How to update rows based on current row data?

Hi... forum newbie here... I tried a couple searches... couldn't really find what I'm looking for...

Here is my question:

Lets say I have the following list of dates in 3 columns

First date, last date, and difference of those dates....

first_timestamp second_timestamp time_stamp_diff
Jul 5 2003 1:43PM Jul 7 2003 1:43PM NULL
Jul 5 2003 2:10PM Jul 7 2003 8:30AM NULL
Jul 5 2003 7:52PM Jul 7 2003 3:07PM NULL
Jul 5 2003 9:12PM Jul 7 2003 8:28AM NULL


How can I write a query that goes in and updates column 3 with the difference of those column 1 and 2?

I tried something like below but obviously it didn't work since the sub query returns more than one result... what am I missing?

select time_stamp_diff = (datediff(hh,(select first_timestamp from db),(select second_timestamp from db)))
from db

Basically the bottom line is I want to update each time_stamp_diff row based on the 2 columns before it. I have no idea how to do that with a for loop, update sub query or anything becuase sql craps out if you return more than one row with a sub expression....

I'm confused!

Any advice is appreciated!

Woops, I knew it was something simple:

Solution:

update blah
set time_stamp_diff = datediff(hh,convert(datetime,feedback_timestamp,11),convert(datetime,app_timestamp,11))

plus other syntax for anyone who cares.