Update & Lack of Error Message

i set up an update statement to update a unit's status
from incomplete to completed.

however, pgsql doesn't kick out an error message when
i try to update something that doesn't exist.

eg, serial number 21 doesn't exist in my table.  when
i "update" serial number 21, it goes through the
motions as though it updated something, but it
actually did nothing to the table data.

is this expected behavior?

i'm using pgsql 8.1 on winxp.

i'm thinking about running a select to check the
existence of the serial number before running the
update.  that way, i can check that the serial 1.
exists and 2. hasn't already been updated and, based
on the result, kick out a message to the user as
appropriate.

tia...

tia...

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