ช่วยเขียน Code ตรวจสอบ การซ่ำของ Username ด้วยครับๆ

<?php
session_start();
include "include/config.php";

$strName = $_POST["tName"];
$strEmail= $_POST['tEmail'];
$strLogin = $_POST['tLogin'];
$strPwd = $_POST['tPwd'];
$strCaptcha=$_POST['tCaptcha'];
$strMode = $_POST["action"];

if ($strMode=="ADD") {
if($_SESSION["security_code"]==$strCaptcha){


$sql_sel= "SELECT *FROM `tbl_member` WHERE `member_login` LIKE '".$strLogin."'";
$result_sql = mysql_query($sql_sel);
$num_rows = mysql_num_rows($result_sql);
if($num_rows>=1){
	echo"Error User";
}

	$sql = "INSERT INTO tbl_member VALUES	(0, '$strName', '$strEmail', '$strLogin', '$strPwd')";					
	$result = @mysql_query($sql) or die(mysql_error());	
	
	if($result) {
		echo "<font color=#009900 size=2>สมัครสมาชิกสำเสร็จ</font>";	
	} else {
		echo "<font color=#FF0000 size=2>ไม่สามารถบันทึกข้อมูลได้</font>";
	}
	
} else {
	echo "<font color=#FF0000 size=2>คุณไม่ผ่านการตรวจสอบ กรุณาลองใหม่อีกครั้ง</font>";
}

}

if ($strMode=="EDIT") {

if($_SESSION["security_code"]==$strCaptcha){

	$sql = "UPDATE tbl_member SET member_name='$strName', member_email='$strEmail', member_pwd='$strPwd' WHERE member_login='$strLogin'";					
	$result = @mysql_query($sql) or die(mysql_error());	
	
	if($result) {		
		echo "<font color=#009900>แก้ไขข้อมูลเรียบร้อยแล้ว</font><br><a href=\"javascript:window.close();\">หน้าต่างนี้</a>";	
	} else {
		echo "<font color=#FF0000>ไม่สามารถแก้ไขข้อมูลได้</font>";
	}
	
} else {
	echo "<font color=#FF0000>คุณไม่ผ่านการตรวจสอบ <br>กรุณาลองใหม่อีกครั้ง</font>";
}
	
}

mysql_close($objConnect);
?>

function getXmlHttpRequestObject() {
 if (window.XMLHttpRequest) {
    return new XMLHttpRequest(); //Mozilla, Safari ...
 } else if (window.ActiveXObject) {
    return new ActiveXObject("Microsoft.XMLHTTP"); //IE
 } else {
    alert("Your browser doesn't support the XmlHttpRequest object.");
 }
}

var receiveReq = getXmlHttpRequestObject();

function checkpass(pwd, rpwd) {
    if (pwd!=rpwd) {
      document.getElementById("msg").innerHTML = '<font color=red size=2 >รหัสผ่านไม่ตรงกัน</font>';
      document.form1.repassword.focus(); 
      document.getElementById('txtCaptcha').disabled = true; 	 
      document.getElementById('btnSubmit').disabled = true; 	  
	} else {
      document.getElementById("msg").innerHTML = '';		
      document.getElementById('txtCaptcha').disabled = false
      document.getElementById('btnSubmit').disabled = false;  
    } 
}

function makeRequest(url, param) {
 if (receiveReq.readyState == 4 || receiveReq.readyState == 0) {
   receiveReq.open("POST", url, true);
   receiveReq.onreadystatechange = updatePage;   

   receiveReq.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
   receiveReq.setRequestHeader("Content-length", param.length);
   receiveReq.setRequestHeader("Connection", "close");

   receiveReq.send(param);
 }   
}

function updatePage() {
 if (receiveReq.readyState == 4) {			 
      document.getElementById('result').innerHTML = receiveReq.responseText;
      document.getElementById("txtname").value = '';
      document.getElementById("txtemail").value = '';	
      document.getElementById("txtlogin").value = '';	
      document.getElementById("password").value = '';	
      document.getElementById("repassword").value = '';		
      document.getElementById('txtCaptcha').value = '';   
  
   img = document.getElementById('imgCaptcha'); 
   img.src = 'member/redcaptcha.php?' + Math.random();
 }
}

function refreshCap() {
   img = document.getElementById('imgCaptcha'); 	
   img.src = 'member/redcaptcha.php?' + Math.random();	
}

function check_data(nm, email, login, pwd, cap, Mode) {
  var cancle=false;
    if (nm.length==0) {
      alert('กรุณาป้อนชื่อด้วย');
      document.form1.txtname.focus(); 
      cancle=true;
    } else 

    if (email.length==0) {
      alert('กรุณาป้อนอีเมลด้วย');
      document.form1.txtemail.focus(); 
      cancle=true;
    } else
	
    if (login.length==0) {
      alert('กรุณาป้อน Username ด้วย');
      document.form1.txtlogin.focus(); 
      cancle=true;
    } else
	
    if (pwd.length==0) {
      alert('กรุณาป้อน password ด้วย');
      document.form1.password.focus(); 
      cancle=true;
    }	
  
    if (cap.length==0) {
      alert('กรุณากรอกตัวอักขระสีแดงที่เห็นในภาพก่อน') + Mode;
      document.form1.txtCaptcha.focus(); 
      cancle=true;
    } 
		
  if (cancle==false) {
	 var url = 'addmember.php';

	 var postStr = "tName=" + encodeURI( document.getElementById("txtname").value ) +
	 	 "&tEmail=" + encodeURI( document.getElementById("txtemail").value ) +
	 	 "&tLogin=" + encodeURI( document.getElementById("txtlogin").value ) +
    	 "&tPwd=" + encodeURI( document.getElementById("password").value ) +
		 "&tCaptcha=" + encodeURI( document.getElementById("txtCaptcha").value )+
		 "&action=" + Mode ;			 	 
	 makeRequest(url, postStr);	
  }
  return false;
}