警告：mysqli_query（）：第21行问题中的空查询

Question

当我打开它然后显示

  

警告：mysqli_query（）：第21行的C：\ xampp \ htdocs \ databasehomework \ check_login.php中的空查询

connection.php

<?php
$con = mysqli_connect("localhost","root","","databasehomework")or die("...");
mysqli_set_charset($con,'utf8');
?

check_login.php

<?php
error_reporting( error_reporting() & ~E_NOTICE );
    require 'dbcon.php';

    session_start();
             $status= filter_input(INPUT_POST, 'status', FILTER_SANITIZE_SPECIAL_CHARS);
             $username= filter_input(INPUT_POST, 'txtUsername', FILTER_SANITIZE_SPECIAL_CHARS);
             $password= filter_input(INPUT_POST, 'txtPassword', FILTER_SANITIZE_SPECIAL_CHARS);

                      if($status=='admin'){
                      $strSQL = "SELECT * FROM data_admin WHERE username = '$username' 
    and password = '$password'";
    }else if ($status=='staff') {
        $strSQL = "SELECT * FROM data_staff WHERE email = '$username' 
        and password = '$password'";
    }else if($status=='student'){
        $strSQL = "SELECT * FROM data_nisit WHERE std_code = '$username' 
        and std_password = '$password'";
    }
                          print($strSQL);
                      $objQuery = mysqli_query($con,$strSQL)or die ();
    $objResult = mysqli_fetch_array($objQuery,MYSQLI_ASSOC);

    if(!$objResult){
        echo "<script type='text/javascript'>alert('กรุณาตรวจสอบข้อมูล อีเมล์ หรือ รหัสผ่านให้ถูกต้อง');</script>";
        echo "<script type='text/javascript'>history.go(-1);</script>";
    }else{
            if($status=='admin'){
                $_SESSION["Status"] = "ADMIN";
                $_SESSION["User"] = $objResult["username"];
            }else if($status=='staff'){
                $_SESSION["Status"] = "STAFF";
                $_SESSION["UserId"] = $objResult["staff_code"];
                $_SESSION["UserName"] = $objResult["tname"].$objResult["fname"]." ".$objResult["lname"];
            }else if($status=='student'){
                $_SESSION["Status"] = "STUDENT";
                $_SESSION["UserId"] = $objResult["std_code"];
                $_SESSION["UserName"] = $objResult["std_tname"].$objResult["std_fname"]." ".$objResult["std_lname"];
            }

            session_write_close();

            if($_SESSION["Status"] == "ADMIN"){
                header("location:index_admin.php");
            }else if($_SESSION["Status"] == "STAFF"){
                header("location:index_staff.php");
            }else if($_SESSION["Status"] == "STUDENT"){
                header("location:index_nisit.php");
            }
    }
    mysqli_close($con);
?>