Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in มีข้อสงสัยครับ

$sqlcu ="select USERNAME from user where USERNAME = '".($_POST['USERNAME'])."'";
$checku = $main->Query($sqlcu);

$sql ="select * from user where USERNAME = '".($_POST['USERNAME'])."'   and PASSWORD = '".md5(md5($_POST['PASSWORD']))."'";
$result = $main->Query($sql);

$sql1 = "UPDATE user SET LOGNOW_DATETIME = '".date("Y-m-d,H:i:s")."' ,LOGIN_SYS_NUM = '".$lognum."' WHERE USERNAME = '".$_SESSION['USERNAME']."' ";
$result1 = $main->Query($sql2);