为什么会出现错误：查询为空？

Question

I am creating a login part to my web page. When a new person registers their details, pressing the register button goes to a register_ok part, showing below:

case 'register_ok':



if (!$_POST['client_username'] || !$_POST['client_password'] ||
     !$_POST['client_email']) {
    die('You did not fill in a required field.');
}

// check if username exists in database.

if (!get_magic_quotes_gpc()) {
    $_POST['client_username'] = addslashes($_POST['client_username']);
}

$qry = "SELECT client_username FROM client WHERE client_username = '".$_POST['client_username']."'";
$result = mysql_query($qry);
    if($result) {
        if(mysql_num_rows($result) > 0) {
    die('Sorry, the username: <strong>'.$_POST['client_username'].'</strong>'
      . ' is already taken, please pick another one.');
    }

}


// check e-mail format

if (!preg_match("/.*@.*..*/", $_POST['client_email']) ||
     preg_match("/(<|>)/", $_POST['client_email'])) {
    die('Invalid e-mail address.');
}

// no HTML tags in username, website, location, password

$_POST['client_username'] = strip_tags($_POST['client_username']);
$_POST['client_password'] = strip_tags($_POST['client_password']);



// now we can add them to the database.
// encrypt password

$_POST['client_password'] = md5($_POST['client_password']);

if (!get_magic_quotes_gpc()) {
    $_POST['client_password'] = addslashes($_POST['client_password']);
    $_POST['client_email'] = addslashes($_POST['client_email']);

}



$insert = "INSERT INTO client (
        client_username, 
        client_password, 
        client_name, 
        client_email, 
        client_last_access)

        VALUES (
        '".$_POST['client_username']."', 
        '".$_POST['client_password']."',  
        '".$_POST['client_name']."', 
        '".$_POST['client_email']."',
        'now()'
        )";

        if(!mysql_query($sql,$con)) {
            die('Error: ' . mysql_error());
        }

        else{

$id= mysql_insert_id();

session_start();


            echo '<script>alert("You May Now Login");</script>';
            echo '<meta http-equiv="Refresh" content="0;URL=pv.php">';
        }


break;
}

When I register a new person, I get the following error:

Error: Query was empty

Why is this?

Answer 1

Do:

if(!mysql_query($sql,$con)) { 

to

if(!mysql_query($insert,$con)) {

your variable name is not correct

Answer 2

在if(!mysql_query($sql,$con)) { ，您的意思是$insert而不是$sql吗？