check user ห้ามซ้ำ...จะเช็ค user login ว่าห้ามซ้ำกับที่มีในระบบยังไงค่ะ

<?php
$sql = "select user_id from user where username = '$username'";
$query_id = mysql_query($sql);
if(mysql_num_rows($query_id) != 0){
มานนนนซ้ำค่ะพี่น้อง
}else{
บันทึก....
}
?>

if(isset($_POST['action'])){
$SQL = "SELECT * FROM member where username = '$username' ";
$result = mysql_query($SQL);
$numrows = mysql_num_rows($result);
if ($numrows > 0 ) {
  echo " <font size=2 color=red><div align='center'><b>Sorry. Your e-mail is already registered</b></div></font><br>";
    echo "<font size=2 ><div align='center'> Plesae try another e-mail</div></font><br>";
	echo "<META HTTP-EQUIV=\"Refresh\" CONTENT=\"5;URL=regist_fr.php \">";
 echo " <font size=2 ><div align='center'><a href='javascript:history.back()'>Back</a></div></font> ";

  $error=1;
}
else {
  echo " ok add";
}

<?php
$sql = "select id from ตาราง where username = '$username'";
$query = mysql_query($sql);
$num=mysql_num_rows($query)
if($num == 0){
echo "User นี้ซ้ำนะจ๊ะ";
}else{
echo "สมัครได้แล้วเย้ๆๆ"

?>

<?php
$sql = "select id from ตาราง where username = '$username'";
$query = mysql_query($sql);
$num=mysql_num_rows($query)
if($num == 0){
echo "User นี้ซ้ำนะจ๊ะ";
}else{
echo "สมัครได้แล้วเย้ๆๆ"
?>