Chapter Notes

Not all versions of ORM allow independent entity types. FCO-IM (Bakema et al., 1994) handles the problem by treating entity types as objectifications of unaries.

Most versions of ER do not allow subset, equality, or exclusion constraints between role-sequences, or attributes, to be included on the schema diagram (see Chapter 8). UML includes graphic symbols for subset constraints only between whole associations, and exclusion constraints only in the context of an exclusive-or constraint between roles (see Chapter 9). Recent versions of ER, as well as UML, offer basic support for subtyping, but ignore the need for formal subtype definitions or context-dependent reference (see Chapters8 and 9).

Some versions of ORM restrict subtypes to derived, active subtypes. Context-dependent reference is not supported in some versions. The PSM variant adopts a different approach to generalization in which the supertype uses a disjunction of the identification schemes of its subtypes (e.g., see ter Hofstede et al. 1993). Although this raises additional implementation problems when a common query is executed on overlapping subtypes, this does provide another viable approach.

For early discussion of subtyping from an ORM perspective, an informal treatment can be found in Halpin (1995) and a formal treatment in Halpin and Proper (1995b). A recent treatment updated for ORM 2 is provided in Halpin (2007a); this paper also distinguishes rigid subtypes from role subtypes, a distinction examined in Chapter 10.