我建立了一个发票系统，现在我尝试查看每张发票的总价格和要支付的金额。
我离开加入了发票上的 invoice_part 表并进行了总结，现在我也想加入付款表，但是当我总结时，有时会多次付款。有没有办法我可以完成加入只收一次付款？
$stmt->from(array('x'=>'invoice'),array('x.id as id','x.ref_id as ref_id','x.start_date as start_date','x.regard as regard','x.project_code as project_code'));
$stmt->joinLeft(array('ip'=>'invoice_part'),'x.id=ip.invoice_id','SUM(ip.price*ip.amount*(100-ip.discount)/100) as price_ex');
$stmt->joinLeft(array('p'=>'payment'),'x.id=p.invoice_id','SUM(ip.price*ip.amount*(100-ip.discount)*(100+tax)/10000)-IFNULL(SUM(p.amount),0) as price_open');
//joins the payment multiple times if there are multiple invoice parts, payment should only be joined once
//note: there can be multiple payments for one invoice
$stmt->group('x.id');
结果查询：
SELECT `x`.`id` , `x`.`ref_id` , `x`.`start_date` , `x`.`regard` , `x`.`project_code` , `o`.`name` AS `contact` , `d`.`name` AS `department` , `c`.`name` AS `company` , `is`.`name` AS `status` , SUM( ip.price * ip.amount * ( 100 - ip.discount ) /100 ) AS `price_ex` , SUM( ip.price * ip.amount * ( 100 - ip.discount ) * ( 100 + tax ) /10000 ) - IFNULL( SUM( p.amount ) , 0 ) AS `price_open`
FROM `invoice` AS `x`
LEFT JOIN `invoice_part` AS `ip` ON x.id = ip.invoice_id
LEFT JOIN `payment` AS `p` ON x.id = p.invoice_id
GROUP BY `x`.`id`
所以当我有 2 个发票部分和 1 个发票付款时。付款被计算两次。我怎么能只算一次呢？