a.asp

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<input name="UserNameQ"   title="请输入用户名"  onBlur="RegUserCheck();" ><span id="chekRegName"></span>
<script type="text/javascript">
function RegUserCheck()
{
var xml = new ActiveXObject("MSXML2.XMLHTTP");
    username2=document.getElementById("UserNameQ").value;
    xml.open("get","b.asp?UserNameQ="+username2+"",false);
xml.send();

   if (xml.responseText == 0)
   {
      document.getElementById('chekRegName').innerHTML="此用户名可以注册";
   }
else if (xml.responseText == 1)
   {
      document.getElementById('chekRegName').innerHTML="此用户已被他人注册，请更换一个再试";
   }
   else
   {
      document.getElementById('chekRegName').innerHTML="用户名检测时发生错误";
   }
}
</script>

b.asp

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<!--#include file="cn.asp"-->
<%
Function codepass()
     UserNameQ=request("UserNameQ")
set rs=server.CreateObject("adodb.recordset")
rs.open "select * from danwei where mingcheng='"&UserNameQ&"'",conn,1,1
  If rs.eof Then
      codepass = True
       Else
         codepass = False
       End If
  rs.close
Set rs=nothing
Set conn=Nothing
End Function
If codepass() then
response.write "0"
else
response.write "1"
end if
%>