how to check data if exist

<?php

$db_host = 'localhost'; // Server Name
$db_user = 'root'; // Username
$db_pass = 'root'; // Password
$db_name = 'node_update'; // Database Name

$conn = mysqli_connect($db_host, $db_user, $db_pass, $db_name);
if (!$conn) {
  die ('Failed to connect to MySQL: ' . mysqli_connect_error());
}
?>
<?php

if(isset($_POST['submit'])){
  $id=$_POST['id'];
  $node=$_POST['node'];
  $interface=$_POST['interface'];
  $filter=("SELECT node from cam where id ='$id'");
  $query = mysqli_query($conn, $filter);

}
if(mysqli_num_rows($query)>1){

 		echo "this file exist".$node;
 		/*
			DELETE CODE HERE!!

 		*/
 }
 else{

 	/*
		update code here !!
 	*/
		echo "update this ! ";

 }
?>