trigger ContactRatingTrigger on Case (after insert, after update, after delete, after undelete) {
    if (Trigger.isAfter) {
        if (Trigger.isInsert || Trigger.isUpdate || Trigger.isUndelete) {
            ContactRatingHandler.updateContactRatings(Trigger.newMap.keySet());
        } else if (Trigger.isDelete) {
            ContactRatingHandler.updateContactRatings(Trigger.oldMap.keySet());
        }
    }
}

public class ContactRatingHandler {
    public static void updateContactRatings(Set<Id> caseIds) {
        List<Contact> contactsToUpdate = new List<Contact>();
        
        // Query related Cases and calculate average rating per Contact
        Map<Id, Decimal> contactAverageRatings = new Map<Id, Decimal>();
        for (AggregateResult result : [
            SELECT ContactId, AVG(Rating) avgRating
            FROM Case
            WHERE Id IN :caseIds
            GROUP BY ContactId
        ]) {
            Id contactId = (Id)result.get('ContactId');
            Decimal averageRating = (Decimal)result.get('avgRating');
            contactAverageRatings.put(contactId, averageRating);
        }
        
        // Update Contact records with the calculated average ratings
        for (Contact contact : [
            SELECT Id, Rating__c
            FROM Contact
            WHERE Id IN :contactAverageRatings.keySet()
        ]) {
            Decimal averageRating = contactAverageRatings.get(contact.Id);
            contact.Rating__c = averageRating;
            contactsToUpdate.add(contact);
        }
        
        if (!contactsToUpdate.isEmpty()) {
            update contactsToUpdate;
        }
    }
}