I need help in writing a batch apex class. I have two custom objects Agg_Datas_c and Agg_Datas_Final_c

In the two objects we have same fields they are called 18 GWP,19 GWP, 18 commission, 19 commission also a picklist field. 

I have a write a batch apex class in such a way that I should fetch the aggregate data from database in start method. 

In execute method I have to calculate variance and variancepercent

In finish method I have to put the data in other object. I am stuck in middle please help me.

global class Batch_AggregateDataTransfer implements Database.Batchable<sObject> {
    //start method to fetch the aggregate data from Database.
    global Database.QueryLocator start(Database.BatchableContext BC){
        string query ='SELECT SUM(X2018_GWP__c),SUM(X2019_GWP__c) FROM Agg_Data_c__c';
        return Database.getQueryLocator(query);
    }
    global void execute(Database.BatchableContext BC, List<Agg_Data_c__c> Batch){
        for(Agg_Data_c__c obj_Agg: Batch){
            AggregateResult ar = (AggregateResult)obj_Agg;
            Double counter1 = Double.valueOf(ar.get('X2018_GWP__c'));
            Double counter2 = Double.valueOf(ar.get('X2019_GWP__c'));
            Double variance=counter1-counter2;
            Double variancePercent= (counter1-counter2)*0.01;
            
        }
    }
    global void finish(Database.BatchableContext BC){
        {
            
        }
        
    }
    
}