need to prevent any January 1st that fall on Monday to Thursday from becoming your week 1 you'd probably have to adjust that implementation and fine tune it further.
Let us know if that will do or what your rule would be otherwise.