A Unified Constraint Model for XML

Integrity constraints are an essential part of a modern schema definition language. They are useful for semantic specification, update consistency control, query optimization, information preservation, etc.,. In this paper, we propose UCM, a model of integrity constraints for XML that is both simple and expressive. Because it relies on a single notion of keys and foreign keys, the UCM model is easy to use and makes formal reasoning possible. Because it relies on a powerful type system, the UCM model is expressive, capturing in a single framework the constraints found in relational databases, object-oriented schemas and XML DTDs. We study the problem of consistency of UCM constraints, look at the interaction between constraints and subtyping, and discuss algorithms for implementing these constraints.