trigger TotalAmountTrigger on Account (after insert, after update, after delete, after undelete)
{
    Set<Id> accountIds = new Set<Id>();
    for(Account c: Trigger.isDelete? Trigger.old : Trigger.new){
        if(c.GroupName__c  != null)                                                     
        {
            accountIds.add(c.GroupName__c );
        }
    }  
    List<CustomerGroups__c> accList = new List<CustomerGroups__c>();
    for(AggregateResult currentAggResult:[SELECT GroupName__c  accId, SUM(Customer_Loan_Amount__c ) sumAmt FROM Account  WHERE GroupName__c  in:accountIds GROUP BY GroupName__c ])
    {
        CustomerGroups__c acc = new CustomerGroups__c();
        acc.Id = (Id)currentAggResult.get('accId');
        acc.Customer_Group_OLB__c = (decimal)currentAggResult.get('sumAmt');
        accList.add(acc);
        accountIds.remove((Id)currentAggResult.get('accId'));
    }
   
    for(Id currAccId : accountIds)
    {
        CustomerGroups__c acc = new CustomerGroups__c();
        acc.Id = currAccId;
        acc.Customer_Group_OLB__c = null;
        accList.add(acc);
    }
    update accList;
}

@isTest
public class TestTrigger1 
{
    static testMethod void testMethod1() 
    {
         CustomerGroups__c p = new CustomerGroups__c ();
       	p.Name='testgroup';
        insert p;
        
        Account c = new Account(GroupName__c = p.id);
		c.Name='CustomerTest';
        c.Date_of_Sale__c=date.today();
        c.Invoice_No__c='123';
        c.GroupName__c='Rotterdam Group';
        c.CurrencyIsoCode ='ZAR';
		insert c;
       
        Opportunity g = new Opportunity (AccountId = c.id);
        g.Name='OpptyTest';
        g.CloseDate=date.today();
        g.CurrencyIsoCode ='ZAR';
        g.StageName='Closed Won';
        insert g;
        
        OpportunityLineItem b = new OpportunityLineItem (OpportunityId = g.id);
        b.Owed_so_far__c=100;
        b.Paid_so_far__c=5;
     	b.Quantity=1;
        b.TotalPrice=1000;
        insert b;
        
        Product2 f = new Product2 (Id = b.id);
        f.Name='ACE1 R1,080  9Month Credit';
        f.CurrencyIsoCode='ZAR';
        insert f;
        
    }
}