直接使用总计，因为您的联接可能会以您想要的组合方式创建更多行。
尝试以下操作：

SELECT id, MAX(Total) as FinalTotal ,MAX(Payment) as FinalPayment
FROM si_invoices a 
    left join 
    (select invoice_id, sum(total) as Total from si_invoice_items group by invoice_id) b 
    on a.id = b.invoice_id
    left join
    (select ac_inv_id, sum(payment) as Payment from si_payment group by ac_inv_id) c 
    on c.ac_inv_id = a.id 
group by id

或者，如果id是唯一的：

SELECT *
FROM si_invoices a 
    left join 
    (select invoice_id, sum(total) as Total from si_invoice_items group by invoice_id) b 
    on a.id = b.invoice_id
    left join
    (select ac_inv_id, sum(payment) as Payment from si_payment group by ac_inv_id) c 
    on c.ac_inv_id = a.id

详细结果：

select a.invoice,ac_inv_id,sum(a.total),sum(b.payment) from
(select 1 as id, 100 as invoice, '2014-05-07' as date, 200 as total union all
select  2, 100, '2014-05-07', 200) as a
left join
(select 1 as id, 100 as ac_inv_id, '2014-05-07' as date, 100 as payment union all
select 2, 100, 2014-05-07, 200) as b
on a.id = b.id

最终查询：

select a.invoice,ac_inv_id,sum(a.total),sum(b.payment) from
(select * from si_invoice_items) as a
left join
(select * from si_payment) as b
on a.id = b.id

结果：

100 100 400 300

选择projects.*，sum(project_tasks.ts_mount)作为Cost_Expend，sum(work_entries.w_mount)作为Actual_Amount from projects左侧加入project_costs上的project_costs。proj_id=projects。proj_id左加入project_costs上的project_tasks。cost_id=project_tasks。cost_id左加入project_costs上的work_entries。cost_id=work_entries。cost_id和work_entries。w_status=projects的‘已批准’组。proj_id；