How to copy data between joined columns?

I have a need to copy/update data from one column to another, based on a
join condition.  Is this easy to do in pure SQL?  I have google'ed
without luck for a easy solution (that's not Microsoft specific, that
is).  For example:

postgres=#
select xx_thing.date_start,xx_note.created_date
from xx_thing
join xx_issue using (thing_id) join xx_note using (issue_id)
where xx_note.created_date is not null;
    date_start     |    created_date
----------------+---------------------2006-03-17     | 1900-01-01
(...many... rows)

I need to copy date_start to created_date, for all records that meet a
particular where condition.
                  -Bryce Nesbitt                 

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