我有一个很大的mysql表（在woocommerce中），人们可以在其中多次购买项目。我想做的是动态生成一个表，在这里我选择要查询的产品（如果可能的话，可以从php的下拉菜单中选择），并按用户id对购买的产品数量进行分组。我找到了这段代码，但无法生成我想得到的表。这里是我当前的测试代码，试图让它工作，但没有表生成；

<?php 
if (!is_user_logged_in() || !current_user_can('manage_options')) wp_die('This page is private.');

$con=mysqli_connect("ip","user","pass","database");
// Check connection
if (mysqli_connect_errno())
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
$result = mysqli_query($con,
"select p.user_id,
       max( CASE WHEN pm.meta_key = '_billing_first_name' and p.ID = pm.post_id THEN pm.meta_value END ) as _billing_first_name,
       max( CASE WHEN pm.meta_key = '_billing_last_name' and p.ID = pm.post_id THEN pm.meta_value END ) as _billing_last_name,
       max( CASE WHEN pm.meta_key = '_billing_address_1' and p.ID = pm.post_id THEN pm.meta_value END ) as _billing_address_1,
       group_concat(distinct oi.order_item_name separator '|' ) as order_items
from wp_posts p join
    wp_postsmeta pm
    on p.ID = pm.post_id join
    wp_woocommerce_order_items oi
where p.post_type = 'shop_order' and
      p.post_status = 'wc-completed' and
      oi.order_item_name = 'Product Name'
group p.user_id");
echo "<table>";
while($row = mysqli_fetch_array($result))
{
echo "<tr style='font-size: 0.665em'>";
echo "<td>"  . $row['billing_first_name'] . "</td>";
echo "<td>" . $row['_billing_last_name'] . "</td>";
echo "<td>" . $row['billing_address_1'] . "</td>";
echo "</tr>";
}
echo "</table>";
?>

我的最终目标是通过一个下拉菜单，根据产品id选择有问题的产品，这样我就可以看到每个用户购买了多少所查询的产品（仅根据上述代码完成订单）；

user ID | Product | quantity | billing address 
23      | chair   | 4        | 22 Bank street
42      | chair   | 12       | 123 Smith Road
88      | chair   | 5        | 3 Parker avenue

等