I have a trigger, the trigger is designed to update the account name field as contacts are added. 

I would like it to be in the form:

Account with one contact called James peterson = Peterson, James Account

Account with contacts james peterson and ellie peterson = Peterson, james and Ellie, Peterson account.

I think I have my syntax off slightly. Please help! I just tried to add a contact to my account and I received this error message:

Error: Invalid Data.
Review all error messages below to correct your data.
Apex trigger ContactTrigger caused an unexpected exception, contact your administrator: ContactTrigger: execution of AfterInsert caused by: System.SObjectException: SObject row was retrieved via SOQL without querying the requested field: Contact.LastName: Trigger.ContactTrigger: line 11, column 1



This is my trigger, your help is very much appreciated

Trigger:

trigger ContactTrigger on Contact (after insert,after update, after delete, after undelete) {
    Set<ID> setAccountIDs = new Set<ID>();
    for(Contact c : Trigger.new){
        setAccountIDs.add(c.AccountId);
    }
 
    List<Account> accounts = [Select ID, Name,(Select Name From Contacts)  From Account WHERE ID IN :setAccountIDs];
    for(Account a : accounts){
        String accName = '';
        for(Contact c : a.Contacts){
        accName +=' and'+c.LastName+', '+c.FirstName;                     
        }
        a.Name=accName;
    }
    update accounts;
 
}


king regards,

Mikie