How to not search the blank in query - php

Question

When I go to this page It's already search the blank string in the variable before pick the value in option. I just want to search the value when the button is click not automatically search when the user go to this page. And Also there's a problem on my "asc" in query.
If I go to this page. It's already search and show up the blank values
Already Search the Blank
And I insert this code $asc but as you can see in the picture it shows up an error but If I pick in the select option the error will vanish and show the value.
this Ascending to Descending
Please Enter Details Below
<div class="nice" style=" width:20%; margin-left:10%; z-index:100px;">
<div class="control-group">
<label class="control-label" for="inputPassword">Search Catalog:</label>
<div class="controls">
<select name="catalogs">
<option></option>
<?php $result = mysqli_query($dbcon,"select * from book_catalog")or die(mysqli_error());
while ($row=mysqli_fetch_array($result)){ ?>
<option value="<?php echo $row['catalog']; ?>"><?php echo $row['catalog']; ?></option>
<?php } ?>
</select>
</div>
</div>
<div class="control-group">
<label class="control-label" for="inputPassword">Author:</label>
<div class="controls">
<select name="status">
<option></option>
<option>Local</option>
<option>Foreign</option>
</select>
</div>
</div>
<div class="control-group" style="margin-left:210%; margin-top:-150px;">
<label class="control-label" for="inputPassword">Ascending-Descending</label>
<div class="controls">
<select name="asc" >
<option></option>
<option value="acc_id">Accession Number</option>
<option value="author">Author</option>
<option value="book_title">Book Title</option>
<option value="publisher_name">Publisher Name</option>
<option value="copyright_year">Copyright Year</option>
<option value="date_added">Date Added</option>
</select>
</div>
</div>
</div>
<input type="submit" name="submit" value="Search" style="margin-left:43%; margin-top:10%;" /> <br>
</form>
</div>
<?php
$selected_stat = "";
$selected_val = "";
$asc =""; // Storing Selected Value In Variable
if(isset($_POST['submit'])){
$selected_val = $_POST['status'];
$selected_stat = $_POST['catalogs'];
$asc = $_POST['asc']; // Storing Selected Value In Variable
}
?>
<br>
<thead>
<tr>
<th>Acc No.</th>
<th>Call Number</th>
<th>Author</th>
<th>Author Category</th>
<th>Book Title</th>
<th>Category</th>
<th>Book Copies</th>
<th>Publisher Name</th>
<th>Copyright Year</th>
<th>Date Added</th>
<th class="action">Action</th>
</tr>
</thead>
<tbody>
<?php $user_query=mysqli_query($dbcon,"select * from book where catalog = '$selected_stat' OR status = '$selected_val' ORDER BY book.$asc ASC")or die(mysqli_error($dbcon));
while($row=mysqli_fetch_array($user_query)){
$id=$row['book_id'];
?>
<tr class="del<?php echo $id ?>">
<td><?php echo $row['acc_id']; ?></td>
<td><?php echo $row['callnum']; ?></td>
<td><?php echo $row['author']; ?> </td>
<td><?php echo $row['status']; ?></td>
<td><?php echo $row['book_title']; ?> </div></td>
<td><?php echo $row ['catalog']; ?> </td>
<td class="action"><?php echo $row['book_copies']; ?> </td>
<td><?php echo $row['publisher_name']; ?></td>
<td><?php echo $row['copyright_year']; ?></td>
<td><?php echo $row['date_added']; ?></td>
<?php include('toolttip_edit_delete.php'); ?>
<td class="action">
<a rel="tooltip" title="Delete" id="<?php echo $id; ?>" href="#delete_book<?php echo $id; ?>" data-toggle="modal" class="btn btn-danger"><i class="icon-trash icon-large"></i></a>
<?php include('delete_book_modal.php'); ?>
<a rel="tooltip" title="Edit" id="e<?php echo $id; ?>" href="edit_book.php<?php echo '?id='.$id; ?>" class="btn btn-success"><i class="icon-pencil icon-large"></i></a>
</td>
</tr>
<?php } ?>
</tbody>
</table>
Sorry for my bad english

Answer 1

When the SQL statement throws an error the best thing to do is log it or dump it to the screen so you can see it as actual SQL. Given the form of the statement I would think that $asc isn't what you think it is, it could possibly even be blank giving:
ORDER BY book. ASC
The book. being something the database can't deal with.
I think the issue with the page running a query automatically on load is because you are calling the query directly in your select element
<select name="catalogs">
<option></option>
<?php $result = mysqli_query($dbcon,"select * from book_catalog")or die(mysqli_error());
while ($row=mysqli_fetch_array($result)){ ?>
<option value="<?php echo $row['catalog']; ?>"><?php echo $row['catalog']; ?></option>
<?php } ?>
</select>
That PHP code isn't waiting for user input, it's just running immediately.

Answer 2

Make your query like this
mysqli_query($dbcon,"select * from book where catalog = '" . $selected_stat ."' OR status = '" . $selected_val ."' ORDER BY book." . $asc . " ASC")or die(mysqli_error($dbcon));