老师，我想问一下

$sql = "select * from imooc_admin limit ".($page-1) * 2 .",2 ";
//把SQL语句传送数据中
$result = mysql_query($sql);
//处理数据
while ($row = mysql_fetch_assoc($result)){
    echo $row['id']. '-'. $row['username']. '-'. $row['email']. "<br>";
}

报错误：Warning:  mysql_fetch_assoc(): supplied argument is not a valid MySQL result resource in D:\APMServ5.2.6\www\htdocs\shopImooc\admin\test.php on line 24

但是改成$sql = "select * from imooc_admin " 可以显示表中所有内容，为什么呢？