为什么连接了mysql却查询不了,该怎么处理

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catalog search
<h2>catalog search</h2>

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<h2>search results</h2>
<!--?php
$searchtype=$_POST['searchtype'];
$searchterm=trim($_POST['searchterm']);
if(!$searchtype||!$searchterm){
    echo"请输入值";
        exit;
}
if(!get_magic_quotes_gpc())
{
    $searchtype=addslashes($searchtype);
    $searchterm=addslashes($searchterm);
}
@ $db=new mysqli('localhost','root','root','books');
if (mysqli_connect_errno()) 
{
echo 'Error: Could not connect to database. Please try again later.';
exit;
}
$query="select * from books where".$searchtype."like '%".$searchterm."%'";
$result=$db--->query($query);
$num_results=$result->num_rows;
echo "<p>Number of books found:".$num_results."</p>";
for ($i=0;$i<$num_results;$i++)
{
    $row=$result->fetch_assoc();
    echo"<p><strong>".($i+1)."title:";
    echo htmlspecialchars(stripslashes($row['title']));
    echo "</strong><br>author:";
    echo stripslashes($row['author']);
    echo"<br>isbn:";
    echo stripslashes($row['isbn']);
    echo"<br>price:";
    echo stripslashes($row['price']);
    echo"</p>";
}
?>