I'm not sure that this is even possible but:

The requirement is to look at the last 90 days of cases by account and determine the average number of cases by account. THis is meant to be a running total - trying to find the top n accounts that require training at any given time. This functionality fires on update of the case.

Then I need to check those cases that match a certain Resolution code. (Training Required)

so:

		for (Case c: cases){
			system.debug('beforeAccountUpdate -->: Case: OwnerId: ' + c.OwnerId + ' Account: ' + c.AccountId + ' LastModifiedById: ' + c.LastModifiedById + ' Updated: ' + c.Updated__c);
			if (c.AccountId != null && c.Status <> 'Cancelled' && c.Type == 'Support'){
				accountIds.add(c.AccountId);
			}
		}
system.debug('\n accountIds -->: ' + accountIds );
		 
		AggregateResult[] allCases = [select accountid, count(id) from Case where CreatedDate > :dtCreate group by accountid];
		for(AggregateResult ar : allCases) {
			System.debug('\n ------------------->intTtlCases = ' + intTtlCases);
			intTtlCases  = intTtlCases + (Integer) ar.get('expr0');
		}
			System.debug('\n ------------------->allCases = ' + allCases);
			System.debug('\n ------------------->allCases.size() = ' + allCases.size());
		
		decAverage = intTtlCases/allCases.size();
			System.debug('\n ------------------->decAverage = ' + decAverage);
		
		AggregateResult[] AccountCases = [select accountid, COUNT(id) from Case where CreatedDate > :dtCreate and accountid in: accountIds group by accountid];
		for(AggregateResult ar : AccountCases) {
			AccCases.put((ID) ar.get('Accountid'), (Integer)ar.get('expr0'));
			System.debug('\n ------------------->Account id = ' + ar.get('accountid'));
			System.debug('\n ------------------->count = ' + ar.get('expr0'));
			System.debug('\n ------------------->AccCases = ' + AccCases);
		}
	 	// find all cases for all of the accounts in the last 90 days
	    if(accountIds != null && accountIds.size() > 0){
	        List<Account> lstAccount = [select id, AccountCases__c, Account_Training_last_90_days__c, (select id from Cases where CreatedDate > :dtCreate and Resolution_Code__c in :lstResCodes) from Account where id in: accountIds];

system.debug('\n lstAccount -->: ' + lstAccount );
	        if(lstAccount.size() > 0){
	            for(Account acc: lstAccount){
	            	if(acc.Cases.size() > 0){
						acc.AccountCases__c = acc.Cases.size();
						acc.Account_Cases_last_90_days__c = AccCases.get(acc.id);
	            	}
				}
system.debug('\n lstAccount -->: ' + lstAccount );
			 	// update the accounts with the number	            
	            update lstAccount;
			}
		}

 What I see is that the AggregateResult gets the first n results - not the whole database of <90 day old cases. That gives me an average which I know to be wrong from drawing a report into excel..

My question: How can I pull all of the cases, count them by account, get the average number for our client base then judge whether this account is in the top n, or not?