无法使MySQL查询也链接WP中的类别/标签

Question

My goal is: allow customers to make no more than 60 pre-orders per day in a given time slot. The way I try to solve the problem is:

• when on checkout page, get amount of previously made orders via php and mysql. Pass it to js array
• user selects date, js compares it to a date in the array, checks the amount of products in a cart, adds
• shows an error message if total exceeds maximum

It seems to work correctly, except it counts all the products, though I need to limit the result to 1 category only. I tried for few hours to link it but with no success.

The main problem is that category info is stored in a table wp_termmeta/wp_terms and I can't understand how is it linked to the product ID or other columns that I currently use in the query.

SQL query:

SELECT REPLACE(a.meta_value,'-','') AS Date, SUM(c.meta_value) AS Quantity
FROM postmeta a, woocommerce_order_items b, woocommerce_order_itemmeta c
WHERE a.post_id = b.order_id AND b.order_item_id = c.order_item_id 
AND b.order_item_type = 'line_item' AND a.meta_key='_deliverydate' 
AND c.meta_key = '_qty' AND a.meta_value='$today'

My MySQL / PHP for the context that works:

add_action( 'wp_enqueue_scripts', 'menu_scripts' );
function menu_scripts() {
// Register the script
    wp_register_script( 'custom_script', get_stylesheet_directory_uri() . '/script.js');

    wp_enqueue_script( 'custom_script' );


    $currenthour = date("H");
    $today = date("Y-m-d");
    $cartcount = WC()->cart->get_cart_contents_count();

    $passphp = array(
        'hour' => $currenthour,
        'today' => $today,
        'incart' => $cartcount
    );

    for($i=0;$i<61;$i++){
        global $wpdb;
        $prefix = $wpdb->prefix;
        $result = $wpdb->get_results("
            SELECT REPLACE(a.meta_value,'-','') AS Date, SUM(c.meta_value) AS Quantity
            FROM ".$prefix."postmeta a, ".$prefix."woocommerce_order_items b, 
            ".$prefix."woocommerce_order_itemmeta c
            WHERE a.post_id = b.order_id AND b.order_item_id = c.order_item_id 
            AND b.order_item_type = 'line_item' AND a.meta_key='_deliverydate' 
            AND c.meta_key = '_qty' AND a.meta_value='$today'");

        foreach($result as $row) {
            $row->Date = str_replace("‐",",",$row->Date);
            $passphp['x'.$row->Date] = $row->Quantity;
        }

        $today = date('Y-m-d', strtotime($today. ' + 1 days'));    

     }

    // Localize the script with new data
 wp_localize_script( 'custom_script', 'passphp' , $passphp);
}

Maybe there is some better workaround to achieve the goal I can't think of?

off-topic: I did try to use LEFT JOIN instead of query you see, but I am new to MySQL syntax and for now, given up breaking the code that works while trying to figure it out.