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");
} 
?>