You have a WHERE clause on the histories but you still miss one on the Account level.
For example this would return only Accounts that have Contacts:
SELECT Id, Name
FROM Account
WHERE Id IN (SELECT AccountId FROM Contact) // try with NOT IN too
With Activities it's trickier because they don't like to be used in WHERE in that way.
http://www.salesforce.com/us/developer/docs/officetoolkit/Content/sforce_api_calls_soql_select.htm
The following objects are not currently supported in subqueries:
- ActivityHistory
- Attachments
- Event
- EventAttendee
- Note
- OpenActivity
- Tags (AccountTag, ContactTag, and all other tag objects)
- Task
Additionally the fine print at the bottom of ActivityHistory definition is also a bit discouraging.
The following restrictions on users who don’t have “View All Data” permission help prevent performance issues:
- In the main clause of the relationship query, you can reference only
one record. For example, you can’t filter on all records where the
account name starts with ‘A’; instead, you must reference a single
account record.
- You can’t use WHERE clauses.
- You must specify a limit of 499 or fewer on the number of rows returned in the list.
- You must sort on ActivityDate in ascending order and LastModifiedDate in descending order; you can display nulls last. For
example: ORDER BY ActivityDate ASC NULLS LAST, LastModifiedDate DESC.
Looks like you will need multiple queries. Go for Task (or Event, depending for which the custom field is visible), compose a set of AccountIds and then query the Accounts?
Or you can manually filter through list from your original query, copying accounts to helper list:
List<Account> finalResults = new List<Account>();
for(Account a : [SELECT...]){
if(!a.ActivityHistories.isEmpty()){
finalResults.add(a);
}
}