I've often faced this problem but don't think ever found a good solution.  Let's say I need an item search that also brings in information about open purchase orders related to those items.  As soon as I add the joined field, of course it only brings back rows for items that have open purchase orders.  How can I keep a row for every item while also bringing in information about the PO?  In other words, I want the base list to include items that have no related purchase orders.