Oracle Week Number From Date ISO Week But From December 1 - oracle

Question

Ok, I've had a query that has been working fine that calculates the week number from December 1 (the start of our Sales Fiscal Year).
Now the requirements have changed. I still need to calculate the week number based on the field (Invoice_Date). However, instead of starting to count from December 1 (Dec 1-7, Week 1, etc.) now I need to start counting on the nearest Monday to December 1st. As I understand it, the ISO week is kind of what I'm looking for but it starts January 1. How do I modify this to work from December 1?
Any help would be greatly appreciated.

Answer 1

select next_day(to_date('0112' || to_char(sysdate, 'YYYY'),'ddmmyyyy') - 1, 'MONDAY') dec_mon from dual; gives you first Monday of December current year
Number of week is just ceil((sysdate - dec_mon)/7).
If you want last Monday before 1st Dec you can get it by:
select next_day(to_date('2511' || to_char(sysdate, 'YYYY'),'ddmmyyyy') - 1, 'MONDAY')
from dual;

Answer 2

In this proposed solution, I build a "helper table" first, showing the Monday_from and Monday_to for each fiscal year (in the third CTE, named ranges). Then I build a few test dates - I was lazy, I should have used to_date() so I can include time-of-day component as well. The join condition in the actual solution (at the end of the code) is written so it works without modification for dates with non-zero "time-of-day" component.
I used the nice feature of Oracle 11.2 which allows us to give column aliases in the declaration of CTEs - otherwise the column aliases would need to be moved inside the respective SELECTs. Otherwise the solution should work at least for Oracle 9 and above (I think).
with
y ( dt ) as (
select add_months(date '2000-12-01', 12 * level )
from dual
connect by level <= 30
),
m ( dt ) as (
select trunc(dt, 'iw') + case when dt - trunc(dt, 'iw') <= 3 then 0 else 7 end
from y
),
ranges ( monday_from, monday_to ) as (
select dt, lead(dt) over (order by dt) - 1
from m
),
test_dates ( t_date ) as (
select date '2013-02-23' from dual union all
select date '2008-12-01' from dual union all
select date '2008-04-28' from dual union all
select date '2016-11-29' from dual
)
select t_date, monday_from, 1 + trunc((t_date - monday_from)/7) as week_no
from test_dates t inner join ranges r
on t.t_date >= r.monday_from and t.t_date < r.monday_to
;
T_DATE MONDAY_FROM WEEK_NO
------------------- ------------------- ----------
2008-04-28 00:00:00 2007-12-03 00:00:00 22
2008-12-01 00:00:00 2008-12-01 00:00:00 1
2013-02-23 00:00:00 2012-12-03 00:00:00 12
2016-11-29 00:00:00 2016-11-28 00:00:00 1

Answer 3

The nearest Monday to any any given date is returned with the following function:
NEXT_DAY(some_date-4,'Monday')
as shown by this query:
with dts(some_date) as (
select date '2006-12-1' from dual
union all
select add_months(some_date,12)
from dts
where some_date <= date '2014-12-1'
)
select some_date
, next_day(some_date-4,'monday') nearest
, some_date - next_day(some_date-4,'monday') dist
from dts;
SOME_DATE NEAREST DIST
----------- ----------- ----------
01-DEC-2006 04-DEC-2006 -3
01-DEC-2007 03-DEC-2007 -2
01-DEC-2008 01-DEC-2008 0
01-DEC-2009 30-NOV-2009 1
01-DEC-2010 29-NOV-2010 2
01-DEC-2011 28-NOV-2011 3
01-DEC-2012 03-DEC-2012 -2
01-DEC-2013 02-DEC-2013 -1
01-DEC-2014 01-DEC-2014 0
01-DEC-2015 30-NOV-2015 1
10 rows selected