sql

public function getUsedCategories() {
        global $wpdb;
        $vouchers_table_name = $wpdb->prefix . 'voucherpress_vouchers';
        $categories_table_name = $wpdb->prefix . 'voucherpress_categories';
        $category_parents_table_name = $wpdb->prefix . 'voucherpress_category_parents';
        return $wpdb->get_results($wpdb->prepare(
            "
            SELECT
                cp.name AS parent,
                GROUP_CONCAT(
                    DISTINCT
                    c.id
                    ORDER BY c.id
                    ASC
                    SEPARATOR ';'
                ) AS ids,
                GROUP_CONCAT(
                    DISTINCT
                    c.name
                    ORDER BY c.id
                    ASC
                    SEPARATOR ';'
                ) AS categories
            FROM $categories_table_name AS c
                INNER JOIN $vouchers_table_name AS v
                    ON c.id = v.category_id
                INNER JOIN $category_parents_table_name AS cp
                    ON c.parent = cp.id
            ORDER BY cp.id
            ASC
            "
        ));
    }