Oracle custom week of the year numbering based on first Monday of the year

Question

I have following sql for generating dates, I wanted to change week of the year numbering based on 'First Monday of the Year'.

Hence numbering would be, 
01-05-15 ---> 01
01-12-15 ---> 02
01-19-15 ---> 03
...
...
...


SELECT TO_NUMBER (TO_CHAR (CurrDate, 'YYYYMMDD')) AS date_key,
   CurrDate AS alt_full_date_key,
   TO_NUMBER (TO_CHAR (CurrDate + 1, 'WW')) AS week_num_of_year,
   TO_NUMBER (TO_CHAR (CurrDate + 1, 'IW')) AS week_num_of_year_iso,
   NULL AS week_num_of_year_custom
FROM (    SELECT LEVEL n, TO_DATE ('28/12/2014', 'DD/MM/YYYY') + NUMTODSINTERVAL (LEVEL, 'day') CurrDate
          FROM DUAL
    CONNECT BY LEVEL <= 18000);

Answer 1

Let me know if you have better solution than this one?

    SELECT TO_NUMBER (TO_CHAR (CurrDate, 'YYYYMMDD')) AS date_key,
       CurrDate AS alt_full_date_key,
       TO_NUMBER (TO_CHAR (CurrDate + 1, 'WW')) AS week_num_of_year,
       TO_NUMBER (TO_CHAR (CurrDate + 1, 'IW')) AS week_num_of_year_iso,
       CASE
          WHEN currdate >= NEXT_DAY (TRUNC (CurrDate, 'YYYY') - 1, 'Monday') AND (CurrDate - NEXT_DAY (TRUNC (CurrDate, 'YYYY') - 1, 'Monday')) < 7 THEN 1
          WHEN currdate < NEXT_DAY (TRUNC (CurrDate, 'YYYY') - 1, 'Monday') THEN TRUNC ( (CurrDate - NEXT_DAY (ADD_MONTHS (TRUNC (CurrDate, 'YYYY') - 1, -12), 'Monday')) / 7) + 1
          ELSE TRUNC ( (CurrDate - NEXT_DAY (TRUNC (CurrDate, 'YYYY') - 1, 'Monday')) / 7) + 1
       END
          week_num_of_year_custom
  FROM (    SELECT LEVEL n, TO_DATE ('01/01/2010', 'DD/MM/YYYY') + NUMTODSINTERVAL (LEVEL, 'day') CurrDate
              FROM DUAL
        CONNECT BY LEVEL <= 18000);

Answer 2

ISO Definition is here, You need to check if Jan-01 is Monday, if it is, then you can use ISO week format model 'IW'. If it isn't you can still use that but subtract 1 from that value.

WITH x AS (SELECT to_char(trunc(SYSDATE,'YYYY'),'DY') y_b FROM dual)
,y AS (SELECT to_date('01-05-15','mm-dd-rr') AS d FROM dual UNION ALL
       SELECT to_date('01-12-15','mm-dd-rr') AS d FROM dual UNION ALL 
       SELECT to_date('01-19-15','mm-dd-rr') AS d FROM dual) 
SELECT y.d your_date
      ,CASE WHEN x.y_b = 'MON' 
            THEN to_number(to_char(y.d,'IW')) 
            ELSE   to_number(to_char(y.d,'IW'))-1 
        END week_num
FROM x,y;

I tested briefly and it looks ok to me, but you should test as well.