php if语句判定my查询是否为空

<?php 

header("Content-type: text/html; charset=utf-8");

$username=$_GET['username'];

$psw=$_GET['password'];

$phone=$_GET['phone'];

include("../ini/inid.php");

$select_admin=mysql_query(" select count(*) c from ofuseradmin where phone='".$phone."' ");//判定是否为空

while($sa = mysql_fetch_array($select_admin)){

if ($sa['c']=='0'){

    mysql_query("INSERT INTO ofuseradmin (username,password,phone) VALUES ('".$username."','".md5($psw)."','".$phone."')");

$aid=mysql_query("select * from ofuseradmin where phone='".$phone."'");

while($aids = mysql_fetch_array($aid)){

//echo $aids['aid'];

//echo "<br>";

echo "<input type='text' value='".$aids['aid']."' id='pid' name='aid'>";

}

}//添加语句

else{

//echo "此组合已经存在！"; 

echo"<script>alert('此手机号已经存在！');history.go(-1);</script>";  

}

//include("../ini/inid.php");

} 

?>