mysql 一张表的多个字段关联另外一张表

SELECT
vtiger_orderitem.orderid,
	(
		SELECT
			vtiger_users.last_name
		FROM
			vtiger_users
		WHERE
			vtiger_orderitem.signatory = vtiger_users.id
	) AS signatory,
	(
		SELECT
			vtiger_users.last_name
		FROM
			vtiger_users
		WHERE
			vtiger_orderitem.servicer = vtiger_users.id
	) AS servicer
FROM
	vtiger_orderitem

SELECT
	n.orderid,
	m.last_name,
	q.last_name
FROM
	vtiger_orderitem n
LEFT JOIN vtiger_users m ON n.signatory = m.id 
LEFT JOIN vtiger_users q ON n.servicer = q.id