谢谢你的回复。真的很感激，但我自己已经弄明白了；）
现在我得到了以下信息：

$currentUserID = get_current_user_id();
$rows = $wpdb->get_results("SELECT * FROM `messages` WHERE `from` = '" . $currentUserID . "' OR `to` = '" . currentUserID . "' ORDER BY `messages_id` ASC");

$messages = [];

foreach($rows as $row) {
    if($row->from == $currentUserID) {
        $messages[$row->to][] .= $row->message;
    }
    else {
        $messages[$row->from][] .= $row->message;
    }
}
print_r($messages);

试试这个：（假设每个字段必须有 Auto-increment_from 或者 to . 对于收到的案件 to 字段有值

SELECT *, if(`Auto-increment_from` = 1 , 'From' , 'From') as meaage_type 
FROM messages WHERE from = 1 OR to = 1 ORDER BY messages_id ASC

几年前我遇到过这样的问题。我做到了：

select 'In' as MessageDirection, `From` as Contact, `To` as UserId, Message 
from Messages
where `To` = 1
union
select 'Out' as MessageDirection, `To` as Contact, `From` as UserId, Message 
from Messages
where `From` = 1
order by Contact