From what I can tell from error messages, you are not allowed to use aggregate functions in subqueries.  Because of this limitation, I have rearranged my query so the aggregate functions are in the main query, but I am getting errors on the relationships.  Can someone help me sort this out?

I have renamed Opportunity as Dealer (plural: Dealers).

I have a Master-Detail relationship between Dealer (master) and custom sObject Car (plural is Cars), where there are many Car sObjects to one Dealer, and many Dealers to one Account.

I am trying to use the aggregate functions to sum up certain fields in the Car sObject for each Dealer.  For example, sum all the Car_Number_Field_1__c fields for all the Cars associated with one Dealer; then elsewhere I will roll up all these Dealer sums for each Account.

There are ways to do this without aggregate functions, but I am interested in getting this to work using aggregate functions.  The following select statement in the for statement will not compile.  I get errors in the subquery where the complier does not understand the relationship with Dealers__r and it doesn't understand Dealer__c.  Can someone help me figure out how to get this to compile?  Thanks.

I have created a Set of relevant Accounts in accountIDs

Map<Id, List<Decimal>> carTotals = new Map<Id, List<Decimal>>();

for (AggregateResult aggregateData : [select Dealer__c,

            (select Id, AccountID from Dealers__r where Id = Dealer__c),
            SUM(Car_Number_Field_1__c) Car1Total, SUM(Car_Number_Field_2__c) Car2Total,
            SUM(Car_Number_Field_3__c) Car3Total from Car__c
            where AccountID in :accountIds Group by Dealer__c])  {

     List<Decimal> carSums = new List<Decimal>{(Decimal)aggregateData.Get('Car1Total'),
          (Decimal)aggregateData.Get('Car2Total'), (Decimal)aggregateData.Get('Car3Total')};
     carTotals.put(AccountID, carSums);

}