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   select sum(whatever)
   from
...
      -- whole pile of stuff omitted --
   group by
      -- another pile of things

  $sth->execute($foo, $bar, $rat) or die $sth->errstr, $/;
  my $sum = $sth->fetchrow_arrayref->[0];
  $sth->finish;

  $sth->execute($foo, $bar, $rat) or die $sth->errstr, $/;
  my $r = $sth->fetchrow_arrayref;
  my $sum = $r ? $r->[0] : 0;
  $sth->finish;

  $sth->execute($foo, $bar, $rat) or die $sth->errstr, $/;
  my $sum = ($sth->fetchrow_arrayref || [0])->[0];
  $sth->finish;