错误警告：mysqli_num_rows（）期望参数1为mysqli_result为什么？

Question

我正在学校学习php，收到此警告

  

警告：mysqli_num_rows（）期望参数1为mysqli_result

为什么？

<?php
require "conn.php";
$user_name = "123";
$user_pass = "123";
$mysql_qry = "SELECT * FROM employeedata where username = '$user_name' and password = '$user_pass';";
$result = mysqli_query($conn, $mysql_qry);
if(mysqli_num_rows($result) > 0) {
    echo "login success";
}
else {
    echo "login not success";
}

?>