How to extract week number in sql

After converting your varchar2 date to a true date datatype, then convert back to varchar2 with the desired mask:

to_char(to_date('01/02/2012','MM/DD/YYYY'),'WW')

If you want the week number in a number datatype, you can wrap the statement in to_number():

to_number(to_char(to_date('01/02/2012','MM/DD/YYYY'),'WW'))

However, you have several week number options to consider:

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.
W   Week of month (1-5) where week 1 starts on the first day of the month and ends on the seventh.
IW  Week of year (1-52 or 1-53) based on the ISO standard.

Try to replace 'w' for 'iw'. For example:

SELECT to_char(to_date(TRANSDATE, 'dd-mm-yyyy'), 'iw') as weeknumber from YOUR_TABLE;