[RESOLVED] Problem with mysql_fetch_assoc

This is the error i am getting

Code:

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Warning: mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource in C:\xampp\htdocs\Student Planner\account.php on line 40

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I am using the example from php.net, i am unsure as to why the application is crashing?

PHP Code:

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if($_POST['email']==""||$_POST['password']=="")
        {
            $berror=1;
            $message.="<p>Please complete all fields</p>";
        }
        else
        {
            $sql="SELECT * FROM student_users WHERE id = '" . $_SESSION['userid'] . "' LIMIT 1";
            
            $result = mysql_query($sql);
            
            if (!$result) 
            {
                $berror=1;
                $message.="<p>Could not successfully run query ($sql) from DB: " . mysql_error() . "</p>";
            }
            else
            {
                if(mysql_num_rows($result)==0) 
                {
                    $berror=1;
                    $message.="<p>No rows found, nothing to print so am exiting</p>";
                }
                else
                {
                    while ($row = mysql_fetch_assoc($result))  // THIS IS LINE 40
                    {
                        if($row['password']==md5($_POST['password']))
                        {
                            $sql="UPDATE student_users Set email = '" . $_POST['email'] . "' WHERE id = '" . $_SESSION['userid'] . "'";
                            
                            $result = mysql_query($sql);

                            if (!$result) 
                            {
                                $berror=1;
                                $message.="<p>Could not successfully run query ($sql) from DB: " . mysql_error() . "</p>";
                            }
                        }
                        else
                        {
                            $berror=1;
                            $message.="<p>Incorrect password</p>";
                        }
                    }
                }
            }
        } 


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Thats the code I am using

Resolved, i used two $results