Edinburgh Napier University

  This paper analyses the problem generated by recursive patterns in typed query languages such as OQL [Cattell '97]. Recursive patterns describe hierarchical structures such as those defining classifications or part-explosion problems. A classification is composed of hierarchies of classes that eventually classify non-class objects. A part-explosion problem is composed of parts that contain other parts, thereby describing a complex object. They are commonplace in database schemas and software models but make it impossible to answer some queries that require the use of either or both attributes from classes and classified objects or attributes from different kinds of parts. The possible approaches to solve this problem are presented and discussed, and a solution proposed. This solution involves the creation of a simple well-defined operator that allows the expression of a type selection coupled with casting and error handling facilities. This way, it becomes possible without type error to query sub-classes of a particular type without writing programs.