http://forums.oracle.com/forums/thread.jspa?threadID=685308

select to_char( date '2008-01-04', 'IW' ) from dual

Basically, there are 3 to_char format models to get week number:

* W - week number in a month

* WW - week number in a year, week 1 starts at 1st of Jan

* IW - week number in a year, according to ISO standard -

alternative: ROUND(TO_NUMBER(TO_CHAR(datefield,'ddd'))/7)