以下函数将为您提供一个数组，其中包含从每个自定义元字段“Brand”值的所有产品变体中计算出的库存量，使用WPDB 类（SQL查询）：

global $wpdb;

$results = $wpdb->get_results( "
    SELECT pm.meta_value as brand, SUM(pm2.meta_value) as stock
    FROM {$wpdb->prefix}postmeta pm
    INNER JOIN {$wpdb->prefix}posts p
        ON pm.post_id = p.post_parent
    INNER JOIN {$wpdb->prefix}postmeta as pm2
        ON p.ID = pm2.post_id
    WHERE p.post_status = \'publish\'
    AND p.post_type = \'product_variation\'
    AND p.post_parent > 0
    AND pm.meta_key = \'Brand\'
    AND pm2.meta_key = \'_stock\'
    AND pm2.meta_value != \'\'
    AND pm2.meta_value != \'NULL\'
    GROUP BY pm.meta_value
");

// Display (example)
echo \'<ul class="stock-by-brand">\';

foreach ($results as $result ) {
    echo \'<li><strong>\' . $result->brand . \'</strong>: \' . $result->stock . \'</li>\';
}

echo \'<ul>\';