我是Codeigniter 4的新手，正在尝试编写SELECT UNION SQL语句。根据codeigniter 4中的用户指南，select（）方法中有第二个参数，允许通过将其设置为false来编写自定义SQL语句。不幸的是，它不起作用。有人能帮忙吗？有没有其他解决方案？

$query = "SELECT acnt_id, acnt_opbal as sum_amount 
                        FROM accounts 
                            WHERE acnt_id = '" . $acnt_id . "' 
                    UNION 
                    SELECT txn_acnt_id_dr as acnt_id, SUM(txn_amount_dr) as sum_amount 
                        FROM transactions 
                            WHERE txn_acnt_id_dr = '" . $acnt_id . "' 
                                AND txn_date < '" . $fdate . "'
                            GROUP BY txn_acnt_id_dr 
                    UNION 
                    SELECT txn_acnt_id_cr as acnt_id, -SUM(txn_amount_cr) as sum_amount 
                        FROM transactions 
                            WHERE txn_acnt_id_cr = '" . $acnt_id . "' 
                                AND txn_date < '" . $fdate . "'
                            GROUP BY txn_acnt_id_cr";
$result = $this->builder()->select($query, false)->get()->getResult();