Hello, I'm sure I'm missing something small. If a category doesn't exist in the database yet, how can I have the query accept a value of 0 without reporting an error? The $subcategory is what the user selected. The result should either be 4 images or a blank place holder with an echo (couldn't figure out the echo either, just got more errors.)

Here is my error Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in /product.php on line 38

Thank you for your help.

<?php 
include "storescripts/connect_to_mysql.php"; 
$tinysubcategory = "";
$tinysql = mysql_query("SELECT * FROM products WHERE subcategory ='". $subcategory ."' LIMIT 4");
$tinyproductCount = mysql_num_rows($tinysql); // count the output amount
if ($tinyproductCount > 0) {
    while($row = mysql_fetch_array($tinysql)){ 
             $id = $row["id"];
             $product_name = $row["product_name"];
             $details = $row["details"];
             $subcategory = $row["subcategory"];
             $date_added = strftime("%b %d, %Y", strtotime($row["date_added"]));
             $tinysubcategory .= 
            '
            <th valign="top">
             <a href="product.php?id=' . $id . '">
             <img style="border:#FFF 1px solid;" src="inventory_images/' . $id . '.jpg" alt="' . $product_name . '" width="70" height="70" border="2px" />
             </a>
            <h3> <a href="product.php?id= ' . $id . '"> ' . $product_name . ' </a> </h3> 
            </th>';
    }
} else {
    $tinysubcategory .= 
    '<img style="border:#FFF 1px solid;" src="inventory_images/productholder.png" alt="Nothing to display!" width="70" height="70"" />
    <br/>';

}
mysql_close(); ?>

<? echo $tinysubcategory; ?>

The mysql_num_rows if failing because your query is failing not because it returned 0 results. Add a die statement to your query to output the error to see what's happening:

$tinysql = mysql_query("SELECT * FROM products WHERE subcategory ='". $subcategory ."' LIMIT 4") or die (mysql_error());

On a side note you're not using your tinyProductCount variable anywhere so you're wasting resources by storing it in a variable. You can combine the two lines into one

$tinysql = mysql_query("SELECT * FROM products WHERE subcategory ='". $subcategory ."' LIMIT 4");

if (mysql_num_rows($tinysql) > 0) {
...

On another side note mysql functions are being depreciated, you should use mysqli or PDO

As GliderPilot says - however, just to clarify as well, the error you are getting is telling you that $subcategory (in your query) is either empty or in an incorrect format.

Where is the value assigend to $subcategory coming from (I don't see anything in your code where a value is assigned to this variable)? I am presuming you are posting the value from a form therefore you need to be using $_POST to obtain the data (for example - please adjust to match your form / variable name):

if (isset($_POST) && !empty($_POST['subcategory']) {
  $subcategory = $_POST['subcategory'];

  // Do rest of query here

}

Thank you! I had to contact the company that hosts my domain to get them to enable errors, it pointed me straight to where the problem was. Thank you all so much.