SELECT TO_CHAR(to_date('07/01/2013', 'dd/mm/yyyy'), 'WW') AS WEEK FROM dual;
-- renvoie 01 ce qui est faut d'après nos calendriers
-- car "WW: Week of year (1-53) where week 1 starts on the first day of the year and continues to the seventh day of the year"

SELECT TO_CHAR(to_date('07/01/2013', 'dd/mm/yyyy'), 'IW') AS WEEK FROM dual;
-- 02 est vrai, la on utilise "ISO WEEK" et donc le numéro de la semaine selon les normes ISO.