Присоединитесь к таблице и получите значение из таблицы, если значение в этой таблице существует в другой таблице

<?php

session_start();
header('Content-Type: text/html; charset=utf-8');
include ("config.php");
error_reporting(0);
$servername = "localhost";
$username = "root";
$password = "root";
$dbname = "data";
$account_Name = $_SESSION['userlogin'];


// Create connection
$conn = mysqli_connect($servername, $username, $password, $dbname);
// Check connection
if (!$conn) {
    die("Connection failed: " . mysqli_connect_error());
}


// get id online

$id_online = $conn->query("select ID from online");

$online = "";
while($row = $id_online->fetch_assoc())
{
	// list account online
	
	$online = $row["ID"];
	echo "- ID  online: $online";
	echo "</br>";
	$id_basetab = $conn->query("select UserID, IP from User where UserID='$online'");
	$ip = "";
	//show same ip list
	while($list = $id_basetab->fetch_assoc())
	{
		$ip = $list["LastIP"];
		echo "<hr>- IP: $ip</br>";
		$queryIP = $conn->query("SELECT UserID, IP FROM User where IP=$ip");
		echo "<hr>";
		//echo " acc Clone: </br>";
		
		while($row1 = $queryIP->fetch_assoc())
		{
				 //echo "id: " . $row1["AccountID"]. " - Name: " . $row1["name"]. " - IP: " . $row1["LastIP"]. "<br>";
		}
		
	
		
	}
	
	// join table
	echo "<hr>";
	$queryshow = $conn->query("select  UserID, IP from online FULL JOIN User ON UserID=ID where UserID=$online ");
		while($result = $queryshow->fetch_assoc())
		{
       // is my $queryshow correct? I want to get value ID is exist or not if exist UserID on User table.
       // How can I create a query for only 3 users online if they have multiple account online.
		  //I will make my custom query here.
		
		}
}


?>