I am not sure that is quite what I am looking for. Consider the following data:
Customer, Date, Activity Type
1, 1/1/10, Mailshot
2, 3/1/10, Post Out
4, 4/1/10, Mailshot
4, 6/1/10, Mailshot
5, 7/1/10, Post Out
1, 8/1/10, Post Out
The filtered list should not show Customer 1 because they have...