如果我有部门的用户名、密码和选项值，如何编写SQL语句

我正在尝试连接到我的数据库，但我不知道如何编写用户名、密码和部门的代码，因为我有4个部门本身的选项值。区域经理、运营经理和管理层这三个部门将被引导到同一页面，但管理员被重定向到admin.html。如何编写正确的代码？

这是我的login.php:

<?php
include("connect.php");
error_reporting(0);
session_start();
$username = $_POST['username'];
$password = $_POST['password'];
if ($username && $password) {
    $query = mysql_query("SELECT *FROM login_user WHERE loginid ='$_SESSION[username]'");
    $numrows = mysql_num_rows($query);
    if ($numrows != 0) {
        while ($row = mysql_fetch_assoc($query)) {
            $dbusername   = $row['username'];
            $dbpassword   = $row['password'];
            $dbdepartment = $row['department'];
        }
        if ($username == "$dbusername" && $password == "$dbpassword" && $dbdepartment == "am") {
            // echo "you're in! <a href='form.php'>click</a> here to enter the member page";
            $_SESSION['username'] = $username;
            echo ("<SCRIPT LANGUAGE='JavaScript'>
                window.alert('Login Success ')
                window.location.href='form.php'
                </SCRIPT>");
        } else if ($username == "$dbusername" && $password == "$dbpassword" && $dbdepartment == "om") {
            // echo "you're in! <a href='form.php'>click</a> here to enter the member page";
            $_SESSION['username'] = $username;
            echo ("<SCRIPT LANGUAGE='JavaScript'>
                window.alert('Login Success ')
                window.location.href='form.php'
                </SCRIPT>");
        } else if ($username == "$dbusername" && $password == "$dbpassword" && $dbdepartment == "m") {
            // echo "you're in! <a href='form.php'>click</a> here to enter the member page";
            $_SESSION['username'] = $username;
            echo ("<SCRIPT LANGUAGE='JavaScript'>
                window.alert('Login Success ')
                window.location.href='form.php'
                </SCRIPT>");
        } else if ($username == "$dbusername" && $password == "$dbpassword" && $dbdepartment == "a") { // echo "you're in! <a href='admin.html'>click</a> here to enter the member page";
            $_SESSION['username'] = $username;
            echo ("<SCRIPT LANGUAGE='JavaScript'>
                window.alert('Login Success ')
                window.location.href='admin.html'
                </SCRIPT>");
        }
        else
            echo ("<SCRIPT LANGUAGE='JavaScript'>
                window.alert('Something went Wrong')
                window.location.href='loginmain.php'
                </SCRIPT>");
    }
}
?>

这是我的HTML代码：

<?php
    include 'login.php';
    session_start();
    $query = mysql_query("SELECT *FROM login_user WHERE loginid='$_SESSION[username]'");
    while($row = mysql_fetch_assoc($query))
    {
        $dbusername =$row['username'];
        $dbpassword =$row['password'];
        $dbdepartment =$row['department'];
    }
    ?>   
     .............
    <form action="login.php" method="post">                     
          <div class="form_settings">   
             <p><span>Username :</span><input type="username" name="username" placeholder ="username" value = "<?php echo $username;?>" /></p>
                    <p><span>Password :</span><input type="password" name="password" placeholder="password"  required value = "<?php echo $password;?> " /> </p>
                    <p><span>Department :</span><select name = "department" value ="<?php echo $department;?>/><option value ="am">Area Manager </option><option value ="om"> Operation Manager</option><option value ="m"> Management</option><option value = "a"> Admin </option></select></p><br>
                        <p><span>Remember Me<input type="checkbox" name="remember" value="1"></span></p>
                <p style="padding-top: 15px"><a href="form.php"><span>&nbsp;</span><input class="submit" type="submit" name="name" value="LogIn" /></a></p>                                                                                                                                 
            </div>
        </form>

这是我的connect.php:

<?php
$mysql_hostname = "localhost";
$mysql_username = "root";
$mysql_password = "";
$mysql_database = "marrybrown_clean";
$bd = mysql_connect($mysql_hostname, $mysql_username, $mysql_password) or die("Could not connect database");
mysql_select_db($mysql_database, $bd) or die("Could not select database");
?>

这是我的form.php:

<?php
    require_once ("login.php");
    include("connect.php");    
?>

我正在尝试创建安全登录页面，但大多数在线代码都强调用户名和密码；如何为部门添加插件？