为＃34;聊天记录选择每个用户ID只有一个结果＆＃34;

Question

尝试仅返回每个UserId的一行（最新）行（如果包含在MySQL数据库中），以使结果显示＆＃34;聊天记录＆＃34;每说。基本上类似于任何其他用户的最新消息的测试历史概述。

<?php
$userId = $_POST['userId'];
$pdo = new PDO('mysql://hostname=localhost;dbname=data_base', 'user', 'password');
$query = $pdo->query("SELECT * FROM `messages` WHERE `from_id`='{$userId}' OR `to_id`='{$userId}' ORDER BY `stamp` DESC");
$data = $query->fetchAll(PDO::FETCH_ASSOC);
echo json_encode($data);

Answer 1

<?php
$userId = $_POST['userId'];
$pdo = new PDO('mysql://hostname=localhost;dbname=data_base', 'user', 'password');
$query = $pdo->query("SELECT * FROM `messages` WHERE `from_id`='{$userId}' OR `to_id`='{$userId}' GROUP BY `from_id`, `to_id` ORDER BY `stamp` DESC");
$data = $query->fetchAll(PDO::FETCH_ASSOC);
echo json_encode($data);