"SELECT FROM table WHERE ".
  ( $form_username ? "username = $form_username " : '' ).
    ( $form_username && $form_password ? 'AND ' : '' ).
  ( $form_password ? "password = $form_password " : '' ).
    ( ($form_username || $form_password) && $form_name ? 'AND ' : '' ).
  ( $form_name ? "name = $form_name" : '' );

##</code><code>##

    a (or not)
    AND if a and b
    b (or not)
    AND if (a or b) and c
    c (or not)
    AND if (a or b or c) and d
    ...
    z (or not)

##</code><code>##

$sql_string = make_anded_statement ( username => $form_username,
                                     password => $form_password,
                                     name     => $form_name );

sub make_anded_statement {
  my ( %args ) = @_;
  my @keys = keys %args;
  my $where_string = '';
  my $ANDING = 0;
  foreach my $i ( 0 .. $#keys ) {
    if ( $args{$keys[$i]} ) {
      $where_string .= $keys[$i] . '=' . $args{$keys[$i]} . ' ';
      $ANDING = 1;
    }
    if ( $ANDING && $args{$keys[$i+1]} ) {
      $where_string .= 'AND ';
    }
  }
  return "SELECT FROM table WHERE $where_string";
}