You may use a Yes/No field to indicate if a case is closed or still open.  Then a simple query would help you find cases that are open or closed: SELECT * FROM Cases WHERE IsOpen=True.  You need to deal with cases, not customers, because a customer could have multiple cases open at the same time, if necessary.  You may even have one case that involves multiple customers, if that is necessary for you.  I'm assuming you have a customer support system or something of that nature.  If may help if each case would have a "priority value" so you could break down the cases further into more urgent cases and less urgent ones.  For instance, cases that remain open for the longest time may have higher priority.