Extract from date field

Dear all,

I want to make a primary key for my table based upon a few other
columns. I do this with the following code:

ALTER TABLE camdengps2
ADD COLUMN camdencrimes_link varchar;
UPDATE camdengps2
SET camdencrimes_link='' || EXTRACT(YEAR FROM date_time) || '0' ||
EXTRACT(MONTH FROM date_time) || '' || EXTRACT(DAY FROM date_time) ||
'' || incident

Essentially I am truncating the year, month, day and another field
called 'Incident'. However I am finding two problems with this:

1) I would like the result to be an integer rather than a varchar.
However when I set the field to be integer rather than varchar the SET
query doesn't work. I guess I maybe need to CAST the date as an
integer somehow?

2) When I extract the DAY from the date field, instead of the 2nd of
the month becoming '02' it actually becomes '2'. I would like it to be
'02'.

Any help greatly appreciated...

Best wishes

James