URGENT: undoing a mistake

Folks,

I just made a mistake that could be quite costly: I did this:

update writer_survey set partid='W41308' where survid in (select survid
from participants where partid='W41291' limit 1);

when I should have done this:

update writer_survey set partid='W41308' where survid in (select survid
from writer_survey where partid='W41291' limit 1);

Is there any way I can undo this, e.g., set partid back to what it was
before I ran this command? I know I should have done it in a transaction,
but I didn't.

More broadly, can someone explain why it worked? There is no survid column
in participants, so I would have expected it to generate an error on the
sub-select, not match all rows!

Thanks for any help.

----------------------------------------------------------------------
Andrew J Perrin - http://www.unc.edu/~aperrin
Assistant Professor of Sociology, U of North Carolina, Chapel Hill
clists(at)perrin(dot)socsci(dot)unc(dot)edu * andrew_perrin (at) unc.edu