Getting the selected query range value in Variable

Hi Experts,

I have one table named "RetailTransactionTable".

In this table we have different customer account.

I want to calculate net amount based on different customers account through Query classes.

I tried like this:

#OCCRetryCount
Query query;
QueryRun queryRun;
QueryBuildDataSource qbds;
QueryBuildRange queryRange;
RetailTransactionTable retailTransactionTable;
real netTotalAmount;


Integer noOfCustomer = _CustomerTransactionContract.parmNoOfCustomer();
query = new Query(SysOperationHelper::base64Decode(_CustomerTransactionContract.parmQuery()));

queryRun = new QueryRun(query);

while (queryRun.next())
{ 
try
{ 
qbds = query.dataSourceTable(tableNum(RetailTransactionTable)); 
qbds.addSelectionField(fieldNum(RetailTransactionTable,netAmount),SelectionField::Sum); 
retailTransactionTable = queryrun.get(tableNum(RetailTransactionTable));

}
catch (Exception::Deadlock)
{
retry;
}
}

info(strFmt("%1",netTotalAmount));
}

Please suggest me the right path

Thanks

Sona Jee