JQuery ajax verify username exists

Actually, there are many examples of this on the Internet, but when I was doing it, I ran into some problems, and finally found a solution to the problem, so I wanted to post it and record it as a warning.

The code is as follows:

<form name="add" method="post" action="test.php">  
  用户名:<input type="text" id="uname" name="username" />  
</form>

$('#cname').blur(function(){  
        if($('#uname').val() == ''){  
            $(&#39;#result&#39;).html("<span style=&#39;color:red;&#39;>不能为空</span>");  
        }  
        else{  
            //var str = &#39;cname=&#39;+$(&#39;#cname&#39;).val();  
            $.get(&#39;test.php&#39;,{username:$(&#39;#uname&#39;).val()},function(data){  
                $(&#39;#result&#39;).html("<span style=&#39;color:red;&#39;>"+data+"</span>");  
            });  
        }  
    });

<?php   
mysql_connect(&#39;localhost&#39;,&#39;root&#39;,&#39;&#39;);  
mysql_select_db(&#39;news&#39;);  
mysql_query(&#39;set names utf8&#39;);  
$a = $_GET[&#39;cname&#39;];  
$sql = "select * from tnewscategory where cCategoryName = &#39;$a&#39;";  
// $sql = "select count(*) from tnewscategory where cCategoryName = &#39;$a&#39;";  
/* 
问题就出在这个地方了，我原来用的是count(*)的查询语句查询的，但是验证的结果都是可以注册，不管是数据库里已存在的，还是未存在的数据，都提示可以注册，后来换成 select * 就正常了。这两种查询方法，最终的查询结果都是一个资源类型的。最终使用mysql_num_rows()转化后，也确实得了一个数值，却不知道为何返回到ajax验证的时候，却无法成功。当初使用count(*);  想着这种效率更快些，不想却出了这样的情况，暂时记录下吧，以后找到原因，再做更新 
*/  
$n = mysql_num_rows(mysql_query($sql));  
if($n > 0){  
   echo "已存在";  
   exit;  
}  
else{  
    echo "可以注册";  
    exit;  
  
}  
?>