I am using apex 4.1. and have a master details form. In the details region (tabular form) I have a column with list. when the user change the list I want to populate other filed base on database table value. e.g
when select a item from the list I want to populate the value of description of the item and qty order from a different table base on the select list value.
i think the better way is Ajax.
Google will give you better examples than me.