使用ajax从数据库中的选定下拉列表中调出数据

<select id="dropdown" onchange="getData()">
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
  <option value="3">Option 3</option>
</select>

<div id="result"></div>

function getData() {
  var selectedValue = document.getElementById("dropdown").value;
  
  // 创建ajax对象
  var xhr = new XMLHttpRequest();
  
  // 设置请求方式和URL
  xhr.open("GET", "backend.php?selectedValue=" + selectedValue, true);
  
  // 设置回调函数
  xhr.onreadystatechange = function() {
    if (xhr.readyState === XMLHttpRequest.DONE && xhr.status === 200) {
      var response = JSON.parse(xhr.responseText);
      document.getElementById("result").innerHTML = response.data;
    }
  };
  
  // 发送请求
  xhr.send();
}

<?php
$selectedValue = $_GET["selectedValue"];

// 连接数据库
$servername = "数据库服务器地址";
$username = "用户名";
$password = "密码";
$dbname = "数据库名";

$conn = new mysqli($servername, $username, $password, $dbname);

// 查询数据库
$sql = "SELECT * FROM your_table WHERE column = '$selectedValue'";
$result = $conn->query($sql);

// 处理查询结果
if ($result->num_rows > 0) {
  $data = "";
  while ($row = $result->fetch_assoc()) {
    $data .= $row["column_name"] . "<br>";
  }
  echo json_encode(["data" => $data]);
} else {
  echo json_encode(["data" => "No data found"]);
}

// 关闭数据库连接
$conn->close();
?>