我在这个问题上做错了什么？主要问题是var_dump 返回null。该查询在phpMyAdmin中运行良好。

global $wpdb;
$users = $wpdb->get_results(
    $wpdb->prepare(
        "SELECT 
            u.ID AS id, 
            u.user_login AS name,
            AVG(b.star) AS rating, 
            COUNT(b.star) AS reviews,
            m.meta_value AS roles
        FROM
            wp_users AS u
                LEFT JOIN
            wp_usermeta AS m ON u.ID = m.user_id
                LEFT JOIN
            wp_bp_activity AS b ON u.ID = b.usercheck
        WHERE
            (m.meta_key = \'wp_capabilities\' AND m.meta_value LIKE \'%subscriber%\') 
        AND 
            (b.is_activated is null or b.is_activated = 1)
        GROUP BY u.ID
        ORDER BY reviews DESC
        LIMIT %d", $limit
    )
);
return var_dump($users);