根据条件

Select Main.MainId,
       Main.PlateNo,
       Info.Location,
       MaintenanceCosts.MaintenanceCost,
       RepairCosts.RepairCost
From   Main
       Inner Join Info
         On Main.MainId = Info.MainId
       Left Join (
         Select MainId, Sum(Cost) As MaintenanceCost
         From   Maintenance
         Where  InvoiceDate >= @StartDate
                And InvoiceDate < @EndDate
         Group By MainId
         ) As MaintenanceCosts
         On Main.MainId = MaintenanceCosts.MainId
       Left Join (
         Select MainId, Sum(Cost) As RepairCost
         From   Repairs
         Where  InvoiceDate >= @StartDate
                And InvoiceDate < @EndDate
         Group By MainId
         ) As RepairCosts
         On Main.MainId = RepairCosts.MainId
Where  Info.Location = @Location 

select 

isnull(sum(a.[ Cost]) + sum(b.[ Cost]),0) cost,
a.[ MainID],a.[ InvoiceDate]

from 

[maintanen] a inner join 
[repairs] b 

on a.[ MainID]=b.[ MainID]


group by a.[ MainID],a.[ InvoiceDate]