Warning: mysqli_error() expects **小白求解

Question

<?php

     $conn=mysqli_connect("localhost","root","123456789") or die("資料庫伺服器連線錯誤".mysqli_error());

# #     mysqli_select_db($conn,"db_library") or die("資料庫存取錯誤".mysqli_error( ));

     mysqli_query($conn,"set names ##     mysqli_query($conn,"set names ##     mysqli_query($conn,"set names ##  );#gt# ;

Warning: mysqli_error() expects exactly 1 parameter, 0 given in *********\conn\conn.php on line 3資料庫存取錯誤