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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