you could also do :

if($id) {
   $sql = "SELECT count(*) FROM category WHERE parent = '$id'";
} else {
   $sql = "SELECT count(*) FROM category WHERE parent = '0'";
}
   $sth = $dbh->prepare($sql) || die "Error: $!";
   $sth->execute or die $dbh->errstr;
   my ($count)  = $sth->fetchrow_array;
   if ($count > 0) {    
    print qq...
    <Then fire the sQL which gets the data>
[download]