Mapping XML DTDs to Relational Schema

we presented a technique to transform an XML DTD to a relational schema considering the structural aspects and the semantic aspects, such as domain constraint, not null constraint, cardinality constraints, ID constraint, and inclusion dependencies. The technique described how the various de¿nitions in a given XML DTD, such as elements, attributes, parent-child relationships, ID-IDREF(s) attributes, and collection types can be mapped to entities and relationships. It described how to handle the Union types that are not present in relational model, and it showed that the XML¿s ordered data model can be e¿ciently supported by the unordered relational data model